Roofing plays a very important part of a building, not only aesthetically but structurally. It is important that a roof, whether flat or pitched, is designed and installed correctly. Rain and wind can play havoc with a roof that has been installed badly or to the incorrect manufacturers’ specification; for example, the incorrect pitch, the incorrect application, or poor workmanship.
With an age of diversity in the building Industry and the abundant choice of design and materials, one tendency has become very clear; the increasing complexity of the geometry of buildings, and more especially of roofs. It is therefore simple to deduce that defects increase in direct proportion to this increase in complexity of geometry of the surfaces of buildings; i.e. at the intersections of different planes and materials. Notwithstanding certain roofs suit only certain designs.
Added to this is the increasing lack of properly skilled artisans in erecting roof structures and the fitment of the desired roof coverings, associated fittings and accessories; highlighting the need for taking extra care in both design and construction, when it comes to roofing.
It is also an element which requires the supervision of an engineer. Local authorities will not issue occupancy certificate to a building without an Engineer’s certificate that the roof complies with the standards as laid out in the National Building Regulations.
One must consider that a roof has numerous functions other than aesthetics that need to be taken into account in the design and erection process:
The top of a truss (roof) where the two chords (rafters) meet.
A barge board is a sloping roof trim fixed in lengths along the edge of a gable to cover the exposed timbers or as some describe as an ornamental board along the gable end of a roof.
Pieces of timber usually SA pine 38mm x 38mm or 38mm x 50mm laid and nailed across the top chords of the truss at 90° at a spacing to suit the roof covering but less than 540mm onto which the roof covering is fixed or hooked and typically used in roof slate or tile applications (see brandering).
The part of the truss that forms the bottom edge that joins the two heel joints and supports the ceiling – also called the tie-beam.
The members fixed to several adjacent trusses usually at a 45° angle to make the roof stable and to prevent buckling of compression members.
Same as battens but fixed to the bottom chord to support the celling (see batten).
Trusses where the top chords (rafters) slope up to the apex at the same angles (pitches) from both ends of the truss.
Trusses where the top chords slope up to the apex at two different angles (pitch) from each end of the truss.
A hip end where the end slope does not reach the apex but the top part of the hip forms a small gable; also called a louver hip.
The under part of a sloping roof that overhangs a wall or a vertical surface (see overhang).
A board or similar trim set on edge along the eaves to cover the rafter ends (sprocket ends) and can also carry a gutter, or a simple horizontal band that projects slightly from the wall surface below the edge of the roof.
A gable is the triangular upper part of a wall at the end of a pitched roof. There are different types of gable ends, and these relate to different architectural styles for example, Cape Dutch.
Small gable on the roof slope, usually formed by a valley set.
A truss (single or multiple ply) used to support other trusses.
A U shaped bracket made of light gauge galvanised mild steel used to support trusses on a girder or beam.
The truss end joint where the top and bottom chords connect, or where the end web joins the bottom chord in stub and mono-pitch trusses.
A hip is an outer intersection between two sloping surfaces of a pitched roof – water flows away from a hip.
A light gauge galvanised mild steel angle bracket used to fix two timber members at 90° to each other.
Insulation in roofing is a material used to prevent heat loss and/or heat gain through a roof. The insulation membrane can also provide fire protection as well as a control against condensation (if correctly installed) in sheeted roof applications.
The smallest end part of a hip corner construction using only single pieces of timber (loose rafter).
The mono pitch trusses of the hip which are supported at the high end by the hip girders.
A truss where there is only one rafter slope.
Extensions of the bottom chord past the truss end, usually to support the truss in brickwork or on a truss hanger.
The part of the truss top chord that extends past the truss heel. Measured horizontally from the truss heel, but from the outside wall face of the building.
The angle of a roof as measured off a horizontal plane.
Top chord (rafter) overhangs are usually cut off vertical, to allow the fixing of fascia boards and/or a gutter at the roof truss ends (sprocket ends).
Trusses with a pitch change in each top chord from a lower pitch to steeper pitch going from heel to apex.
A frame of two columns with one horizontal roof beam between them, or two sloping rafters that join in the middle.
These are pieces of timber usually SA pine 76mm x 50mm laid and nailed at 90° across the trusses or beams onto which the roof covering is fixed and typically used in sheeted roof applications.
A sloping roof beam, usually from eave to ridge. The term rafter is also applied to all types of trussed rafter, the sloping beam of a portal frame, and the principal rafter of a truss – also known as the top chord.
A roof can be defined as the upper structure for and covering of a building, and roofing as the materials which form that covering.
The horizontal line on top of a pitched roof (the apex) usually covered, for example with ridge tiles.
The distance between the centres of two of the same elements, i.e. trusses.
The span is the distance between two supports (usually walls) onto which a truss or beam rest.
Also called a stub heel; where the top and bottom chords are some distance apart and connected by the first truss web.
The bottom horizontal member of a roof truss equal in length to the span of the roof – also known as the bottom chord.
Also known as a tilting fillet; the tilting batten is the last batten at the bottom edge of the rafter. It is larger than the battens and usually 76mm x 50mm and sometimes cut to triangular shape in cross-section. It lifts the end of the last tile or slate.
Also called a rafter; the part of a truss, which forms the top edge usually at a slope and has the battens or purlins fixed to it to carry the roof covering.
A truss is a rigid structural framework of timbers or steel members bridging a space with each end of the truss resting on some form of support, usually walls. The trusses provide support for battens or purlins which in turn support the roof covering, e.g. roof tiles. Trusses are often prefabricated and brought to site, but can also be built on site.
Underlays can be fitted under the tiles or roof covering to assist with waterproofing should the concrete tile or other roof covering fail; the underlay can also act as an insulating membrane or vapour barrier. An underlay in roof tile applications is also used to prevent dust from entering the ceiling/roof space.
A valley is an inner intersection between two sloping surfaces of a pitched roof – water flows towards a valley.
Impervious barrier that prevents the passage of water vapour through building components.
The sloping edge of a pitched roof above a gable. A verge can overhang or be flush with the wall.
A piece of timber usually SA pine 76mm x 38mm placed flat on top of the supporting wall (completed brickwork) on the inside skin acting as a flexible and bearing surface for the trusses or beams to be placed and fixed and to spread the load.
The truss members that connect the top and bottom chords together.
SANS 10400 establishes general requirements for satisfying the National Building Regulations issued in terms of the National Building Regulations and Building Standards Act, 1977 (Act No. 103 of 1977), and some of these requirements that are deemed to satisfy the following parts of such Regulations of Part L: Roofs are highlighted below:
The roof of any building shall be so constructed that it will:
The requirements of subrules LL3.4 and LL3.5 shall apply only to single or double pitched Howe-type trusses, with a span of not more than 10 m, supported at heel joints only and having bays of equal lengths of not more than 1.5 m.
Table 15.1 may seem confusing or even incorrect since the same nominal timber sizes appear to be capable of supporting either a Class A (light) or a Class B (heavy) roof covering and in the case of the heavier covering, greater truss spans are permitted. This apparent anomaly is negated when this table is read in conjunction with subrule LL3.6 (see below), as the centre-to-centre spacing of trusses supporting Class B roof covering is limited to a maximum of 760 mm compared with 1400 mm for Class A roof covering. The load per truss for a roof supporting a Class B covering is therefore less than that for a truss supporting a Class A covering.
It would appear from the table that in certain cases the use of a better grade timber is of no advantage as it does not lead to any increase in the maximum span that is permitted. The figures have been included in order to complete the table but the restriction on span is artificial in the sense that it is not directly related to the capability of the timber. The fact that no span of more than 10m has been included in the table serves only to draw attention to the fact than any roof supported on walls complying with the rules contained in Part K of SANS 10400 is restricted to a maximum span of 10m. It is also important to note that the values quoted in this table should only be used subject to the further limitations that –
The centre-to-centre spacing of trusses relevant to the roof covering to be applied shall not exceed:
|Sheets, either metal or fibre cement||1400mm|
|Concrete tiles, clay tiles or tiles of similar material||760mm|
Roofs used in domestic and small commercial buildings are usually constructed by using timber roof trusses and/ or rafters and in certain applications timber poles. While in larger commercial and Industrial buildings, portal frames or girders using steel are more commonly used.
The need for taking care in the design, the construction and erection of roofs and fixing of the roof covering is essential to insure water penetration into the interior of the building is avoided. There is an absolute requirement in roof construction to prevent water reaching the interior of a building and where two kinds of rainfall intensity need to be considered:
Both categories contribute to the total quantities of rainwater needing disposal, but the second category particularly affects the weather tightness of lapped roofing, such as tiles and slates, and even the direction and extent of lap of larger sheets; with many manufacturers recommending the use of underlays, fixing of tiles etc. in these applications.
One must remember that rain falling while the wind is blowing affects pitched roofs more than flat (and walls even more so), it is therefore important to consider ones geographical location, associated weather patterns and not only your desired roofing requirements when deciding on an appropriate roofing design. Roofs can be flat or pitched and are made up of a variation of elements, types and shapes.
The most commonly found forms of roof construction include the following:
A roof truss is a coplanar (forces lying in the same plane) assembly of structural members made from timber or steel joined at their ends. The diagonals form triangles, producing a rigid framework. Triangulation is the fundamental concept behind the design of a roof truss; a triangle is the only multisided geometric figure that is rigid and will not move or rotate provided a structural member doesn’t bend or a connection (join) fail.
Roof trusses act like beams and support a roof and all imposed loads. A roof truss consists of top and bottom chords and a system of webs. The slope of the top chord (the pitch) can vary as required to suit the roof form, design and roof covering.
Some of the more commonly used truss configurations are illustrated in figure 15.4 below.
Timber trusses are widely used in the construction of roofs and for all types of buildings. They can be easily erected on site, are relatively easy to handle and can span the width of most buildings without interior load-bearing walls being required.
Site made trusses
Roof trusses manufactured on site must comply with the minimum “deemed to satisfy” requirements for nailed and bolted trusses of SANS 10400 or be designed by a professional engineer or other competent person.
Some local authorities will require an engineer’s certificate for the roof truss construction before issuing an occupancy certificate and similarly some financial institutions won’t make final payment without an engineer certifying the roof construction.
The requirements of the National Building Regulations are that the truss, single or double pitch, shall be a ‘Howe’ type truss (see figure 15.4) with a span not exceeding 10m for double pitch trusses and 6m for single pitch trusses.
The trusses must be supported at heel joints only and have bays of equal lengths not greater than 1.5m.
