5.1 Substructure and ground bearing floors
(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.
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.
(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.
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 1.1||200|
|1.1 to 1.4||300|
|1.4 to 1.7||400|
|1.7 to 2.0||500|
This table is only applicable to the temporary condition and where problems such as hydrostatic pressure are not present.
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.
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:
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.
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.
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).
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.
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.
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 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.
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.
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.
Where dwellings are stepped down a sloping site, the dpcs and dpms should be linked so that all parts of each dwelling are protected.
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).
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.
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.
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).
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:
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.
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.