NHBC Standards 2011

5.1 Substructure and ground bearing floors


5.1 - S1Sitework standards
5.1 - S2Walls below DPC
5.1 - S3Ground below fill
5.1 - S4-S5Fill supporting ground bearing slabs
5.1 - S6Trench backfill
5.1 - S7Blinding
5.1 - S8Damp-proofing floors
5.1 - S9Damp-proof course
5.1 - S10Laying thermal insulation
5.1 - S11Laying the slab
5.1 - S12Basements

5.1 - S1
All sitework shall:
(a) meet the Technical Requirements
(b) take account of the design
(c) follow established good practice and workmanship

Sitework that complies with the design and the guidance below will be acceptable for substructure and ground bearing floors.


5.1 - S2
Walls below dpc shall be constructed in accordance with the design

Items to be taken into account include:

(a) concrete cavity fill

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

Concrete cavity fill

(b) construction sequence

Where backfill is placed and compacted in one side of the foundation trench before the other side is backfilled, the wall will be acting as a temporary retaining wall.

Backfill placed before concrete cavity fill

Backfill place after concrete cavity fill

In such cases, the wall should be either designed by an Engineer in accordance with Technical Requirement R5 or the thickness (T) should be increased as indicated in the following table:

Depth (D) of
filled trench [m]
Minimum thickness (T) of
wall leaf supporting fill [mm]
Up to 1.1200
1.1 to 1.4300
1.4 to 1.7400
1.7 to 2.0500


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

(c) workmanship

Workmanship below dpc level should be of a high standard to avoid the necessity for costly repairs. Details of cavity wall construction are given in Chapter 6.1 'External masonry walls' (Sitework).

(d) wall ties

Where cavity insulation batts or slabs start below dpc level, the vertical and horizontal spacing of wall ties should be compatible with the spacing to be used above dpc level.

(e) services and service entries

Underground services should be installed as described in Chapters 8.1 'Internal Services' (Sitework) and 5.3 'Drainage below ground' (Sitework), or provision made for their later installation. Where services penetrate walls, the following alternatives are acceptable:

Pipes passing through lintelled opening

Lintel over multiple services

Pipe bedded in walls

Services should be sleeved where they pass through a structural element.

When unidentified services, ducts, cables or pipes are exposed, advice should be sought from local offices of statutory undertakings and service supply companies.


5.1 - S3
Ground shall be adequately prepared to provide consistent support to the fill and the ground bearing slab

All vegetable soil and organic matter, including tree roots, should be removed to provide an even bearing surface.

Ground bearing floors may be used only where:

  • trenches are backfilled with properly compacted material
  • infill is less than 600mm in depth and properly compacted
  • the ground is suitable to support floor loads and any other loads.
Properly compacted fill and backfill

Where more than 600mm of infill is required at any point within a self-contained area, or the bearing capacity and nature of the ground varies, the floor over the self-contained area should be of suspended construction. Reference should be made to Chapter 5.2 'Suspended ground floors' (Sitework).

Suspended floor where infill is more than 600mm


5.1 - S4
Fill, including made ground, trench backfill and infill, shall provide full and consistent support to any ground bearing slab

Well graded inert fill without hazardous materials, which passes a 150mm x 150mm screen in all directions, will normally be suitable as support for a ground floor slab.

Fill should be placed and mechanically compacted in layers not exceeding 225mm deep, to form a stable mass.

5.1 - S5
The performance of the substructure and ground bearing slab shall not be affected by any hazardous materials in the fill

Types of fill which require special precautions to be taken are given in Appendix 5.1-A.

Where sulfates are present in the ground at levels likely to be harmful:

  • the floor slab should be of the appropriate mix to resist sulfate attack and be protected by an impervious layer of 1200 gauge (0.3mm) polyethylene sheet (or 1000 gauge (0.25mm) if assessed in accordance with Technical Requirement R3) which may also serve as a dpm
  • mortar in substructure walls should contain sulfate resisting cement
  • concrete blocks in substructure walls should have a sulfate resistance appropriate for the level of sulfate in the fill or ground.

Expansive materials are not acceptable for use as fill supporting ground bearing slabs.


