Definition of measurement – Measurement of quantities for construction works.
Measurement of quantities for construction works is the transformation of drawn information into descriptions and quantities, undertaken to value, cost, and price construction work.
It is not just about a quantity surveyor producing a bill of quantities for contractors to price during tendering. It is used in both pre-and post-contract work, helping assess the likely cost of the works, and determining what contractors and subcontractors should be paid for work that has been completed.
Pre-contract measurement
During the early design stages, the quantity surveyor will measure the dimensions of the building to produce budget estimates, perhaps based on bench-marking against similar buildings.
As the design develops, they will measure more detailed approximate quantities for cost planning purposes, ensuring that the design can be achieved within the budget.
The quantity surveyor then measures the completed working drawings to produce a bill of quantities. Contractors tender for the job by pricing the work described in the bill of quantities.
Post-contract measurement
The contractor may use measurement for:
Preparing a construction methodology.
Ordering goods and materials.
Procuring subcontract works.
Calculating the effects of any variations.
Assessing work done against the construction programme.
Making payments to subcontractors.
Preparing or assessing valuations of work completed for interim payments.
The quantity surveyor may undertake measurement for:
Cost control.
Estimating the cost of variations to the work.
Calculating the value of nominated subcontractors’ and suppliers’ work.
Preparing or assessing interim valuations to pay the contractor for work done.
Levels of detail for measured information
The degree of detail to which construction work can be measured varies according to its use and the stage in the project. In the very early design stages, there is not much detail available, so estimates are based on general parameters, such as:
Functional unit: For example, cost per school pupil, cost per theatre seat, cost per hospital bed, and so on.
Floor area: Cost per sq. m gross floor area.
As the design progresses and more information is known, estimates can become more detailed, such as elemental estimates (for walls, floors, roof, frame, etc.).
During the later stages of the design, the work required to construct the building may be measured by:
Itemised specification: A detailed cost plan which is broken down into a series of elements. Initially, the elemental cost plan will simply be the total construction cost for the project divided into elements on a percentage basis.
Approximate quantities: A first attempt to measure defined quantities from the drawings (or to take them off from a building information model (BIM)). This should be accompanied by a schedule of assumptions made.
Bills of quantities: The work is measured in detail, usually in accordance with a standard method of measurement.
Taking off
The term ‘taking off’ refers to the process of identifying elements of construction works that can be measured and priced. Those elements can be measured in number, length, area, volume, weight or time, then collated and structured to produce an un-priced bill of quantities. This process is sometimes referred to as ‘working up’.
Standard measurement conventions
Always measure gross building area and then deduct items such as exterior walls to find floor space area. Always measure on the centre line of the material.
Calculating girths and centre lines
The centre line is halfway between the external and internal girth.
Centre line (CL) = (Internal girth + external girth)/2
CL = Internal girth + (No. of corners) x 2(wall width)/2
or
CL = External girth – (No. of corners) x 2(wall width)/2
Calculating girths for irregular shaped buildings
CL = Internal girth + (No. of corners) x 2(wall width)/2
Number of external corners = 5
Number of internal corners = 1
External corners – Internal corner = 4
This occurs regardless of shape providing the walls encompass 360°.
In the example above, the internal girth was 8.00, and the external girth was 10.00.
Difference = 2.00 m = 4 x 2(wall width)/2
Buildings with an inset
Girth = 2(length + width) + 2(depth of inset)
= 2(6.00 + 5.00) + 2(2.60)
= 27.20 m
BIM
Increasingly, software packages are available to assist in the preparation of bills of quantities, and building information modelling systems can be used to produce bills of quantities from information already contained within the model.
At Take off Bill, we use the latest Bill of Quantities software when Taking off construction works. This ensures Accuracy, Efficiency and Transparency, our motto here at Take off Bill.
We produce Take off’s and Bills of Quantities for all aspects of construction. No Job is too big or small. We typically work for Developers, Architects, Main Contractors, Sub Contractors and Home Builders.
Feel free to contact us here at Take off Bill at plans@takeoffbill.com or check out our website at http://www.takeoffbill.com .
P.S. In relation to this post, we tried to upload some really useful images to help explain the measurement process, but they would not upload, sorry guys!!
