Taking off construction works

The term ‘taking off’ refers to the process of identifying elements of construction works that can be measured and priced. This is necessary to produce bills of quantities and requires that the design is complete and a specification has been prepared.

Analysis of drawings and specifications allows the cost consultant (usually a quantity surveyor) to prepare a taking off list, which lists all of the individual elements that comprise the works.

These elements can then be measured in number, length, area, volume, weight or time then collated and structured to produce an unpriced bill of quantities. This process is sometimes referred to as ‘working up’.

Where there may be uncertainty about some elements, they may be entered onto a query sheet that can be issued to the appropriate members of the consultant team for clarification.

The unpriced bill of quantities will have price and rate columns for tenderers to complete.

The bill of quantities is issued to tenderers for them to prepare a price for carrying out the works. The bill of quantities assists tenderers in the calculation of construction costs for their tender, and, as it means all tendering contractors will be pricing the same quantities (rather than taking off quantities from the drawings and specifications themselves), it also provides a fair and accurate system for tendering.

Best practice for the preparation of bills of quantities is set out in the New Rules of Measurement (NRM) published by the Royal Institute of Chartered Surveyors and replacing Standard Method of Measurement, seventh edition (SMM7). Adopting the New Rules of Measurement means that all projects are taken off in the same way and in the same order following consistent rules.

Increasingly, software packages are available to assist in the preparation of 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. The process in Building Information Modelling (BIM) is often referred to as material or quantity takeoff, which uses the underlying 3D parametric model to calculate materials takeoff for cost estimation, as well as in some cases embodied carbon calculations by adding the density factor and CO2 e per kg or volume of material.

In this process care must be taken that digital models are a fair representation of the actual product or component, for examples metal window frames being modelled as hollow extrusions rather than solid elements, the later can impact the material take-off dramatically increasing for example the assumed aluminium used by weight.

Bills of quantities are normally only prepared on larger projects. On smaller projects, or for alteration work, the contractor can be expected to measure their own quantities from drawings and schedules of work. Schedules of work are 'without quantities' instructional lists that allow the contractor to identify significant work and materials that will be needed to complete the works and to calculate the quantities that will be required.