Then depending on the class of roof covering, the size of the rafter (top chord), tie-beam (bottom chord) and the grade of timber to be used it must be selected from table 15.1 in such a way that the desired truss span does not exceed the relevant figure – see LL3.4 under standards above.
For more on timber grades and sizes see the next sub-section on materials.
The spacing of site made roof trusses must be in accordance with SANS 10400 Part L: Roofs – LL3.6 – as described under standards above.
The number of connecting devices to be used at each intersection between two members at any heel joint or any splice in a site made truss shall be determined from table 15.3. In the case of any joint other than a heel joint or splice, one 10mm bolt plus four clinched 90 x 4mm nails shall be used.
To ensure a high standard of finish, it is essential that the roof structure is properly constructed. Poor workmanship and warped timber will reflect on the finished product and may result in deflection and distortion of the roof.
A network of prefabricated timber roof truss manufacturers can be found throughout South Africa. These fabricators operate under licence to suppliers of nail-plate connectors and use computer design programs devised by professional engineers. These fabricators are trained and equipped to offer advice and solutions for any shape of roof, any pitch or span for new structures, or to match existing roofs and since they are carefully engineered and factory built, prefabricated trusses have a consistent, reliable quality.
For the purposes of municipal approval (when required), design calculations can be issued, together with the appropriate roof layout, truss diagrams and any explanatory notes that may be needed.
When placing orders with a truss fabricator, or when a quotation is required, the fabricator must be provided with the following minimum information or given a detailed roof drawing covering the following:
In most instances the truss fabricator would make a site inspection prior to manufacture of the trusses, to ensure dimensions and building work match that of the drawings originally provided.
Structural timber suitable for all normal building work is covered by SANS 1783-2004, which defines visual stress grades and mechanical stress grades. The compulsory grading of timber prescribes that under grade structural sized timber is clearly marked with black crosses (known as black cross) on both ends to distinguish it from graded timber bearing the appropriate SANS mark for the various visual grades.
In order to obtain the permissible stresses, stress grades are adjusted by means of the appropriate modification factors which relate to the specific conditions of use. Structural timber can be used to its full safe load-bearing capacity, and hence more economically, in engineered structures if it is stress-graded. S5 is the most commonly used on site timber; and S7 or mechanically graded timber is used for engineered designed prefabricated trusses.
Timber is available in three different types of finish, to suit specific applications;
Rough sawn timber (RGH)
Timber sold as rough has no further planing done to it after milling; also known as fine sawn. Structural sizes 38x152,
38x228, 50x152, 50x228 and 76x228.
Structural sizes 38x38, 38x50, 38x76 and 38x114 are generally sold as “Sized”. These products have been planed slightly to a consistent size to allow accurate truss manufacture and exact framing of buildings. The afore mentioned sizes are generally planed to 36x36, 36x47, 36x73, 36x111.
Planed timber (PAR)
Is a smooth finish which suits certain applications e.g. exposed trusses requiring painting. Planed timber has to be ordered as the process involves rough or sized timber, placed in a planing machine which then planes each side of the timber. As planing reduces the size of timber a decrease in strength can be expected.
Generally, the SABS allow a +3 -2mm tolerance on the thickness of rough sawn, kiln dried SA Pine. The lengths could be up to 50mm longer than that stated. 0.900 to 2.700 metre lengths are referred to as short lengths and 3.000 to 6.600 metre lengths as long lengths; which are also priced differently to that of short lengths. Lengths longer than 6.600 metres are available as finger- jointed timber.
A number of steel truss systems are available for roof construction. Included are roof trusses, open-web joists and space frames. Open-web joists are a type of truss. They are light-weight structural members made from steel angles and bars. They, like other trusses, are end bearing and can span long distances in roof construction. Space frames are a three dimensional truss system. They provide a structural frame that spans in several directions and over large areas with a minimum number of vertical supports (columns). Steel trusses consist of top and bottom chords and a system of webs. Steel trusses are manufactured using galvanized mild steel and depending on the system and manufacturer.
the material sizes and thicknesses differ quite considerably. Steel trusses are designed and usually prefabricated although for low-cost housing or geographically remote projects steel trusses can be supplied in kit-form and assembled on site. The biggest advantage of using steel is that it is lightweight and compact which improves handling and erection on site and is economical to transport. Another is the span capability of steel where clear spans of up to 40metres are possible. Lastly steel is non-combustible and is more suitable for certain applications. Some examples of steel truss components showing sizes are illustrated below:
The difference between a rafter and beam in roofing is a matter of description and use. A rafter is a sloping roof beam typically in timber, which is fitted from eave to ridge and used in place of roof trusses for aesthetic reasons by design or purpose. Whereas a beam can be described as a structural member that carries a load and can be in timber or steel. Rafters can also be described as girders, joists or bearers, the latter describing supports for a suspended floor. Beams can also be used to carry timber trusses or poles and are then built into the structure (walls) of the building and/or spanned between columns to support a roof. Materials used for rafters and beams Timber Laminated beams Laminated beams can be used for beams and rafters as their properties provide to carry greater loads and spans (up to 15 meters) than that of normal structural pine and are available in the following popular sizes – these sizes can vary slightly from manufacturer to manufacturer.
Extract from SANS 10400 Part L: Roofs
Where rafter construction is used in place of roof trusses and the roof covering is of the class given in column 1 of Table 15.6 the size of rafter and grade of timber to be used shall be selected from such table in such a way that the rafter span does not exceed the relevant figure for maximum rafter span given in columns 3 to 14, as the case may be. Where rafter spacing differs from that in Table 15.6, intermediate values of maximum rafter span may be interpolated within the range of values given, for the relevant timber grade.
Steel is an alloy of carbon and iron, which is hard, tough, elastic, and capable of resisting various stresses and loads. Mild steel is the most popular steel used for general structural beams used in roofing. Steel sections come in a number of different shapes and sizes and the most popular sections used in roofing are the I-Beam; Channel; Angle Iron and T-Section. We have only highlighted these components very briefly in the section as timber is the more commonly used material used in residential and smaller type commercial buildings.
Note: For more on steel sections and sizes see Metalwork section.
Commonly referred to as ‘Gum’ poles from the Eucalyptus (Gum) tree from which poles are cut. (Eucalyptus – Grandis/Saligna)
Round poles are typically used, for the timber frame construction of thatched roofed buildings. They are of course used for many other applications e.g. Props, fencing, bearers, pergolas etc. Timber poles and associated products are available untreated but are usually treated in one of the following:
Nail lengths for different tile profiles
Round Poles are available in the following diameters from nominal Min. diameter to Max. diameter.
Available in lengths starting at 1.800m up to 7.800m – in 600mm increments. Longer lengths are available up to 13.000m
Slabbed and split poles
Laths or saplings serve the same purpose as battens and are used for tying/fixing thatch in thatch roof applications; also used for fencing and decorative applications. Available in diameters from 15mm up to 30mm; and in lengths from 1.800m up to 4.500m.
The following sub-sections discuss briefly the various types of roof coverings in common use; also showing the three different classifications of roof covering as described in table 15.1.
Concrete (Class B)
Concrete roof tiles are a common and cost effective form of roof covering. Concrete roof tiles consist of a mix of cementitious materials, such as Portland cement, sand, hydraulic cements, fly ash, pozzolans, fine aggregates and pigments, producing a durable lightweight tile that is fire resistant. Some are pigmented in the mix process known as ‘through colour’, others are coated after moulding.
Concrete roof tiles are manufactured in an extensive range of profiles, colours and finishes (finishes will vary from one manufacturer to another) which enhance the visual appearance of any roof and provide designers with a wide scope for expression.
Truss and rafter centres when using concrete roof tiles are spaced to a maximum of 760mm using 38 x 38mm battens – refer Standards LL3.6.
Underlay – a suitable underlay is in all cases recommended. Modern practice has demonstrated that the underlay is a fundamental part of a tiled roof at pitches below 26° and for pitches above 45°, and at all pitches in exposed and coastal areas. (See underlays later in this section for more information)
Clay (Class B)
Clay roof tiles enhance warmth and character to a building with permanent colour which weather and age over time, but never fade. Clay roof tiles are natural and durable which elegantly enhances the appearance of roofs, not only withstanding the elements, but actually improving with age from exposure.
Clay roof tiles are available in the ever-popular Broseley, Constantia & Cordova as well as the cost effective Portuguese and Marseilles. They are available from various different manufacturers in South Africa, in various colours like terracotta and multi flashed colours. Various bonds are used when laying different types of clay tiles, from; Straight bond or mock interlocking, broken bond, flat and semi-circular over under; the over-tile and under-tile are of roughly the same shape, the under-tile being larger; these tiles form a beautiful roof which is flexible in both sidelap and headlap.Truss and rafter centres when using clay roof tiles are the same as that of concrete roof tiles.
Clay cover tiles are usually used to cover IBR or Brownbuilt roof sheeting, to provide an attractive covering when low roof pitches are required. They can also provide thermal insulation and acoustic benefits. In certain instances it is required that IBR roof sheeting be covered with a cover tile by local authorities.
The truss and rafter centres when using cover tiles over sheeted roofing will reduce from the maximum of 1 400mm as directed by the cover tile manufacturer.
Pressed Metal (Class C)
Pressed metal roof tiles are strong and light weight which significantly reduces the quantity of timber required in the support structure and provides easy installation. They allow for ease of delivery and don’t require large trucks to transport them, making this a popular roof covering in remote lying areas. A further advantage being steel-based, no breakages occur during transit or storage on site.
Metal roof tiles come in different profiles and finished with either a standard acrylic coating or a textured coating and available in various colours and are also manufactured from different materials.
Due to their lightweight attributes, tiles are the ideal application
in the re-roofing market where they can be laid over the existing
roof without removing the old roofing material. Truss and rafter centres when using pressed metal tiles are spaced to a maximum of 1 050mm using 38 x 38mm battens – refer Standards LL3.6. Pressed metal tiles can be used without underlay on roof pitches between 15° and 45°; and with underlay with pitches from 10° to 15°.
Sheeted roofing systems come in various different profiles, e.g. corrugated, IBR and folded steel. And manufactured from numerous different types of material to suit specific applications, all having certain advantages and unique properties with metal sheeting being the most commonly used.
Metal (Class A)
Metal Roof sheeting is available in a wide variety of profiles, thicknesses and types of material and coatings. Usually all roof sheeting materials are manufactured from hot-dipped galvanized steel and fall within internationally accepted specifications and tolerances.
