NHBC Standards 2011

5.2 Suspended ground floors

SCOPE

This Chapter gives guidance on meeting the Technical Requirements and recommendations for suspended ground floors of in-situ and precast concrete and timber joists.

DESIGN STANDARDS

General
5.2 - D1 Design standards
5.2 - D2 Statutory and other requirements
5.2 - D3-D4 Resistance to ground contaminants
In-situ concrete
5.2 - D5 Structural stability
5.2 - D6 Resistance to moisture
5.2 - D7 Thermal insulation
5.2 - D8 Floor finishes and decking
Precast concrete
5.2 - D9 Structural stability
5.2 - D10 Resistance to moisture
5.2 - D11 Thermal insulation
5.2 - D12 Floor finishes and decking
Timber
5.2 - D13 Structural stability
5.2 - D14 Structural timber
5.2 - D15 Trimmers
5.2 - D16 Strutting
5.2 - D17-D18 Joist hangers
5.2 - D19 Joist support at separating walls
5.2 - D20 Intermediate support
5.2 - D21 Floor decking
5.2 - D22 Damp-proofing
5.2 - D23 Thermal insulation
Information
5.2 - D24-D25 Provision of information

5.2 - D1
Design shall meet the Technical Requirements
 

Design that follows the guidance below will be acceptable for suspended ground floors.

STATUTORY AND OTHER REQUIREMENTS

5.2 - D2
Design of suspended ground floors shall comply with all relevant 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:

RESISTANCE TO GROUND CONTAMINANTS

5.2 - D3
Design shall ensure that adequate measures are taken against adverse effects of ground contaminants and radioactive materials
 

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.

5.2 - D4
Design shall provide adequate protection against radon gas
 

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

5.2 - D5
In-situ concrete suspended ground floors shall be designed to transmit all loads safely to the supporting structure without undue movement
 

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:

(b) end bearings

Bearings on supporting walls should be designed either:

RESISTANCE TO MOISTURE

5.2 - D6
In-situ concrete suspended ground floors shall be designed to resist the passage of moisture to the inside of the building
 

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

5.2 - D7
Thermal insulation of in-situ concrete suspended ground floors shall be designed to comply with statutory requirements
 

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 placed below the floor slab
 

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.

(b)
insulation placed above the floor slab
 

For guidance on insulation above the floor slab, reference should be made to Chapter 8.3 'Floor finishes' (Design and Materials).

(c)
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 substructure with insulation.

FLOOR FINISHES AND DECKING

5.2 - D8
Finishes and decking to in-situ concrete suspended ground floors shall be suitable for their intended use

 

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

5.2 - D9
Precast concrete suspended ground floors shall be designed to transmit all loads safely to the supporting structure without undue movement
 

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

5.2 - D10
Precast concrete suspended ground floors shall be designed to resist the passage of moisture to the inside of the building
 

Items to be taken into account include:

(a)
damp-proofing
 

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).

(b)
ventilation of underfloor voids
 

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.

(c)
damp-proofing of suspended floors
 

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)).

(d)
linking dpm with dpc
 

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

5.2 - D11
Thermal insulation of precast concrete suspended ground floors shall be designed to comply with statutory requirements
 

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

5.2 - D12
Finishes and decking to precast concrete suspended ground floors shall be suitable for their intended use
 

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

5.2 - D13
Timber suspended ground floors, including the decking material, shall be designed to:
(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

5.2 - D14
Structural timber grades and sizes shall be adequate for the spans and imposed loads
 

For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (Design and Materials).

TRIMMERS

5.2 - D15
Where trimming is necessary, adequately sized timbers shall be used
 

For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (Design and Sitework).

STRUTTING

5.2 - D16
Strutting shall be adequate to limit the twisting of joists
 

For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (Design and Sitework).

JOIST HANGERS

5.2 - D17
Joist hangers shall be suitable for the joist width and depth, the strength of masonry and the required load
 

For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (each section).

5.2 - D18
Adequate end bearings shall be provided for joists and joist hangers
 

For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (Design and Sitework).

JOIST SUPPORT AT SEPARATING WALLS

5.2 - D19
Joists shall be correctly supported at masonry separating walls
 

For guidance, reference should be made to Chapter 6.4 'Timber and concrete upper floors' (Design and Sitework).

INTERMEDIATE SUPPORT

5.2 - D20
Sleeper walls shall adequately support the floor joists
 

For guidance, reference should be made to Chapter 4.4 'Strip and trench fill foundations' (Design and Sitework).

FLOOR DECKING

5.2 - D21
Appropriate materials for floor decking shall be used
 

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

5.2 - D22
Design of timber suspended ground floors shall ensure that the floors are not adversely affected by moisture
 

Items to be taken into account include:

(a)
damp-proofing of supporting structure
 

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.
(b)
resistance to ground moisture
 

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.

(c)
ventilation of underfloor voids
 

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

5.2 - D23
Thermal insulation of timber suspended ground floors shall be designed to comply with statutory requirements
 

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

5.2 - D24
Designs and specifications shall be produced in a clearly understandable format and include all relevant 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.
5.2 - D25
All relevant information shall be distributed to appropriate personnel
 

Ensure that design and specification information is issued to site supervisors and relevant specialist subcontractors and/or suppliers.