Measurement conventions

Figure 5: Ground floor plan showing external and internal walls

Figure 6: External walls - Section A-A

Figure 7: Internal walls - Section B-B

Figure 8: Trench

Figure 8 shows a cross section through the trench and reduced level excavation required for the external wall in the example bill of quantities for substructure. Note that the levels have been reduced internally by 150mm to allow for a 150mm thick bed of hardcore.

The top of the hardcore bed when compacted will be covered or blinded with sand to prevent the damp proof membrane, a layer of polythene sheet with a minimum thickness of 0.30mm, being perforated by the hardcore. It is important that the material used as hardcore is inert and free from chemicals, vegetable or other deleterious matter.

It is a requirement of the Building Regulations that insulation is incorporated into the floor construction and in this case 50mm thick rigid insulation board has been used. The decision as whether to position the boards above or below the slab is one of personal choice. The bottom of the trench excavation when completed will be compacted prior to the concrete being poured, this is to prevent the soil being incorporated into the concrete and weakening the mix. This is particularly important when reinforced concrete is being used, where it is common to blind the bottom of the excavation with a weak mix concrete before the reinforcement in placed in position.

When measuring quantities using NRM2 the following items are deemed to be included, that is, they are not required to be measured in the bill of quantities/work package:

• disposal of surface water;
• earthwork support – unless not at the discretion of the contractor;
• working space; and
• levelling, grading, trimming and compacting surfaces exposed by the excavation – for example the bottom of trench excavations.