Other types of material are manufactured like Supergalm which is an aluminium zinc alloy coated steel and Zincalume® a composite of aluminium and zinc. Typical types of material include; Commercial quality, high tensile and econogalv, while types of coatings include Chromadek. There is a variety of sheeted roof systems designed for larger type commercial buildings like shopping centres and industrial buildings known as ‘concealed fixing’ or ‘standing seam’ roof systems and are quite different to ‘exposed fixing’ roof sheets used for smaller type buildings and residential buildings. We have concentrated more on the pierced/exposed fixing systems used in residential building projects and only provide a brief overview of concealed fixing systems.
IBR and Widespan sheets must be laid with one corrugation and corrugated sheets with one and a half corrugations, side lap with the narrow flute uppermost and should be fixed through the crests of alternate flutes to purlins – see figures on next page (all fixing holes should be drilled and not punched).
Truss and rafter centres when using metal roof sheeting are spaced to a maximum of 1 400mm using 76 x 50mm purlins – refer Standards LL3.6.
The name IBR is abbreviated from ‘Inverted Box rib’ and has become a household name in the South African building industry. The deep, broad flute design offers excellent drainage characteristics combined with optimum weight versus load/span capabilities.IBR sheeting has a square fluted profile usually with an effective covering width of 686mm designed for use as side cladding or roofing material in commercial, industrial and domestic applications.
Widespan is a roofing and cladding profile designed to provide an economic alternative to deeper Box rib profiles without losing the aesthetic appeal of a square fluted sheet profile. The widespan profile offers greater spans and lower roof pitches than corrugated sheeting but provides the same covering width, being 762mm.
Corrugated (iron) is the traditional and familiar S-Rib profile for roofing and cladding applications. It is the oldest and most commonly used roofing profile because it is easy to handle, has excellent fixing properties and commensurate strength. The S-Rib is derived from the sinus curve and offers strong structural properties. The 8.5/76 represents 8.5 corrugations over the width of the sheet and the 76 refers to the distance in millimeters between two consecutive curves. The overall width of a traditional 8.5 corrugated sheet is 700mm and a 10.5 corrugated sheet is 840mm.
Concealed fixing is where a clip or special cleat is fixed to the purlin which securely holds the sheets in position and usually lock-in both the sidelap and centre ribs of the sheet; or where the cleat fastens to the purlin and folds over the flanges of the sheet and a cap snaps over this assembly. The main advantage of this type of roof system over others is the water-tightness achieved with very long spans at low pitches. Another advantage with concealed fixing (standingseam) is corrosion and failure at fixing points is eliminated. Brownbuilt metal sections pioneered the manufacture of concealed/secret fix roof profiles in SA with their “Brownbuilt” profile. Today other types of profiles are available and manufactured by a number of different manufacturers.
Fibre cement roofing sheets essentially consist of an inorganic hydraulic binder or calcium silicate formed by a chemical reaction of a siliceous and a calcareous material, reinforced by natural synthetic organic fibres. The sheets are supplied in their natural grey colour and can be painted (where required) with a 100% acrylic PVA after installation. Tinted sheets using a ultra-violet resistant pigment and branded as ‘Vineyard’ sheets are also available and recommended for Tuscan roofs where they are used as underlay for clay tiles.
Fibre cement roof sheeting is available in two basic profiles;
The Victorian sheet is a popular profile and it has been designed to recreate the appearance and character of a traditional Victorian style roof and with its reduced dimensions and mass, makes handling and laying relatively easy. Victorian sheets are particularly suitable for coastal areas where corrosive conditions prevent the use of many other products.
Onduline is an extremely tough lightweight corrugated roofing and wall cladding material manufactured from bitumensaturated organic fibres under intense pressure and heat. It is flexible, economical and virtually indestructible. It provides a high degree of weather protection and thermal insulation- even in the most extreme climatic conditions. Colour stability is also ensured through a unique pigmentation process which “stains” the colour into the sheet.
Translucent (Class A)
Translucent or GRP (Glassfibre reinforced Polyester) sheeting is manufactured using UV stabilised unsaturated polyester resin and glass fibre reinforcement material. The weathering surface is covered with a highly UV stabilised gelcoat layer. This new generation technology offers superior weathering characteristics to conventional methods of surface protection in the GRP sheeting industry.
GRP products are specifically designed to withstand the harsh South African climatic conditions and a range of colours are manufactured, with the more popular being clear, white (opal 50), green and blue. All the generally used profile shapes, to match those of other roofing and cladding materials are usually manufactured.
Slates can be described as any rectangular sheet of roofing material, whether of natural slate, stone, cast stone, fibre cement, or metal.
Fibre Cement (Class B)
Fibre cement roofing slates essentially consist of an inorganic hydraulic binder or calcium silicate formed by a chemical reaction of a siliceous and a calcareous material, reinforced by natural synthetic organic fibres. Fibre cement roof slates offer designers and specifier’s an alternative to other slates with a modern clean look and are available in a number of bright colours including white.
Fibre cement roof slates are designed for a minimum roof pitch of 10° using and underlay and 17° without and underlay and in high wind areas the slates may no longer provide a waterproof covering and a waterproof underlay must be installed. It is however important to be aware of the fact that any distortion or unevenness in the roof structure and/or battens will reflect in the final appearance of the application. Time spent to ensure that the structure and battens are accurate and sound is therefore a small investment in the process of achieving an excellent result.
Natural Slate (Class B)
Slate is commonly described as a dark grey natural stone made up of many thin layers which can be split (riven) into thin sheets, then cut to size to create tiles. Slate is a heavy material and the roof structure must be designed to carry the weight – see Table 15.18. Slates are fire resistant and have a long life but are more costly than most other roofing materials.
Features and Benefits of Marley Roofing’s P75 Fibre Cement Roof Sheets:
Dimensions and Properties
Refer to Table 11.49.
Refer to Fig 11.12.
Wood: Pozigrip screw with PVC washer (dimensions: 6.3mmx120mm)
Steel: Hook bolt and nuts – length determined by depth of steel purlin (+90mm)
Refer to Table 11.50.
NOTE: Although Marley Roofing’s P75 FC Roof Sheets are fitted with security strips that can prevent a person from falling through the roof during roof maintenance works, it is recommended that duckboards are used as walking areas to avoid damage to sheets and injury to workers as per legislative requirements.
Features and Benefits of Kalsi FIbre Cement Boards
Dimensions and Properties
Refer to Table 11.51.
Refer to Table 11.52.
32mm x 2.5mm serrated ceiling nails must be placed 150mm away from the edges of the board OR 12mm x 40mm brass woodscrew with countersunk head
Dimensions and Properties
Refer to Table 11.54.
Suitable for virtually any roof (includes tiles and sheeting), providing a functional finish while protecting the roof’s substructure.
These boards can be used as a facade system or for cladding in a lightweight steel frame (LWSF) structure. Boards can be fixed as an expressed joint system or flush plastered.
Dimensions and Properties
Refer to Table 11.55.
All fibre cement products sold by Marley Roofing are warranted against defect in material and manufacturing processes. This warranty does not apply to defects resulting from any distributor or customer actions, such as mishandling, misapplication and improper installation. Our factories are also ISO 14001 certified, demonstrating our ongoing commitment to minimising waste, energy consumption, water consumption and pollution to the environment.
In addition to various international certifications, Marley Fibre Cement adheres to SABS specifications SANS 802 and SANS 10904.
Natural slates are available in a number of different sizes and in a variety of colours usually originating out of the rock from which it was quarried; below are the more commonly used colours. See colours to the right. Slates are usually applied over solid sheathing covered with bitumen saturated roofing felt underlayment; commonly referred to as the Alumaz system. Slates can also be laid conventionally with no underlayment known as the conventional system.
Shingles are made using an organic felt base saturated with bitumen and coated with a mineral-stabilized coating of bitumen on both sides. The exposed top side is coated with mineral granules. They are flexible, robust and lightweight and available in a range of colours.
Tile strips are available in a variety of styles, most common are rectangular and oval and they are laid on a boarded roof deck. Shingles can be laid on roof pitches of between 12° and 60°.
Construction methods for commonly used roof decks related to materials and types of construction are detailed in other sections in this book. A properly functioning roof depends on a structurally sound deck that is compatible with the roofing system and the installation of an approved waterproofing system.
Thatch is classified as a class B roof covering and is probably the oldest type of roofing still used today. It is a popular and indigenous solution in particular areas where materials and skills are available and is the roof covering of choice in many African bush lodge designs and applications.
Thatch roofs should be constructed at a minimum angle/pitch of 45°. If good quality, clean, dry grass is used, the lifespan of the thatch will be increased (in general the top layer of thatch will need to be replaced and dressed every 12 to 15 years). Techniques, and to some extent materials, vary according to locality or region, and it is not possible in this section to provide more than an outline of what a thatch roof should contain.
Thatching is a natural insulator; cool in summer and retaining maximum internal heat in winter. Thatching also creates a relatively low cost means of additional living areas. The required pitch of a thatched roof allows this space to be used for a loft, by simply building a slab or suspended floor. The greatest disadvantage of thatch is the fire risk, although fire prevention in thatch has developed over the years and includes items like:
The designer of a roof of a building needs to know a lot about the conditions that will be imposed on that structure in service. Factors such as loading, environment and durability all have to be understood and assimilated into the design process, and considered in relation to behaviour in fire. For example, firewalls or fire stops that restrict the spread of fire in a roof assembly must extend continuously from a structure’s foundation to or through its roof.
If the fire wall stops at the roof, the roof assembly must then consist of non-combustible materials. The fire wall should be able to remain intact even if the structure on either side collapses due to fire damage.
The National Building Regulation’s fire protection standard – SANS 10400 Part T stipulates requirements for fire ratings and approved non-combustible materials; and contain content that is relevant to roofs and their fire protection – see some extracts below.
Note: A test result C is equivalent to an untreated roof.
Note: Safety distances or fire separation specifies the distance between adjacent buildings; it is used to reduce the risk of fire spreading from one building to another.
An underlay can be described as any layer or sheet material layed under another material. Typically a roofing underlay is a weather-proofing membrane, which prevents moisture, wind and dust entering the roof space. Roofing underlays can at times be confused with insulation, although in some cases they can perform dual functions, for example alububble® where the underlay is an effective water vapour barrier under roof tiles or roof sheeting and at the same time insulates the roof. Underlays for supported sheeting applications like shingles are usually of inodorous felt while tiles and slates are underlain with plastic sheet known as undertile plastic except on low pitches (typically in natural slate applications) bitumen felt is used.
