5.2 Suspended ground floors
SCOPE
DESIGN STANDARDS
Design that follows the guidance below will be acceptable for suspended ground floors.
STATUTORY AND OTHER REQUIREMENTS
Design should be in accordance with relevant Building Regulations and other statutory requirements.
NHBC requires ground floors to be constructed as suspended floors in the following situations:
- where the depth of fill exceeds 600mm, as described in Chapter 5.1 'Substructure and ground bearing floors' (Design)
- where soil swelling may occur, as described in Chapter 4.2 'Building near trees'
- on sites which have been subject to vibratory ground improvement, as described in Chapter 4.6 'Vibratory ground improvement techniques'
- where ground or fill is not suitable to support ground bearing slabs. For suitable fill providing temporary support to suspended floors, refer to Chapter 5.1 'Substructure and ground bearing floors' Appendix 5.1-A.
RESISTANCE TO GROUND CONTAMINANTS
Any contaminants in or above the ground should be identified to the satisfaction of NHBC, following the guidance given in the appropriate British Standard.
Where necessary, precautions against danger to health caused by contaminants should be taken, as described in the Appendix 'Introduction to remedial measures' to Approved Document C1/2/3 and other Building Regulation documents.
In certain parts of the country, special precautions may be necessary to reduce the entry of radon gas. Areas in England and Wales where special precautions are necessary are detailed in BRE Report 211.
When precautions are necessary, they should be acceptable to NHBC.
In-situ concrete
STRUCTURAL STABILITY
Items to be taken into account include:
(a) dead and imposed loads
Loads should be calculated in accordance with BS EN 1991-1-1.
Suspended in-situ concrete ground floors should be designed either:
- by an Engineer in accordance with Technical Requirement R5, or
- in accordance with BS 8103 Part 4.
(b) end bearings
Bearings on supporting walls should be designed either:
- by an Engineer in accordance with Technical Requirement R5, or
- in accordance with BS 8103 Part 4.
RESISTANCE TO MOISTURE
Items to be taken into account include:
(a) damp-proofing
Dampness from the ground and supporting structure should be prevented from reaching the floor by use of dpms and dpcs.
(b) linking dpm with dpc
Damp-proofing of suspended ground floors should be linked with any damp-proofing of the supporting structure in order to provide continuous protection.
Where there is a risk of sulfate attack, a polyethylene sheet dpm, not less than 1200 gauge (0.3mm) (or 1000 gauge (0.25mm) if assessed in accordance with Technical Requirement R3) should be used, properly lapped.
THERMAL INSULATION
The BRE report 'Thermal insulation: avoiding risks' discusses aspects of insulation relevant to suspended ground floors. In England and Wales account should be taken of Accredited Details.
Items to be taken into account include:
Insulation below the ground floor slab should:
- be placed on a suitable compacted and even substrate
- have low water absorption
- be resistant to ground contaminants
- be strong enough to support wet construction loads
- be compatible with any dpm.
Suitable insulating materials are described in the Materials section.
For guidance on insulation above the floor slab, reference should be made to Chapter 8.3 'Floor finishes' (Design and Materials).
The design should ensure that any risk of cold bridging is minimised, giving particular attention to junctions between floor and external walls.
Precautions include:
- extending cavity insulation below floor slab level
- linking floor and wall insulation
- providing perimeter insulation to floors
- facing the supporting substructure with insulation.
FLOOR FINISHES AND DECKING
Details of finishes and decking are given in Chapter 8.3 'Floor finishes' (each section) and Chapter 6.4 'Timber and concrete upper floors'.
Precast concrete
STRUCTURAL STABILITY
Items to be taken into account include:
(a) dead and imposed loads
Loads should be calculated in accordance with BS EN 1991-1-1
Precast concrete suspended ground floors should be:
- designed by an Engineer in accordance with Technical Requirement R5, or
- proprietary systems which have been assessed in accordance with Technical Requirement R3, or
- chosen from manufacturers' details which are based on recognised Standards and Codes of Practice.
(b) end bearings
Bearings on supporting walls should be as recommended by the manufacturer, and in no case less than 90mm.
RESISTANCE TO MOISTURE
Items to be taken into account include:
The supporting structure should, wherever necessary, incorporate adequate damp-proofing measures to prevent dampness from reaching the floor as described in the following Chapters:
5.1 'Substructure and ground bearing floors' (each section)
6.1 'External masonry walls' (each section).
A minimum void of not less than 150mm should be provided below the underside of floor slabs and beams.
On shrinkable soil where heave could take place, allowance should be made for the void to accommodate the following movements according to the shrinkage potential of the soil:
- high potential - 150mm
- medium potential - 100mm
- low potential - 50mm.
Voids should be ventilated by openings providing not less than 1500mm2 per metre run of external wall or 500mm2 per m2 of floor area, whichever gives the greater opening area.
Ventilation openings should be provided on at least two opposite sides. Where this is not possible, effective cross ventilation from opposite sides should be provided by a combination of openings and air ducts.
Where the finished level below the floor is lower than the finished adjoining ground level, appropriate drainage should be provided.
It may not be necessary to provide additional damp-proofing 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).
Vapour control layers may be necessary to protect floor finishes and should be positioned in accordance with the manufacturer's recommendations (reference should be made to Chapter 8.3 'Floor finishes' (each section)).
