Suspended ground floors

Also see:

5.2.10Damp-proofing and ventilation

Suspended ground floors shall be designed and constructed to resist the passage of moisture into the building. Issues to be taken into account include:

  1. damp-proofing
  2. ventilation.


Where DPMs are required, they should be linked with any DPCs in the supporting structure, in order to provide continuous protection from moisture from the ground or through the supporting structure.

DPMs should be properly lapped in accordance with Chapter 5.1 ‘Substructure and ground-bearing floors’.

In-situ concrete:

Dampness from the ground and supporting structure should be prevented from reaching the floor by using linked DPMs and DPCs to provide continuous protection.

Where there is a risk of sulfate attack, in-situ or oversite concrete should be protected with polyethylene sheet that is a minimum:

  • 1200 gauge (0.3mm), or
  • 1000 gauge (0.25mm) if assessed in accordance with Technical Requirement R3.

Precast concrete:

Additional damp-proofing may not be necessary where:

  • the underfloor void is ventilated and DPCs are provided under bearings of precast floors in accordance with CP 102
  • ground below the floor is effectively drained, if excavated below the level of the surrounding ground.

Where proprietary floor systems are used, adequate moisture-resistant membranes should be installed in accordance with the manufacturer’s recommendations.

Vapour control layers may be necessary to protect floor finishes, and where used, should be positioned in accordance with the manufacturer’s recommendations.

Timber ground floors:

Timber used for suspended ground floors should be treated or naturally durable, in accordance with Chapter 3.3 ‘Timber preservation (natural solid timber)’, and the ground below the floor covered with:

  • 50mm concrete or fine aggregate on a polyethylene membrane laid on 50mm sand blinding, or
  • 100mm concrete.

In Scotland, the deemed-to-satisfy specification of the building regulations should be followed.


Ventilation should be provided to precast and timber suspended floors. This is generally provided by ventilators on at least two opposite external walls, with air bricks properly ducted in accordance with Chapter 6.1 ‘External masonry walls’. Where this is not possible, suitable cross ventilation should be provided by a combination of openings and air ducts. Ventilation should not be obtained through a garage.

Sleeper walls and partitions should be constructed with sufficient openings to ensure adequate through ventilation. If necessary, pipe ducts should be incorporated in adjoining solid floors, separating walls or other obstructions. Where underfloor voids adjoin ground bearing floors, ventilation ducts should be installed.

Void ventilation should be provided to whichever gives the greater opening area:

  • 1500mm2 per metre run of external wall
  • 500mm2 per m2 of floor area.

In the case of timber floors, ventilators should be spaced at no more than 2m centres and within 450mm of the end of any wall.

A minimum ventilation void of 150mm should be provided below the underside of precast concrete and timber suspended floors. On shrinkable soil where heave could take place, a larger void is required to allow for movement according to the volume change potential.

  • high volume change potential – 150mm (300mm total void)
  • medium volume change potential – 100mm (250mm total void)
  • low volume change potential – 50mm (200mm total void).