The National Building Regulations covers various types of roof coverings and pitches where underlays must be used; they all fall under class B roof coverings as described in the table below.
Discussed ahead are the two types of underlay most often used in roofing applications; plastic and felt. For other underlays used for insulation properties, see thermal Insulation.
Plastic underlays are a polyethylene membrane that has been specially formulated for use under roof tiles and slates, usually (250μm – micron) in thickness and made in rolls of 1.5 x 30 m and must be SABS approved.
The membrane is designed to prevent moisture from warm damp air reaching and condensing on ceiling boards and other vulnerable points in the building fabric. It is also used for the prevention of draughts and dust penetration into the roof space through tiles and slates, and prevents damage to ceilings and rotting of timbers.
The underlay further prevents strong wind from lifting and ripping off roof tiles. The underlay must be fixed horizontally between rafters and battens with 150mm minimum overlaps and carried well into the gutters. Avoid allowing the underlay to sag and form water traps – particularly behind fascia boards. A strip of underlay, at least 600mm wide, must be placed at hips so as to overlap each side for its full length. A similar strip must be laid for the full length of each valley ensuring that the opposing slopes overlap the edges.
Felt underlays are a sheet made from cellulose fibres of organic materials such as kraft paper and glass fibres saturated with bitumen and coated with a layer of thin mixed asphalt and crushed gravel or sand which can be laminated with other materials like aluminium foil or polyethylene. Felts are usually supplied in rolls of 10m and 20 m x 450mm and 900mm wide. In natural slate roofing applications the felt underlay is typically faced over half its width with a heat laminated layer of aluminium foil, used as a waterproofing membrane which is long lasting and also prevents the ingress of dust.
Vapour barriers are a plastic or foil sheet that resists diffusion of moisture through the roof assembly and typically used in sheeted metal roofing applications to prevent moisture from penetrating other inaccessible building components and condensing as moisture on insulation and ceilings. If condensation occurs in a building it is both difficult and costly to eradicate so it is always recommended to take precautionary measures during construction and design. To avoid condensation in a roof, moist air must be prevented from contacting the underside of the sheeting.
Thermal insulation acts as a barrier to heat flow and is essential in keeping a building or home warm in winter and cool in summer. A well-insulated and well-designed building or home will provide all year comfort. Climatic conditions will influence the appropriate level and type of insulation; which are covered in SANS 204 under Standards and in the “Insulation installation” section that follows. It is important to establish whether the Insulation will be predominantly needed to keep heat out (summer) or in (winter) or both. Insulation must cater for seasonal and daily variations in temperature. Ceilings and roof spaces account for 25 to 35% of winter heat loss and must be well insulated. To prevent heat loss, locate most of the insulation next to the ceiling as this is where the greatest temperature control is required.
Insulation products come in two main categories – bulk (cellular) and reflective. Examples of Bulk insulation are – Mats e.g. Aerolite; Loose fill e.g. Thermguard; and Insulating boards e.g. Isoboard (polystyrene). They have many tiny air or gas bubbles, which reduce conductivity and commonly make them lightweight. Whereas reflective insulation, such as aluminium foils and low-emissivity coatings, reduces heat loss or gain by radiation. These are sometimes combined into a composite material. There are many different products available, like alububble®.
See next sub-heading ‘Insulation types and Properties’
To compare the insulating ability of the products available look at their R-value, this measures resistance to heat flow – the higher the R-value the higher the level of insulation.
Note: Products with the same R-value will provide the same insulating performance only if installed as specified.
R-values can differ depending on the direction of heat flow through the product. The difference is generally marginal for ‘bulk’ insulation but can be pronounced for ‘reflective’ insulation. Some products must be installed professionally while others can be defined as DIY – some types of insulation require the use of masks and protective clothing. Ensure products chosen or specified suit the particular application and will fit within the space available. The appropriate mass or amount of insulation depends on a number of factors like climate, building type and size, and whether the building is naturally or mechanically ventilated. The National Building regulations – SANS 10400: PART XA sets out minimum requirements – see Standards.
Bulk insulation mainly resists the transfer of conducted and convected heat, relying on pockets of trapped air within its structure. Its thermal resistance is essentially the same regardless of the direction of heat flow through it. Bulk insulation includes materials such as glasswool, wool, cellulose fibre, polyester and polystyrene. All products come with one material R-value for a given thickness.
Reflective insulation mainly resists radiant heat flow due to its high reflectivity and low emissivity (ability to re-radiate heat). It relies on the presence of an air layer of at least 25mm next to the shiny surface. The thermal resistance of reflective insulation varies with the direction of heat flow through it. Reflective insulation is usually shiny aluminium foil laminated onto Kraft paper and or plastic (polyethylene or polypropylene) and reinforced using a scrim, all bonded together and available as sheets (sarking). Together these products are commonly known as reflective foil laminates or RFLs – See following example.
Dust settling on the reflective surface will greatly reduce performance. Face reflective surfaces downwards or keep them vertical. The antiglare surface of single sided RFLs should always face up. The Total R-values for reflective insulation are supplied as up and down values. Total values depend on where and how the reflective insulation is installed. Ensure system values provided by the manufacturer relate to your particular installation situation – See thermal resistance of systems below. Composite bulk and reflective materials are available that combine some features of both types.
Manufacturers of bulk insulation products typically quote the thermal resistance, R-value and thermal conductivity, k-value and thickness, of their products. Confusion arises when suppliers of reflective insulation quote thermal resistance values which are inclusive of air gaps. Such thermal resistance values are in fact the resistance of a total system and cannot be compared to individual product R-values. RFLs, e.g. foil laminates, are typically very thin with high thermal conductivities, and thus have a very poor thermal resistance. For this reason, radiant barriers should always quote an associated air gap.
A word of caution must be sounded with respect to air gaps. Although it is acknowledged that static air is a very good insulator owing to its very low thermal conductivity, it must be borne in mind that thermal gradients can induce natural convection, in which case convection becomes the main mechanism of heat transfer and not conduction. When this takes place, the thermal performance of air gaps is drastically reduced. Many published system thermal resistances for reflective insulation ‘systems’ are based on ‘trapped’ air gaps above and below the barrier, which is sometimes impossible to achieve in practical installations. Thus thermal resistance values, inclusive of air gaps, determined under ideal, experimental conditions, tend to overstate the performance of the system. It is recommended that the total thermal resistance of systems be calculated on the basis of internationally accepted standards and procedures to ensure that the required thermal performance is achieved and can be guaranteed.
Firstly it is important to understand that roofs and ceilings work in conjunction when it comes to insulation and for this reason are referred to as roof assemblies in building standards.
Note: One can save up to 45% on heating and cooling energy with correctly specified and installed roof and ceiling insulation.
Many existing buildings were built before even basic thermal insulation provisions were introduced and many before any standards were considered or discussed. There are considerable opportunities for improving energy efficiency in existing buildings. These opportunities depend upon how individual buildings are constructed both for the practical difficulty of carrying out the improvements and their cost effectiveness.
Insulation can be added to existing buildings with varying effectiveness and cost depending on the construction type and where the insulation is being placed.
It is usually easy to insulate pitched roofs with accessible roof spaces, giving highly cost effective results. Flat roofs are more difficult to insulate as the space between ceiling and roof covering is often inaccessible and where the internal ceiling or external roof sheeting must be removed before insulation can be installed; which in most cases would not be cost effective. Although, it is possible to apply special coatings on the roof covering which does then provide better insulation. In so far as concrete slabs are concerned inverted insulation can be installed, which could be more cost effective when the waterproofing needs to me renewed.
Extracts from SANS 10400XA
22.214.171.124.3 Reflective insulation shall be installed and supported:
126.96.36.199.4 Bulk insulation shall be installed so that
Note: See table 15.24 regarding typical R-values for roof/ceiling construction and the resulting typical intervention insulation thicknesses.
Climatic zone: The first fundamental matter that needs to be determined before applying the DTS provisions is the climatic zone in which the building is to be located. The climatic zone map of South Africa (see Foundation Section) shows diagrammatically the extent of each zone and the table detailing the applicable climatic zone for common locations. In this case, the applicable climatic zone for Cape Town is 4.
Insulation: Roofs in climatic zone 4 are required to achieve a minimum total R-value of 3,7 in the upwards direction (see table 15.19). A pitched tiled roof with a flat ceiling in climatic zone 4 achieves a total R-value of 0,35. This means that additional insulation that achieves a minimum R-value of 3,35 (3,7 to 0,35) in the upward direction is required to be installed in the roof. This can be achieved by installing bulk insulation or a combination of bulk and reflective insulation
Compression of bulk insulation: The R-value of bulk insulation is reduced if it is compressed. The allocated space for bulk insulation must therefore allow the insulation to be installed so that it maintains its correct thickness. This is particularly relevant to wall and cathedral ceiling framing whose members can only accommodate a limited thickness of insulation. In some instances, larger framing members or thinner insulation material, such as polystyrene boards, may be necessary to ensure that the insulation achieves its required R-value.
As already established installing insulation can make a significant difference to the overall energy efficiency of a home or building. Roofs and ceilings or as referred to here and in the standard as a roof assembly are a very significant element in the consideration of energy efficiency of any home or building, more particularly when that building is single storeyed. All the components and materials of a roof and ceiling, and even the spaces between layers of the materials (cavities), contribute to the overall energy efficiency or thermal performance of a roof assembly. Installing roof and ceiling insulation can save up to 45 % on heating and cooling energy.
What follows in this section is to briefly describe how to install insulation showing different products and the different uses they have in their application in different roof construction and assemblies, also providing installation tips and typical solutions.
Pitched roofs with flat ceilings
This is the most common type of roof assembly and the easiest to insulate.
A layer of reflective foil laminate (RFL) is an effective barrier to radiant heat and as a vapour barrier. Reflective insulation gives excellent insulation performance for downward heat flow (summer heat gain), but only moderate performance for upward or horizontal heat flow (slowing heat losses in winter) and requires an air space between the foil and solid surfaces to achieve full insulation qualities. RFLs should be installed in conjunction with conventional bulk insulation, to achieve optimum energy efficiency.
