Substructure and ground-bearing floors

Also see:

5.1.12Walls below the DPC

Substructure and walls below the DPC shall be suitably constructed. Issues to be taken into account include:

  1. construction of walls acting as temporary retaining walls
  2. concrete cavity fill.

Construction of walls acting as temporary retaining walls

Backfill should be placed in layers of equal thickness to both sides of the substructure walls, so that compaction on one side is not more than one layer ahead of the other. Where backfill is placed and compacted on one side of the foundation trench before the other side is backfilled, the wall will be acting as a temporary retaining wall.

In such cases, the wall should either be designed by an engineer in accordance with Technical Requirement R5 or the thickness (T) should be as indicated in Table 2.

Table 2: Acceptable D:T of temporary retaining walls

Depth (D) of filled trenchMinimum thickness (T) of wall leaf supporting fill
Up to 1100mm200mm

This guidance is only applicable to the temporary condition and where problems such as hydrostatic pressure are not present.

Concrete cavity fill

A minimum 225mm clear cavity below the DPC should be maintained. When specialised foundations are used, including those for timber framed buildings, the minimum clear cavity depth may be reduced to 150mm below the DPC, provided that weep holes and other necessary measures are taken to ensure free drainage.