Substructure and ground-bearing floors

5.1.20Damp proofing concrete floors

Ground-bearing floors shall resist the passage of moisture to the inside of the home.

Ground-bearing concrete floor slabs should be protected against ground moisture by providing a continuous damp proof membrane (DPM). The DPM should:

  • have sealed laps of at least 300mm wide
  • link with wall DPCs to form an impervious barrier to prevent moisture reaching the interior of the dwelling
  • take account of possible differential movement.

Care should be taken not to trap moisture when a combination of damp proofing and vapour control layers are used.

When the DPM is located below the slab, a blinding layer of sand should be provided to fill voids in the hardcore and to minimise the risk of puncturing the membrane.

A clear cavity of at least 225mm below the DPC should be maintained. When specialised foundations are used, including those for timber framed buildings, this depth may be reduced to 150mm below the DPC where weep holes are provided and other necessary measures are taken to ensure that the cavity can drain freely.

Where homes are stepped down a sloping site, the DPCs and DPMs should be linked so that all parts of each home are protected. The guidance in Chapter 5.4 ‘Waterproofing of basements and other below ground structures’ should be followed where steps between floor slabs are greater than 150mm.

Suitable materials for DPMs include:

  • 1200 gauge (0.3mm) polyethylene sheet
  • minimum 1000 gauge (0.25mm) polyethylene sheet where it complies with Technical Requirement R3
  • bitumen sheet to BS 6398
  • materials that comply with Technical Requirement R3.