The most common method of installation in domestic applications is where the RFL is stapled with industrial staples to the top of the rafter (with timber trusses) before fixing of the batten or purlin. The insulation (RFL) material is then laid horizontally (gable to gable) commencing at the eaves an ensuring that subsequent sheets overlap the previous sheet by 100mm. With this method the RFL acts as both insulation and as a waterproofing membrane.See figure 15.17
A second layer of RFL (either sarking or foil batts) beneath the roof will increase resistance to radiant heat. This may be useful in hot climates. However, ensure that there is at least a 25mm gap between reflective surfaces.
Place ceiling insulation between the joists. Suitable bulk insulation includes bulk batts, mats, cellulose loose fill and polystyrene boards. In colder climatic regions two layers of bulk insulation may be necessary to increase thermal performance, one between the rafters and the second on top. Other insulation types include flexible composite bulk and reflective materials which combine some features of both types; and can be installed in the same way as bulk insulation except that the reflective foil would determine its position i.e. facing downwards. See figure 15.18
Note: The National Building regulations Part XA (SANS10400XA) sets out minimum requirements for materials R-values for different climatic zones used in roof assemblies – refer Standards
Ceilings that follow the roof line
These include sloping ceilings, cathedral ceilings, vaulted ceilings, and flat or skillion roofs, where there is no accessible roof space.
A layer of reflective foil laminate (RFL) is an effective barrier to radiant heat and as a vapour barrier. Reflective insulation gives excellent insulation performance for downward heat flow (summer heat gain), but only moderate performance for upward or horizontal heat flow (slowing heat losses in winter) and requires an air space between the foil and solid surfaces to achieve full insulation qualities. RFLs should be installed in conjunction with conventional bulk insulation, to achieve optimum energy efficiency.
Foil backed or faced blankets (composite insulation) can also be used (although they are primarily designed for application where no ceilings exist) where the outer jacket reflects heat and controls condensation under steel sheeted, fibrous cement and tiled roofs (acting as a vapour barrier); and where the bulk insulation provides thermal and acoustic insulation mainly to reduce the noise associated with metal roofing. When used for thermal insulation, compression of the blanket over the battens or purlins lowers the total R-value.
Consult the insulation manufacturer or installer about installation clearances. As a rough guide, minimum clearances (batten height) for ceilings with exposed rafters are:
Suitable composite insulation for exposed rafter applications includes foil faced polystyrene boards (it is a lot easier to install); the minimum batten height should be 65mm to allow for two 25mm reflective air spaces either side of the boards where 25mm boards are being used. 25mm foil faced polystyrene boards and RFL insulation will give a total R-value of approximately 1.7 up, 2.9 down.
In South Africa, many low cost homes and retail and industrial buildings are built with no ceilings, where as much as 70% of the heat gain is through the roof. The over purlin under roof sheet insulation is the most cost effective method of achieving thermal efficiency in these building types. A layer of reflective foil laminate (RFL) is an effective barrier to radiant heat and as a vapour barrier. Reflective insulation gives excellent insulation performance for downward heat flow (summer heat gain), but only moderate performance for upward or horizontal heat flow (slowing heat losses in winter) and requires an air space between the foil and solid surfaces to achieve full insulation qualities. RFLs should be installed in conjunction with conventional bulk insulation, to achieve optimum energy efficiency. Foil backed or faced flexible bulk blankets (composite bulk insulation) are primarily designed for applications where no ceilings exist and where the outer jacket reflects heat and controls condensation under steel sheeted, fibrous cement and tiled roofs (acting as a vapour barrier); and where the bulk insulation provides thermal and acoustic insulation mainly to reduce the noise associated with metal roofing. When used for thermal insulation, compression of the blanket over the battens or purlins lowers the total R-value. There are various application options available when installing the above two types of insulation (which can include applications where ceilings are installed) depending on the particular roofing material and or design, construction method and project requirements i.e. Insulation only; insulation and waterproofing membrane; ceiling substitution, noise etc.
RFLs can either be laid horizontally i.e. in the same direction as the purlins or battens which is normal in domestic applications where the truss or rafter centres are usually less than 1500mm. While in industrial applications RFLs are laid vertically as the truss and rafter centres are usually greater than 1500mm, where the RFL is then laid vertically using straining wires. The most common domestic application of installing RFLs is illustrated in figure 15.17 – i.e. stapled with industrial staples to the top of the rafters before fixing the battens. Material is laid ‘horizontally’ (Gable to Gable) commencing at the eaves an ensuring that subsequent sheets overlap the previous sheet. With this application the RFL acts as both insulation and a waterproofing membrane. The RFL can also be laid and stapled ‘horizontally’ on top of timber rafters and battens or purlins before fixing of the roof covering (usually sheeting). See figure 15.24 on the following page.
Composite bulk insulation can either be laid horizontally which is normal in domestic applications where the truss or rafter centres are usually less than 1500mm. While in industrial applications composite bulk insulation is laid vertically as the truss and rafter centres are usually greater than 1500mm, where the composite bulk insulation is then laid vertically using straining wires.
Only some composite bulk insulation products are recommended as water vapour barriers and if this is a requirement; this needs to be checked with the manufacturer, before specifying the product.
Composite bulk insulation in domestic applications can be installed in the following ways:
See schematic – Figure 15.25.
The most common domestic method of installation is where the blanket is installed as (2) above; where the insulation (the fibre side facing upwards) is stapled with industrial staples to the top of the rafter or truss before the fixing of the batten.
Material is laid ‘horizontally’ i.e. in the same direction as the battens, commencing at the eaves an ensuring that subsequent sheets overlap the previous sheet with the edge without the loose lap laid over the lap of the previous sheet, depending on product and manufacturer.
When installed horizontally the blanket needs to be fixed firmly so as to prevent sagging.
When used for thermal insulation, compression of the blanket over the battens or purlins as per fixing details 1, 2 & 4 as described above, the total R value of the blanket is lowered.
In sheeted roof applications, fix the roof sheeting as soon as possible after the fitting of composite bulk insulation using the specially designed roofing screws normally recommended by the manufacturer of the product. Care should be taken to ensure that weight is applied to the drilling point to facilitate drilling without distortion of the composite bulk fibre. Where the roofing is in sheet form and with trusses spaced at greater than 1500mm centres the industrial method of installation is normally used with straining wires – See Figure 15.26.
The thermal insulation values of materials reduce with increased moisture content so that materials which do not absorb water are needed where prolonged wetting e.g. through condensation is inevitable. Some thermal insulation materials are more vulnerable than others; for example cellulose.
The overall insulation value of a roof assembly can be degraded by thermal bridges where high thermal transmission material penetrates layers of low thermal transmission material. Thermal losses due to cold bridges are often ignored in calculations, especially where thin sections are involved; but these and other materials such as steel roof trusses or steel beams become more important in the overall R-Values of the roofing assembly in terms of the “deemed-to-satisfy” requirements of the standard.
Some cold bridges are also important because they produce inside surface temperatures below the dewpoint of the air, leading to selective condensation on parts, for instance, of a ceiling. Suitable designs can overcome or reduce cold bridging to acceptable levels. Air movement into and within the roof space, and especially through layers of low density insulation material, can reduce thermal efficiency considerably. Sealing at joints and around areas where services penetrate the insulation is important.
Condensation occurs on a surface when its temperature falls below the dew point of the air for a sustained period. In these conditions, the relative humidity of the air in direct contact with the cold surface rises to 100% and the moisture in the air condenses on the surface.
On a clear night, the outer roof covering radiates heat to the sky and its temperature may fall even below that of the surrounding air. If the relative humidity of the air inside the building is greater than about 60%, there may then be a risk of condensation on the underside of the roof unless precautions are taken to avoid it; the risk is especially high in profiled sheet roofs.
Interstitial condensation occurs out of sight within the thickness of a roof or other part of the building structure when water vapour diffuses through the internal fabric, including any insulation, and condenses on a cold surface beyond. Interstitial condensation can rot and or weaken roofing components especially timber and can lead to deterioration in other organic materials; it can also corrode metal and other components.
There are three main ways in which the risk of condensation in roofs can be reduced:
With today’s development of window and glazing systems with low U-values and low energy losses, it makes it possible for large glazed surfaces to be used in buildings. However, large glazed surfaces need solar protection or shading, since otherwise there is a risk of overheating in summer and noncompliance in terms of the National Building Regulations – Part XA. The term shading refers here to the roof overhang or any shading device like awnings, pergolas, horizontal slatted baffles (aluminium louvres), roller blinds, venetian blinds, shutters, etc. Some shading devices can also act as overnight insulation for windows, like roller blinds or shutters.
During the design phase it is recommended to assess the comfort and energy required for heating or cooling, if the building is to function properly. However, shading and/ or shading devices are seldom designed during the design phase and are typically installed as an afterthought when problems are encountered, i.e. after the first summer that the building has been in use. Although with the proper implementation of SANS 10400 XA many of these problems should be avoided – see the sub section on standards that follows after shading guidelines.
In general it is best to block the sun before it reaches the window. However when choosing an appropriate shading device one must also consider whether a fixed device or moveable device will be better; as the need for protection will be seasonally dependent in its effects. For example, a roof overhang (eave) may block high summer sun while allowing through low-angle winter sun. (We use the term window which could include a door (glass) or an opening). Some form of adjustable or moveable shading device is usually the better option, as it provides the home owner some control over the effects of the shading device and especially where glare is of concern. There are a variety of shading strategies that are discussed below under shading guidelines.
North-facing elevations (and south facing ones above the tropic of Capricorn) receive higher angle sun and therefore require narrower overhead shading devices than east or west facing openings; as the angle of incidence is greater, much of the radiation is reflected from the glass or blocked by the walls on either side of the window, especially if the window is fitted deep into the reveal. Fixed horizontal shading devices are often adequate above north facing windows, which could include eaves, awnings, and pergolas with beams spaced at correct centres or louvres set to the correct angle, to allow sufficient winter sun through.
East-facing elevations require a different approach, as low morning sun from these directions is normally most welcome in winter and in many cases also welcome in summer, but can become a problem in so far as glare is concerned and therefore more difficult to shade. This morning sun is not as intense as the afternoon west sun; however in certain coastal areas in South Africa especially in climatic Zone 5 this morning sun can be problematic, presenting overheating problems that would require the use of an appropriate shading device.
South-facing elevations don’t usually require any shading apart from certain locations like climatic Zone 4, where the afternoon summer sun sets very late and could present some glare.