Where provided, damp-proofing of suspended ground floors should be linked with any damp-proofing of the supporting structure in order to provide continuous protection.
THERMAL INSULATION
The BRE report 'Thermal insulation: avoiding risks' discusses aspects of insulation relevant to suspended ground floors. In England and Wales account should be taken of Accredited Details.
Items to be taken into account include:
(a) insulation above floor slab
For guidance on insulation above a floor slab, reference should be made to Chapter 8.3 'Floor finishes' (each section).
(b) cold bridging
The design should ensure that any risk of cold bridging is minimised, giving particular attention to junctions between floor and external walls.
Precautions include:
- extending cavity insulation below floor slab level
- linking floor and wall insulation
- providing perimeter insulation to floors
- facing the supporting structure with insulation.
FLOOR FINISHES AND DECKING
Details of finishes and decking are given in Chapter 6.4 'Timber and concrete upper floors' and Chapter 8.3 'Floor finishes' (each section).
Timber
STRUCTURAL STABILITY
(a) support self weight, dead loads and imposed loads
(b) transmit loads safely to the supporting structure
(c) not deflect unduly
(d) take account of the adverse effects of shrinkage and movement
For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (Design).
STRUCTURAL TIMBER
For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (Design and Materials).
TRIMMERS
For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (Design and Sitework).
STRUTTING
For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (Design and Sitework).
JOIST HANGERS
For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (each section).
For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (Design and Sitework).
JOIST SUPPORT AT SEPARATING WALLS
For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (Design and Sitework).
INTERMEDIATE SUPPORT
For guidance, reference should be made to Chapter 4.4 'Strip and trench fill foundations' (Design and Sitework).
FLOOR DECKING
Items to be taken into account include:
(a) decking and joist centres
(b) resistance to moisture
(c) fixing
For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (each section).
DAMP-PROOFING
Items to be taken into account include:
The supporting structure should include damp-proofing to prevent moisture penetrating to the suspended floor, as described in Chapter 6.1 'External masonry walls' (each section).
Where the finished level below the floor is lower than the finished adjoining ground level:
- appropriate drainage should be provided, or
- the structure should be tanked.
Provision should be made to prevent ground moisture affecting timber floor construction.
This can be achieved by either:
- 50mm concrete or 50mm fine aggregate on a polyethylene membrane laid on 50mm sand blinding, or
- 100mm concrete.
In Scotland, the deemed-to-satisfy specification of the statutory regulations should be followed.
Where necessary, oversite concrete should be protected against sulfate attack by the use of a polyethylene sheet dpm, not less than 1200 gauge (0.3mm) (or 1000 gauge (0.25mm) if assessed in accordance with Technical Requirement R3) properly lapped.
A minimum ventilation void of 150mm should be provided below floor joists or 75mm below any wall plate.
On shrinkable soil where heave could take place, an allowance for movement should be added to the underfloor ventilation requirement to determine the minimum dimension of the underfloor void. The allowance for movement relates to the shrinkage potential of the soil as follows:
- high potential - 150mm
- medium potential - 100mm
- low potential - 50mm.
Voids should be ventilated by openings providing not less than 1500mm2 per metre run of external wall or 500mm2 per m2 of floor area, whichever gives the greater opening area.
Ventilators should be spaced at not more than 2m centres and within 450mm of each end of any wall. Air bricks should be ducted through cavities and be unobstructed.
Every part of the void under a timber suspended ground floor should be thoroughly ventilated through openings on at least two opposite sides. Where this is not possible, effective cross ventilation from opposite sides should be provided by a combination of openings and air ducts.
Provision should be made for ventilation through partitions and sleeper walls. If necessary, pipe ducts should be incorporated in adjoining solid floors, separating walls or other obstructions. Ventilation should not be obtained through a garage.
THERMAL INSULATION
The BRE report 'Thermal insulation: avoiding risks' discusses aspects of insulation relevant to suspended ground floors. In England and Wales account should be taken of Accredited Details.
Items to be taken into account include:
(a) positioning of insulation
Insulation may be:
- insulation quilt, or
- rigid insulation.
Insulation quilt should be supported between joists.
Rigid insulation boards should be adequately supported on battens fixed to the sides of joists.
(b) cold bridging
The design should ensure that any risk of cold bridging is minimised, giving particular attention to junctions between floor and external walls.
Precautions include:
- extending cavity insulation below floor level
- linking floor and wall insulation
- providing perimeter insulation to floors
- facing the supporting substructure with insulation.
Information
PROVISION OF INFORMATION
Clear and fully detailed drawings should be available on site to enable work to be carried out in accordance with the design.
Design drawings should include:
- all necessary plan dimensions and levels related to identified benchmarks
- details of loadbearing walls
- minimum bearing dimensions
- information on all proposed underground services
- points of entry to the building for services
- penetration of services through the substructure, including support of the structure above
- details of trench backfill, infill and void formers
- the required sequence of trench backfill if this is relevant to the design of the walls below dpc
- work required to maintain the integrity of dpcs and dpms
- details of junctions between dpm, dpc and tanking
- details of underfloor and floor edge insulation and cavity insulation, where relevant.
Ensure that design and specification information is issued to site supervisors and relevant specialist subcontractors and/or suppliers.