5.1 - S6
Backfill to trenches shall be adequately compacted

To avoid settlement at junctions between the substructure wall and the ground bearing floor, trenches should be backfilled with solid material, graded and compacted according to the guidance given in Clause S4. Alternatively, concrete may be used (see Chapter 2.1 'Concrete and its reinforcement' (Sitework)).

Fill compacted in equal layers on both sides

Fill should be placed in layers of equal thickness to both sides of substructure walls so that compaction on one side is not more than one layer ahead of the other. Where this is not possible, the wall will be acting as a retaining wall and the advice given in Clause S2(b) should be followed.


5.1 - S7
Blinding shall provide a suitable surface for the materials above

Fill should be blinded sufficiently to receive concrete (or dpm, if required) using the minimum thickness necessary to give a suitable surface. Concrete blinding may be needed where voids in the fill could result in loss of fines from the blinding. Where hardcore fill is used, smooth blinding, eg sand or other suitable fine material, is essential to avoid puncturing a sheet dpm. Where the ground floor is to be reinforced, the blinding should be firm and even to give good support for the reinforcement and to maintain the design cover, using reinforcement stools, where necessary.


5.1 - S8
Ground bearing floors shall be resistant to the passage of moisture from the ground

A ground bearing concrete floor should be protected from ground moisture by a continuous damp-proof membrane.

The continuity of the membrane should be maintained as follows:

  • laps in polyethylene should be at least 300mm and joints sealed, where necessary
  • membranes beneath the slab should link with wall dpcs to form an impervious barrier to prevent moisture reaching the interior of the dwelling
  • linking should take account of possible differential movement.
Dpc laps dpm

Where dwellings are stepped down a sloping site, the dpcs and dpms should be linked so that all parts of each dwelling are protected.


5.1 - S9
Moisture from the ground shall be prevented from reaching the inside of the building

A damp-proof course should be positioned at least 150mm above finished ground or paving level and should link with any ground floor dpm.

The dpc should be of the correct width and fully bedded.

For guidance concerning setting and bedding dpcs, reference should be made to Chapter 6.1 'External masonry walls' (Sitework).


5.1 - S10
Installation of thermal insulation shall ensure that the full thermal performance of the floor is achieved

Insulation boards should be tightly butted together to maintain insulation continuity.

Where the insulation is turned up vertically at the edge of the slab, the edge should be protected whilst concrete is being poured and tamped.

Where edge insulation is within the cavity external wall, the insulation material should be supported on wall ties as described in Clause S2(d) and Chapter 6.1 'External masonry walls' (Sitework).

Dpm trimmed to avoid bridging cavity


5.1 - S11
Ground bearing floors shall be reasonably level and effectively impervious to moisture

All underfloor services and ducts should be installed and, where appropriate, tested before concreting is started.

Care should be taken to ensure that all joints and junctions between damp-proofing membranes, wall dpcs or tanking in substructure walls are undamaged, especially while the concrete for the ground slab is being poured.

Dpm protected by board

Details of screeds laid monolithically with floor slabs are given in Chapter 8.3 'Floor finishes' (Sitework).

Details of mixing, placing and curing concrete are given in Chapter 2.1 'Concrete and its reinforcement' (Sitework).


5.1 - S12
Basements shall be constructed in accordance with the design

Appendix 5.1-B gives details of the different types of basement construction that may be acceptable to NHBC subject to appropriate detailing.

Items to be taken into account include:

detailing and application of tanking materials

Appendix 5.1-C gives typical details of the critical areas in basement construction to avoid water penetration.

The design will specify the method of tanking to be used. This should not be changed without consulting the designer.

Where required by the design the waterproofing system should be applied by a specialist contractor experienced in that system. Installers should be made fully aware of the design and the manufacturer's recommendations for the preparation and installation.

Tanking which is being installed or is complete should be protected to prevent damage.

A suitable protection board should be provided against the waterproofing material to avoid damage. Where the waterproofing system is protected by backfilled material this should be placed carefully in layers to prevent damage.

junctions, abutments and services

Details of how junctions and abutments are formed should be provided to site personnel.

Where services pass through the waterproofing system they should be in accordance with the design.

The basement waterproofing system should be linked to the damp proofing arrangements for the superstructure. Any lap joints should be protected against damage during construction.

See Appendix 5.1-C.