West-facing elevations present the biggest problem with solar heat gain. This hot summer afternoon sun from these directions is quite intense, causing overheating, fading etc. and added to this, the challenge of screening glare with this direct low sun. The sun can be low enough in the sky that only a very wide overhang can be effective. Adjustable shading is the optimum solution for these elevations. Appropriate adjustable systems include sliding screens, louvre screens, shutters, retractable awnings and adjustable external blinds. Deep verandahs, balconies or pergolas can be used, but may still admit very low angle summer sun, where using trees in combination with other planting to filter unwanted sun can help solve this problem.
North-East and North-West elevations require an integrated approach as they receive a combination of high and low angle sun throughout the day. Adjustable shading devices are recommended for these elevations and it is further recommended to select systems which allow the user to exclude all sun in summer; choose full sun in winter; and manipulate sun levels at other times. Figure 15.27 shows the path of the sun in summer and winter in order to illustrate the impact of orientation on solar shading. As illustrated in this figure one can see that the sun is at its lowest in the sky on the winter solstice (21 June) and where shading is most likely to occur. Whilst in summer and at the summer solstice (21 December) the sun is at its highest where very little shading is likely to occur In the table that follows (Table 15.25) the angles of elevation of the sun over the horizon as seen by the observer at different cities in South Africa at different times of the day is illustrated.
Use external shading devices wherever possible over openings, windows and doors; lighter-coloured shading devices reflect more heat. Internal shading will not prevent heat gain unless it is reflective. Use plants to shade the building, particularly windows, to reduce unwanted glare and heat gain. Evergreen plants are recommended for humid sub-tropical climates (Zone 5) and some hot and dry climates (Zone 3). For all other climates use deciduous vines or trees to the north, and deciduous or evergreen trees to the east and west. For latitudes north of 27.5°S the response varies with climate. For humid sub-tropical climates and hot and dry climates with no passive heating requirements, shade the whole building at all times. For hot and dry climates with passive heating requirements allow some low angle winter sun to reach walls, concrete floors and windows. The simplified climatic zone table below is for quick reference for the application of solar shading.
Fixed shading devices (eaves, pergolas and louvres) can regulate solar access on northern elevations throughout the year, without requiring any user effort. Summer sun from the north is at a high angle and is easily excluded by fixed horizontal devices over openings. Winter sun from the north is at a lower angle and will penetrate beneath correctly designed fixed horizontal devices (see figure 15.27 – above). For locations north of the tropic of Capricorn (23.5°S) in high humid climates or hot dry climates with warm winters, the building and outdoor living spaces should generally be shaded throughout the year. Listed below are the more common types of fixed shading systems.
Correctly designed eaves are generally the simplest and least expensive shading method for northern elevations, and are all that is required on most single storey houses. Having said that, permanently shaded glass at the top of the window can become problematic if not designed correctly as it is a significant source of heat loss, especially in temperate and cold climates (Zone 1 & 2). To avoid this, distances between the top of glazing and the eave underside (G) needs to be calculated correctly (See Figure 15.28 – Method of measuring P and H) this calculation is further dependent on latitude and the extent of summer shading required.
However, this calculation is often not as simple as it seems, as the National Building Regulations have a prescribed minimum roof height of 2.400m, with a normal window head height set at 2.100m. This renders the above solution then almost unworkable; with the only solution being to increase the window head to eave distance i.e. to raise the roof height which will increase costs.
Awnings and pergolas
Awnings and pergolas need to extend beyond the width of the opening by the same distance as their outward projection. Pergolas covered with deciduous vines provide self-adjusting seasonal shading.
Fixed horizontal louvres set to the noon midwinter sun angle and spaced correctly allow winter heating and summer shading in locations with cooler winters. Midwinter and midsummer noon sun angles for locations (x) can be calculated using the formulas below, where (L) is the latitude of your home.
Adjustable shading allows the user to choose the desired level of shade while enabling daylight levels and maximizing views. This is particularly useful in spring and autumn when heating and cooling needs are variable. External adjustable shading systems appropriate for northern elevations include retractable awnings or horizontal and facade louvre systems, while removable shade cloth over pergolas offers a flexible and low cost shading solution. Other external adjustable systems include sliding screens, louvre screens and shutters. Controllable louvre systems with controllable fins mounted vertically in front of windows or fitted horizontally above a window, reduce the likelihood of ‘overshading’ or ‘undershading’ that frequently happens with fixed shading devices and will result in the optimum shading angle all year round.
Match plant characteristics (such as foliage density, canopy height and spread) to shading requirements. Choose local indigenous species with low water requirements wherever possible. In addition to providing shade, plants can assist cooling by transpiration. Plants also enhance the visual environment and create pleasant filtered light.
Extracts from SANS 204
Note: An adjustable device that is capable of completely covering the glazing may be considered to achieve a P/H value of 2.
Roofing accessories are the items other than the roof covering that can or should be used to complete the roofing installation; for example, installing a facia board at the eaves or a metal flashing to a parapet wall.
Note: In this book we cover or illustrate the two main different roof coverings being tile or sheeted; although many roofing systems have a number of different and special application accessories they are too broad to try and discuss here in detail.
Ridge tiles and verge tiles
The ridge is the horizontal junction between two roof slopes at the apex. Ridges should be covered with ridges or ridge tiles of similar colour and finish to those of the roof covering and in the case of ridge tiles they should be edge-bedded onto the last course of tiles in tinted 3:1 sand/cement mortar, strictly in accordance with the manufacturer’s recommendations. All ridge tiles should overlap the last course of tiling by a minimum of 75mm and the exposed mortar must be neatly pointed.
A strip of approved DPC sheeting 150mm wide should be placed lengthwise under the ridge tiles, overlapping the top course of tiling on each side by 25mm. Lapped ends must be supported underneath and the overlap should not be less than 150mm. The end ridge tiles at gable ends should be solid bedded with mortar inset with pieces of tiles and neatly pointed
at fair ends. All ridge tiles shall be neatly cut and mitred at intersections with hips, intersecting ridges etc.
In the case of sheeted roofing applications – see the following sub-section – Flashings – Ridging. All ridges do however need to be installed and fixed in strict accordance with the manufacture’s recommendations. Monoridges are to be formed with purpose made monoridge tiles edge-bedded onto the top course of tiling as described for ridge tiles strictly in accordance with the manufacturer’s recommendations.
The Verge of a roof can be described as the edge of a roof surface at a gable. And unless otherwise specified, verges should be formed with purpose made verge tiles of similar colour and finish to those of the main roof covering and must be fixed strictly in accordance with the manufacturer’s recommendations. In sheeted roof applications where the roof is installed between parapets flashings would need to be installed – See flashings in the following sub-section.
Hips are covered with ridge tiles/hip tiles of similar colour and finish to those of the main roof tiles and usually the same as the ridge tiles. In the case of sheeted roofing applications the hip is usually the same profile to that used as described above under ridges. For tiled roofs the hip tiles should be cut closely to the rake of the hip, and the hip tiles should be edge-bedded onto the tiles as described for ridge tiles.
The first hip tile should be shaped at the foot to the line of tiling at the eaves and the fair end filled with mortar inset with pieces of tile and neatly pointed. Alternatively hip starters can be used – most manufactures do supply hip starters. For vertical hips and at steep pitches above 45°, hip irons should be used and fixed to the hip tree with two screws or nails to support the first hip tile. All other hip tiles must be nailed to the hip tree and bedded as detailed for hips and ridges.
Fascia and barge boards
They provide a functional finish by protecting the underlying exposed timber structure from the elements at roof ends, as well as adding visual impact to the building. In high wind areas barge boards form an important component of the roof structure, as they protect slates or tiles from being dislodged by the wind at the gable ends.
A Fascia Board; is a trim set on edge along the eaves, to cover the rafter ends and can also carry the gutter or can also be described as the flat horizontal surface immediately below the edge of a roof. A Barge Board; is a sloping roof trim fixed in lengths along the edge of a gable to cover the exposed timbers or as some describes as an ornamental board along the gable end of a roof.
To ensure a high standard of finish, it is essential that the supporting timber structure is accurate. Warped, twisted or poor quality timber work will reflect in the final appearance of fascia’s and barge boards. Fascia and Barge boards are available in different types of materials, sizes and shapes. With fibre cement products being the most popular.
A gutter can be described as a gently sloping channel to collect water and lead it to an outlet or drain. Roof gutters are no different and it is not always appreciated that gutters and downpipes form a system; roof gutters can be described in four specific functions:
Many systems that have adequately sized gutters and downpipes fail in their function because of incorrect falls, insufficient downpipes or incorrect outlet locations, incorrectly installed box gutters or conditions at the outlets causing water to back up and spillover.
A roof guttering system serves three basic purposes:
Sizing of gutters and downpipes for domestic installations has become relatively straightforward, although the design of such systems for larger buildings is rather more than a simple calculation of roof area and water load.
The rainfall pattern over most of South Africa is variable in the extreme, so that even in so called dry areas, high intensity rainfall of short duration can be experienced. It is the intensity of rainfall, rather than the total amount, which is the important factor in designing roof drainage for roofs and paved areas.
The flow capacity of a gutter will vary according to the fall of the gutter and the positioning of downpipes and gutter angles, e.g. an outlet situated centrally in a length of gutter drains water from the gutter at twice the rate provided by a single end outlet. See size selection table below.
Tests show that the performance of eaves gutters is improved by 20% at a fall of 1 in 600. Excessive falls on the other hand should be avoided because, apart from the bad appearance, a wide gap between the roof edge and the gutter results in wind driven rain water missing the gutter altogether. The presentation of data relating to the flow capacities of gutters varies considerably from simple tables to the more complex. In many cases the architect , building contractor or whoever installs the gutters has the task of deciding exactly what to use, and becomes quite confused and relies in the end on instinct. In general a feeling of distrust prevails where on the one hand the design data is too brief, or at the other extreme, the thesis outlining the principles of design is too academic and complex. And in many instances the cost is the ultimate deciding factor even if it means the installation of a guttering system is at risk of functional failure.
The chart given below has been based on the assumption that 10cm² of gutter and 8cm² of downpipe is sufficient to effectively drain 10m² of roof area under average rainfall conditions. To calculate for heavy rainfall conditions, however, sizes should be based on a figure of 10cm² of gutter and 8 cm² of downpipe for 7m² of roof area. Conversely, if dry conditions prevail, calculations should be based on 10cm² of gutter and 8cm² of downpipe being sufficient for 14m² of roof area. If, therefore, gutter sizes are required for a roof under these rainfall conditions, the measured area of the roof may be increased by 43% for heavy rainfall conditions or reduced by 28% for dry conditions and the gutter and downpipe areas required read off the chart as for normal conditions. Example: Roof Area 150m² Gutters 150cm² use either 200mm half round gutter or 150 x 125 x 150 VHV (square) gutter. Downpipes 120cm² use 100mm round downpipes.
