Chapter 6
Cost Estimating
Overview
The Three Rules To Cost Estimating
Understanding the Building Process and What Makes Up a Structure
Building Drawings and Specifications
Some Basic Principles of Measuring Quantities
Resources and Building Elements (Rate Build-Ups)
References
Overview
In this short section the intention is not to teach one to be a cost estimator but rather to give an overview of the intricacies involved in measuring quantities and estimating costs. Most importantly, to stress the importance of measuring costs accurately in the first place and not undertaking construction projects based on guess work.
There is no such thing as a standard building cost/rate per square meter of floor area. Because no two structures are the same, no two building costs are the same. This variation in cost does not only relate to the finishes in a building (finishes on average only make up between 20 – 30% of the total building cost), but also the shape and design of the building. Quantity surveyors and cost estimators offer comprehensive services that cover measuring quantities and allocating costs and one is advised to employ the services of these professionals.
The Three rules to Cost Estimating
Estimating must be quick.
If one can’t complete an estimate quickly that person might not bother doing it at all. Quickly doesn’t mean performing all the functions quickly, it means working cleverly. One of the best ways to keep estimating quick is to keep and reuse your elements/rate build-ups (see examples under this section). Once you have worked out and compiled a rate build-up you can use it again and again.
Estimating must be consistent
When estimating one must assume a number of quantities and be consistent with this. As an example the quantity of bricks required to build 1m2 of wall will differ depending on the size of the brick and the mortar joint.
It can generally be assumed that in a 110mm wall (half-brick wall), 50 to 55 bricks will be used per m2 and in a 220mm (1 brick wall), 100 to 110 bricks will be used per m2. Chose a quantity per m2 and always use this when compiling a rate build-up/cost estimate.
Estimating must be accurate
While taking into account that estimating must be quick, it obviously must be accurate. Great care must be taken when measuring from the plan and when doing all the calculations. Care must also be taken when compiling elements/rate buildups or any other information that is used on a continuous basis.
Understanding the building process and what makes up a structure
An understanding of the building process is important when measuring quantities for a building. It is unfortunately common practice that not all the required information is on the drawings or in the specification. The estimator then has to make assumptions and understanding the building process makes that task a lot easier. For example, the drawings don’t show skirtings and assume there is no specification. Because of his/her knowledge of the building process, the estimator will know that there is a finishing carpenter involved and that skirtings are required. The estimator can therefore measure the skirting, assume a specification and allow a value. Below is a summerised framework of the building process.
- Plant Hire
- Site Works
- Foundations & Superstructure
- Aggregates
- Piling
- Brick work
- Cement
- Concrete
- Formwork
- Mortar
- Labour
- Sub Contractors
- Sundries
- Damp proofing
- Reinforcing
- Concrete Slabs
- Waterproofing
- Windows and doors
- Roofing
- Plastering
- Concrete fibre cement other mouldings
- Plumbing
- Electrical
- Carpenter and joinery
- Ironmongery
- Tiling
- Painting and Wall-coatings
- Glazing
- Flooring
- Kitchen units, tops and cupboards
- Ceilings and insulation
- Specialised sub-contractors (swimming pool)
Another important aspect of understanding the above process is that it allows the estimator to follow a logical procedure when measuring any thereby not leaving anything out i.e. starting at A and ending at Z.
Building drawings and specifications
Drawings and specifications are obviously the most important reference documents to the estimator. An example of a set of drawings is shown in this publication under Drawings. The drawings should contain all the information required for measuring e.g. walls, floor areas, roof areas. Other information where quantities are simply counted should also be on the drawings e.g. doors, light points. Unfortunately as mentioned above, many drawings don’t contain all the information required for estimating and the estimator therefore has to make some assumptions.
It is obviously important to compile an accurate cost estimate before a contract price is agreed or before building commences. The more detailed the specification the more accurate the cost estimate will be. The specification is usually a document separate from the drawings that lists finishes, details and specifications that might not be on the drawing e.g. type of sanitaryware to be used. The information in the specification is very helpful to the estimator.
Some basic principles of measuring quantities
Measuring should be kept as simple as possible and each measurement should contain as many resources as possible. As an example you don’t want to count all the bricks in a structure one by one. Its better to measure the m2 area of brickwork and then knowing how many bricks are in a m2 you can calculate the number of bricks. To know the m2 area of brickwork in linear (running) metre of wall makes the job] of measuring even easier. The areas of work to be measured in a structure that require time and care are the structural areas e.g. brickwork, concrete work, roofing. Most other quantities are simply counting e.g. light points, bathroom points. Although counting is simple one must remember that for some items there will be related items e.g. for every piece of sanitary-ware, there is plumbing material and labour required to install the sanitary-ware. The material may include the item itself, a waste plug and chain, a trap, taps.
Centre-line or outside-line measuring
When measuring, the most accurate way is to measure to the centre of the wall. Unfortunately this is not practical because all construction drawings don’t show dimensions to the centre line of the wall, meaning that a lot of time consuming calculations will have to be done.
Always remember it is preferred that one uses dimensions on the drawings before using a scale ruler. The common and accepted way of measuring from a drawing is to measure the outside line of all walls. When measuring interior walls the measuring process is more logical. In the floor plan example below the length of external walls is 31040mm (31.04m). Measure the 110mm interior walls which should add up to 18820mm (18.82m). Remember to measure over windows, doors and openings.
Measure over windows and doors
When measuring the walls one must measure over windows, doors and openings. Each door and window occupies a space in the brickwork and this space needs to be deducted from the brickwork quantities. These calculations are done separately and then deducted from the brickwork. Window and door specifications can be done at this point as well as quantifying related items such as door handles, window sills, glazing etc.
Resources and building elements (rate build-ups)
Resources are the smallest “purchasable” objects that can go into a structure e.g. a bag of cement or a m3 of sand. The reason the term “purchasable” is used is because it is not practical to purchase a litre of cement or a wheelbarrow of sand.
Elements or rate build-ups (for ease referred to as elements) are a combination of resources with related quantities. The example below indicates the difference between a resource and an element:
| Door – Resource | Quantity | Price |
| 813mm X 2031mm masonite hollwcore door | I each | R450.00 |
| Door – Element | Quantity | Price |
| 813mm X 2031mm masonite hollwcore door | I each | R450.00 |
| Meranti door frame | I each | R350.00 |
| Brass butt hinges | I each | R80.00 |
| 2 lever lockset and handles | I each | R120.00 |
| Labour to hang door | I each | R200.00 |
Note: The prices used are only for demonstration purposes
Figure 6.1 – Floor plan for estimating
The reason elements are used is because once they are compiled one doesn’t have to do this again each time measurements are taken from a set of drawings. Some elements can be complicated and once calculated and documented can be used again and again. See some examples of slightly more complicated elements in the pages that follow. Elements can be large and complex made up of many resources and smaller elements or sub elements. In the Figure 6.2 on the following page it shows how 1 lineal metre of wall can be broken up into smaller components or sub-elements. These sub elements contain resources. The reason sub elements are used is because they can be compiled and calculated once an be used in several different main elements. In the tables to follow there are 7 sub elements that can be used on their own or within an element.
Figure 6.2 – Example of sub-elements for 1 linear meter of wall
Sub Elements
References
Alexander G D & Alexander G C. 2010. Buildaid Building and Pricing Guide 2010/11. Johannesburg: Buildaid Publishing.
Alexander G D & Van As F. 2008: Civil Technology. Johannesburg: EWA Publishing.
Alexander G D & Van As F. 2008: Civil Technology. Johannesburg: EWA Publishing.