If required consult your local authority for rainfall conditions in your area to determine your gutter requirements or the SA weather services. www.weathersa.co.za www.southafricanweather.co.za
It is good roofing practice that all exposed terminations and abutments are either flashed or counter flashed. A variety of flashing methods and systems are available. The nature of the roof covering and vertical surface including aesthetic requirements will determine the flashing type. Flashings at a roof/wall abutment may be in two parts, with an up-stand flashing turned up the wall and a counter flashing turned down over it.
Generally consisting of either galvanised sheet, copper or lead in various shapes and forms; these flashings are of a permanent nature and are installed by a plumber or roofer.
This is one of the most commonly used and effective flashing methods, used in flat roof applications. The asphalt is a trowelable mixture of solvent-based bitumen, mineral stabilizers, other fibres and or fillers. Fibreglass or other fibre reinforcing is sandwiched between two coats of asphalt cement, applied by trowel or brush over the terminating edge of the felt and sealed to the wall. The flashing compound may be blended with fine sand to ensure a key for subsequent painted finish. The terminating edge of the felt is trimmed to ensure a smooth, neat and effective finish.
Fibre reinforced acrylic flashing
Similar to bitumen flashing above but incorporating liquid acrylic, commonly referred to as ‘Flash Harry’. This provides an aesthetic finish and may extend to the top of the wall/ parapet. If applied as intended this product can assist in waterproofing roof transitions and parapets effectively and in difficult places, however this product is so often used in applications that are not fit for purpose, resulting in a number of failures.
A resin bonded cement reinforced with a polypropylene fabric provides reasonable adherence to brickwork or plastered vertical surfaces and provides a tough finish that may be plastered, tiled or painted; however not recommended for use at transitions between different materials.
Note: No nails should pass through any part of a flashing.
Formed flashings are available from various different manufacturers; all manufactured with the same fundamental shapes as described below, and made for the following purposes.
The purpose of ridging is to effectively seal the ridge of a dual pitch roof using sheeting as a covering.
Is used to effectively seal a roof against a wall and is fitted over the overtile or undertile flashing depending on the detail required and roof covering.
Is used where a roof abuts against a parapet wall (sidewall or headwall); and needs to be effectively sealed. This type of flashing is mainly used on roofs with metal sheeting as a covering. The flashing is visible after installation.
Is used mainly to seal the roof where the roof abuts against a parapet wall (side wall).This type of flashing is installed against the side wall and fits underneath the roof covering – typically concrete roof tiles; the short lip hooks underneath the tile to secure the flashing.
Is used in the valley of a roof where two roof planes join together. It usually has 225mm on either side of the bend with a short lip on both sides to prevent water running off the flashing. Valley flashing is manufactured in one size only.
Note: Roll top ridging is also known to be used as a valley lining, provided the ends are bent over creating a lip.
For open valleys the adjacent tiling must be neatly cut on both sides to form an open channel of at least 100mm wide. The cut tiles must be well fixed to the battens. For closed valleys the adjacent tiles must be neatly cut on both sides to form a close fit and a straight line. The tiles must be holed and secured by nailing. If the cut tiles are very small and cannot be nailed they must be secured to the battens by means of bailing wire.
Waterproofing tends to be one of the trades that are mostly overlooked when building, not only in the design phase, but throughout the building process resulting in a number of failures directly attributed to poor or incorrect waterproofing detail, and (or) the incorrect choice of materials or system being used (not fit for the purpose intended). Added to these; poor workmanship and the use of inferior products, in order to save money just make matters worse.
Trying to save money on waterproofing a roofing slab is very short sighted, considering the cost vs. the performance criteria of incorrect or inferior waterproofing systems/ products or poor workmanship or lack of detail after the building is complete, is very costly and in some instances impossible, unless one literally starts all over again. Waterproofing has advanced tremendously in recent years, from membranes, coatings and systems. Leaving the contactor or home owner with many different options available, ensuring a roofing slab is completed free of waterproofing defects and the ingress of water. If you are unsure of what system would best suit your specific application, consult with your architect or structural engineer or a reputable waterproofing contractor or manufacturer for advice.
Waterproofing can be used in a number of different applications, which include vertical and horizontal surfaces:
Although in this section we only describe roofing slab applications and therefore only horizontal surfaces.
In the design, choice and specification of any roofing slab waterproofing system, the following basic principles and questions must be considered.
The following questions need to be answered before choosing a waterproofing system.
Membrane waterproofing systems whether torch-on, acrylic or other, are the only certain method of waterproofing roof slabs. Systems are essential in a variety of applications, as they provide durability, dimensional stability at different temperatures (thermal movement) and most are installed with a guarantee. Manufacturers also provide assistance from specification support at design phase to on-site support during material/system application.
Torch-on membranes can be described as sheets of fibres matted into felt and bonded by saturating with bitumen, bitumen-polymer or other modified bitumen’s and other components such as rot-proof polyester reinforcements. The fibres are usually a non-woven polyester fleece, or glass wool, which both have better resistance to puncturing or tearing. They are mostly applied by torching-on and can be used in dry applications like roofing and damp proof membranes. Torch-on systems are generally the most suitable and the most commonly specified waterproofing systems used for concrete roofs. Torch-on systems are basically bitumen felt sheets welded together to create a waterproofing membrane to a concrete substrate, that has been primed using a bituminous solution with petroleum solvents.
4mm membranes are the most commonly used, with the ideal system being; a double layer system which incorporates the application of two torch-on membranes; the under layer being a 3mm membrane and the top layer a 4mm membrane, which are typically used in roofing and balcony applications. There is a proliferation of membranes being imported into South Africa. With some not suitable for use in our climate and are only suitable to use as a “cap sheet” or top layer.
Covered grade membranes (CG) are typically available in the following thicknesses.
Exposed grade membranes are a range of superior quality special polyester (SP) dual reinforced membranes to suit exposed applications and available in the following thicknesses.
This application refers to general low-slope (less than 5° pitch) concrete roofs screeded to falls and cross falls to rainwater outlets where the roofing membrane is exposed to the elements and temperature fluctuations. The waterproofing membrane, correctly selected, installed and maintained, is now considered as a permanent structural element with a life expectancy equal to that of the building it is protecting. Simplicity in design together with proper preparation of the substrate are key to successful roofing.
As with most membranes and coatings, the substrate plays an extremely important part in the success or failure of the waterproofing system. Too often Waterproofing Contractors do or are compelled to install the waterproofing system on inadequate or poorly prepared surfaces.
Falls and Drainage
It is generally accepted that low-slope (flat) roofs should have positive drainage. A finished fall of 1:80 is generally acknowledged as the minimum to ensure adequate rainwater runoff. To achieve this minimum finished fall, the roof should be designed with a fall of 1:60 allowing for directional changes and constructional inaccuracies. It is good practice to ensure that ponding of rainwater does not occur. Whilst most membranes are unaffected by standing water, it should be noted that ponding, besides being unsightly, could result in rapid deterioration of the final painted finish.
There are circumstances where the falls may be as low as 1:100 such as where the roof slope is in a single direction and great care has been taken to ensure there are no undulations in the screed. If no drainage falls and cross falls exist (not recommended) a duel layer system must be used.
A layer of light coloured stone chips laid loose over the waterproofing membrane provides a number of advantages:
The crushed stone covering should be clean washed 19- 25mm in size and usually 50mm to 75mm deep. A stone-guard or band of larger stones should be positioned around the outlets so as to ensure that the smaller stone is not ‘washed’ down the outlets. It is considered good practice to apply a coating of bituminous aluminium paint on the waterproofing prior to laying the stone protection.
This application typifies pedestrian-trafficable areas such as balconies, patios and terraces under fully bonded tiles or pavers. These areas present some of the most difficult situations from a waterproofing point of view insomuch as there are often myriad complexities in usually confined areas. The cost of the waterproofing is often but a fraction of the cost of the finishes – consider the cost/risk ratio and give high priority to the design, surface preparation and installation. Most waterproofing contractors recommend the use of a dual-layer waterproofing system to obviate the risk of labour error and reduce the possibility of subsequent damage.
Isolation Layer/Separation sheet
Tiles or Pavers bedded in mortar are to be laid on an isolating membrane, such as Interdek or a PVC sheeting, so as to reduce stresses placed on the waterproofing by possible movement in the paved finish. This further will facilitate future removal and replacement of the tiled finish (if required). Movement joints (soft joints) between the tiles/pavers are to be created at approximately 3 metre centers and at all abutments.
In small areas (less than 10m²) the waterproofing may be installed directly to the concrete and the fall created in the mortar bedding without the use of an isolating layer. In such cases, the membrane is to be blinded with course sand in an emulsion binder. This will act as a key for the mortar bed. It is recommended that the minimum thickness of the mortar bed be 45mm.
The height of the door threshold is of paramount importance in that it will be determined by the thickness of the membrane, the mortar bedding and the tiles and also the necessary drainage falls. The minimum height of the waterproofing turn-up should be 100mm. The finished height of the waterproofing (turn-up) is to be higher than the finished paving surface. This is to ensure that surface water does not penetrate behind the skirting. Special precautions are to be taken so that wind-driven rain is prevented from entering the interior. Door and window frame fixing methods require careful detailing and should be done in consultation with the frame manufacturer. Too often, the frame is fixed mechanically through the waterproofing membrane.
In South Africa, under the National Building Regulations, there are only two legal methods by which to design and construct timber roofs:
Tacit approval has been given by local authorities in general to pre-fabricated truss fabricators who use a suite of design programs supplied by system suppliers to design roofs up to 10m in span for non-public buildings.
Most of these system suppliers have been accredited by the Institute for Timber Construction. Larger buildings and those to whom the public has access are designed under the supervision of professional engineers using the same design programs.
Trusses designed by a competent person in accordance with Part B of the National Building Regulation are not limited to the span, pitch and geometric configuration of trusses specified in Part L of SANS 10400.
The Institute for Timber Construction works closely with both the Timber Division and the Civil Engineering Division of the SABS on grading specifications and design codes for structural timber as well as on matters affecting the National Building Regulations and the application thereof.
The Institute for Timber Construction have instituted a Certificate of Competence scheme for timber truss fabricators who design, manufacture and supply prefabricated nail-plated trusses.
Stringent auditing of the truss fabricators’ operations and key personnel before awarding these certificates is an assurance of quality trusses for specifiers and for the general public. The accuracy and performance of prefabricated timber trusses exceeds that of bolted trusses and cost savings are often significant.
All timber used for the construction of roof trusses, Rafters and beams should be structural SA pine complying with the requirements of SANS 1783-2/1460/10149, and bear the full standardisation mark. Timber used for roof battens should comply with SANS 1783-4 and bear the full standardisation mark. Timber used for the construction of roofs on site must be ordered in the dimensions in which it be used and must not be resawn into smaller cross-sectional sizes on site, as this will cause the grade, strength and dimensional tolerances to change.
Refer Roof Construction – Roof trusses – Timber Trusses
In certain regions in South Africa, it is illegal to use timber for structural purposes, which has not been treated against biological attack. Treatment can be either with CCA or Boron in accordance with SANS 10005 “Treatment of timber.”
The South African Wood Preservers Association, discusses the basics of preservative treated timber and or wood and the subject of wood preservation. Wood preservation can be categorised into two types; primary (industrial) and secondary (DIY). Primary wood preservation involves an industrial process whereby wood is impregnated with an industrial biocide-containing wood preservative to render the timber durable and resistant to biological attack, i.e. decay fungi, and wood destroying insects such as wood borers and termites. The high pressure processes, involving the application of waterborne preservatives such as CCA and Creosote by means of vacuum and pressure cycles in autoclaves, are predominantly used. Other approved methods of primary wood preservation include immersion methods such as the hot and cold open tank process using Creosote; diffusion methods using Borates; and low pressure or double vacuum processes using light organic solvent preservatives (LOSP) such as TBTN-P or Azole Permethrin. Visit goo.gl/qWeHZ9 for an animation of the high pressure process used.
Primary wood preservation is a pre-treatment whereby the timber is treated prior to its intended end use application, and therefore is regarded as a preventative measure. It is not supplemental or remedial (after the fact). Chemical retention (take up), depth of penetration and the processes used are prescribed in SANS standards, and mandatory compliance is controlled through regulations and compulsory specifications, wherein compulsory third party product certification is specified.
Secondary wood preservation comprises the hand application of supplemental or remedial preservatives that contain active ingredients (biocides). These preservatives use mainly solvents as carrier and in some cases form part of protective wood finishes, i.e. wood sealers, and therefore used as a dual purpose supplemental or remedial preservative against biological attack as well as a protective finish against weathering factors such as UV, precipitation and temperature changes etc. Supplemental or remedial preservatives are mainly applied as a corrective measure to stop existing biological attack in untreated timber already in use, but can also be both corrective and preventative, i.e. treating exposed ends of pre-treated timber that has been modified or cut. Bandages, pastes and rods (sticks) with diffusible borate as the active ingredient also fall under the remedial preservatives, which are also used to augment and maintain primary treated timber exposed to high risk environments.
Supplemental or remedial preservatives usually require periodic maintenance to remain effective. Protective wood penetrating finishes and sealers that are applied by hand, i.e. by brush, paint, and spray, but which contain no active ingredients (biocides) are not regarded as preservatives but merely wood protective finishes, and as already eluded to, these type of products protect against weathering factors only.
Its application is however crucial when exposed timber (including preservative treated timber) is required to retain its natural wood luster and not acquire the grey to silvery weathered look.
Why preserve timber?
The natural durability of our commercially grown and used plantation species like Pinus and Eucalyptus, is low, rendering it susceptible to insect and fungal attack; therefore it is imperative to preserve the timber. Timber preservation also enhances durability and confidence in using timber and extends the life of timber, as well as providing the added benefit of increasing the carbon sink. Preservation of timber and the sale and use of preservative treated timber are regulated in South Africa. Regulation A13 1(b) in SANS 10400-A, as well sections in the NHBRC home builder’s manual, requires the use of primary preservative treated timber when used in a permanent structures in specific areas of South Africa. In addition compulsory specifications i.e. VC 9092 regulated by the National Regulator for Compulsory Specifications (NRCS) control the sale of preservative treated timber. All of the aforementioned regulations refer to SANS10005, The preservative treatment of timber, which specifies exactly in which specified areas the use of primary treated softwoods and hardwoods are compulsory.
Choosing the correct treated timber
SANS 10005 and the SANS product standards for preservative treated sawn timber (SANS 1288) and poles (SANS 457-2 or SANS 457-3) specify a Hazard Class system (H Classes), which categorises treated timber into different end-use applications based on the following:
Product use information
Be sure to specify and use the correct H class timber for your intended application. Apply remedial preservative to all cross-cut and exposed areas. Under no circumstances must cross cut areas be exposed to ground contact unless retreated by pressure impregnation. Ensure that when poles THE HOW TO OF BUILDING 442 or posts are planted that in the ground that it is done in such a way as to allow for drainage of moisture or water taken up by the timber (see below).
The diagrams do not relate to structural details. Please consult a structural engineer.
When machining (e.g. sanding and sawing) CCA treated wood, be sure to wear a dust mask and work in a wellventilated area to avoid inhalation of treated wood particles, and wear safety eyewear to protect your eyes from flying particles. Wear suitable safety gloves when working with freshly treated wood. Do not make toys or furniture from treated wood that may be chewed on by infants, or make any food utensils from treated wood. Do not use treated wood for firewood, or cooking purposes. Treated wood shavings or sawdust should not be used for animal litter or where it can become a component of animal feed.
Treated timber waste is not regarded as hazardous waste material; however, treated wood off-cuts and waste should not be allowed to accumulate, but should be disposed of at a registered disposal or landfill site. It is important not to burn treated wood off-cuts as this will allow the release of hazardous chemicals, which are tightly bound to the wood, into the smoke. The ashes will also contain residual chemicals.
The primary wood preservation industry currently boasts ± 115 certified timber treatment plants in South Africa, consisting predominantly of CCA and Creosote plants. In 2013, the total volume of preservative treated timber manufactured in South Africa was in the region of 1 065 580m3. For more information on wood preservation and where to find a SAWPA member in South Africa, please contact SAWPA at 0119741061 or firstname.lastname@example.org or visit our website at www.sawpa.co.za.
Structural timber stored on site should be stacked on level ground on bearers and adequately protected against the weather by covering with a waterproof material. Air must be allowed to circulate through the timber stacks. Strapping around bundles of battens should not be removed until the battens are to be fixed. This will prevent excessive warping of the battens.
Refer Roof Construction – Roof trusses – Steel Trusses
The roof as a whole should be designed to with stand the minimum design loadings in accordance with SANS 10160. “The general procedures and loading to be adopted in the design of buildings.” When considering wind forces acting on a pitched roof, the pressure on the windward slope is dependent on the pitch. If the pitch is less than 30°, the windward slope may be subjected to severe suction or to negative pressure. If the pitch is steeper than 35°, the roof generally presents sufficient obstruction for a positive pressure to develop on the windward slope but, even in this case, there is an area near the ridge where suction develops. The leeward side is always subject to suction, though it is usually not as strong as the produced near the windward edge.
When considering pitches of roofs, conditions such as wind speed, shape and locality of building, height and exposure of the roof must be taken into consideration. Recommended minimum pitches for the various types of roofs are described in Table 15.2 – Minimum roof slopes and sheet end laps. Where rain and wind conditions are known to be severe, the roof pitch should, as a rule, be increased by 5° above the minimum.
In current building practice, the undertile membrane has become an integral part of any tiled roof. When properly laid, it will provide a highly effective barrier against the ingress of wind-driven rain and dust and the pressures exerted by wind forces will be reduced substantially due to pressure equalisation. The use of a suitable undertile membrane which complies with requirement type E of SANS 952 – 1985 or Agrément approved having a nominal thickness of 250 microns (0,25mm) is recommended for all pitches in all areas.
In valleys, a similar strip should be used and tucked under the undertile membrane of the main roof. See figure 15.37.
Note: It is essential to install a undertile membrane for roof pitches below 26° and above 45° and for all roof pitches in coastal and other exposed and windy areas.
Refer to – Flexible reflective foil laminate insulation (RFL) figures 15.17 and 15.24
Failure of roofs can often be attributed to poor workmanship and the disregard of simple erection procedures. Trusses should be protected against any damage on site whilst awaiting erection. They should be stacked on level ground on timber bearers and covered with a waterproof material but with adequate ventilation.
When handling trusses care must be taken to avoid any damage to the timber and to the joints.
The use of rafters spanning from wall to wall is a regular feature of modern architecture. The ceiling follows the slope of the roof, or it can be fixed on top of the rafters. Refer to Figure 15.43.
The rafters and wall plate are anchored as previously described, using either 30x1,6mm metal straps or 4mm diameter galvanised wire built into the wall. The rafters must be designed simply supported and the loading uniformly distributed over the full span, in accordance with SANS 10400 Part L.
Timber used for battens and purlins must comply with SANS 1783-4. The battens and purlins should be straight, free from major defects and in long lengths. Joints in battens and purlins should be staggered on rafters.
Size of battens must be in accordance with spacing of trusses. Refer to Table 15.26.] Batten centres will depend on the type of roof tile which is to be used – refer to the manufacturer of the roof tile chosen.
Refer to Superstructure section – Brickwork – Roof anchoring – for more information
Any purlin shall have a minimum nominal width and depth of 50 mm and 76 mm respectively and the maximum centre- to-centre spacing between purlins shall be 1,2 m – refer to the manufacturer of the roof sheeting chosen.
The following checks should be carried out before fixing of
the battens or purlins commences.
Note: The tiling procedure and the fixing of tiles must all be in accordance with the manufacturers recommendations. Refer to Roof construction at the beginning of this section for more information under rafters and beams and more especially Table 15.6 Maximum spans for rafters.
The roof is now ready for the roof sheeting. For sheeting used at the minimal pitch for the profile being used, consideration should be given to the use of a lap sealant at end and side laps.
Note: While we have only briefly described the erection procedure of the two most commonly used roof types – being tile or sheeted – other roofing types are also used which have not been described above; although the fundamentals of each roofing type remain the same it is up to the reader to seek the advice from the manufacturer according to the type of roofing being used for specific specifications and recommendations.
Note: Most pierced fixed roof sheeting profiles should only be fixed at every second crest on internal purlins and at every crest at eaves and ridge (end) purlins. Side laps should be stitched at 500mm centres between purlins all in accordance with the manufactures recommendations.