Section 4 Floors
4.1 This section gives guidance for five situations:
a. ground supported floors exposed to moisture from the ground (see paragraphs 4.6 to 4.12);
b. suspended timber ground floors exposed to moisture from the ground (see paragraphs 4.13 to 4.16);
c. suspended concrete ground floors exposed to moisture from the ground (see paragraphs 4.17 to 4.20);
d. the risk of interstitial condensation in ground floors and floors exposed from below (see paragraph 4.21);
e. the risk of surface condensation and mould growth on any type of floor (see paragraph 4.22).
4.2 Floors next to the ground should:
a. resist the passage of ground moisture to the upper surface of the floor;
b. not be damaged by moisture from the ground;
c. not be damaged by groundwater;
d. resist the passage of ground gases. To meet requirement C1 (2) floors in some localities may need to resist the passage of hazardous ground gases such as radon or methane.Remedial measures will include a gas resistant barrier which, with proper detailing, can also function as a damp proof membrane. For specific guidance for methane and other gases refer to paragraphs 2.25 to 2.38, and for radon refer to paragraphs 2.39 to 2.40. Guidance is provided in reports BR 414 and BR 211 respectively.
4.3 Consideration should be given to whether 4.2(a) need apply to a building used wholly for:
a. storing goods, provided that any persons who are habitually employed in the building are engaged only in taking in, caring for or taking out the goods; or
b. a purpose such that the provision would not serve to increase protection to the health or safety of any persons habitually employed in the building.
4.4 Floors next to the ground and floors exposed from below should be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial condensation.
4.5 All floors should not promote surface condensation or mould growth, given reasonable occupancy conditions.
*Ground supported floors (moisture from the ground) *
4.6 Any ground supported floor will meet the requirement if the ground is covered with dense concrete laid on a hardcore bed and a damp- proof membrane is provided. Suitable insulation may be incorporated.
Technical solution
4.7 Unless it is subjected to water pressure, which is likely in the case of buildings on very permeable strata such as chalk, limestone or gravel (in which case see Alternative approach, paragraph 4.12), a concrete ground supported floor may be built as follows (Diagram 4):
a. well compacted hardcore bed, no greater than 600mm deep, of clean, broken brick or similar inert material, free from materials including water-soluble sulphates in quantities which could damage the concrete (BRE Digest 276; and
b. concrete at least 100mm thick (but thicker if the structural design requires) to mix ST2 in BS 8500 or, if there is embedded reinforcement, to mix ST4 in BS 8500; and
c. damp-proof membrane above or below the concrete, and continuous with the dampproof courses in walls, piers and the like. If the ground could contain water soluble sulphates, or there is any risk that sulphate or other deleterious matter could contaminate the hardcore, the membrane should be placed at the base of the concrete slab.
Diagram 4 Ground supported floor – construction (see paragraph 4.7)
4.8 A membrane below the concrete could be formed with a sheet of polyethylene, which should be at least 300um thick (1200 gauge) with sealed joints and laid on a bed of material that will not damage the sheet.
4.9 A membrane laid above the concrete may be either polyethylene sheet as described above (but without the bedding material) or three coats of cold applied bitumen solution or similar moisture and water vapour resisting material. In each case it should be protected by either a screed or a floor finish, unless the membrane is pitchmastic or similar material which will also serve as a floor finish.
4.10 Insulants placed beneath floor slabs should have sufficient strength to resist the weight of the slab and the anticipated floor loading as well as any possible overloading during construction. In order to resist degradation insulation that is placed below the damp proof membrane should have low water absorption. If necessary the insulant should be resistant to contaminants in the ground.
4.11 A timber floor finish laid directly on concrete may be bedded in a material which may also serve as a damp-proof membrane. Timber fillets laid in the concrete as a fixing for a floor finish should be treated with an effective preservative unless they are above the damp- proof membrane. Some preservative treatments are described in BS 1282:199986.
Alternative approach
4.12 The requirement can also be achieved by following the relevant recommendations of Clause 11 of BS CP 102:197387. BS 8102:199088 includes recommendations for floors subject to water pressure.
Suspended timber ground floors (moisture from the ground)
4.13 Any suspended timber floor next to the ground will meet the requirement if:
a. the ground is covered so as to resist moisture and prevent plant growth; and
b. there is a ventilated air space between the ground covering and the timber; and
c. there are damp-proof courses between the timber and any material which can carry moisture from the ground.
Technical solution
4.14 Unless it is covered with a floor finish which is highly vapour resistant (in which case see the Alternative approach in paragraph 4.16), a suspended timber floor next to the ground may be built as follows (Diagram 5):
a. Ground covering either:
i. unreinforced concrete at least 100mm thick to mix ST 1 in BS 850089. The concrete should be laid on a compacted hardcore bed of clean, broken brick or any other inert material free from materials including water-soluble sulphates in quantities which could damage the concrete; or
ii. concrete, composed as described above, or inert fine aggregate, in either case at least 50mm thick laid on at least 300µm (1200 gauge) polyethylene sheet with sealed joints, and itself laid on a bed of material which will not damage the sheet.
To prevent water collecting on the ground covering, either the top should be entirely above the highest level of the adjoining ground or, on sloping sites, consideration should be given to installing drainage on the outside of the up-slope side of the building (see Diagram 6).
b. Ventilated air space measuring at least 75mm from the ground covering to the underside of any wall-plates and at least 150mm to the underside of the suspended timber floor (or insulation if provided). Two opposing external walls should have ventilation openings placed so that the ventilating air will have a free path between opposite sides and to all parts. The openings should be not less than either 1,500mm2/m run of external wall or 500mm2/m2 of floor area, whichever gives the greater opening area. Any pipes needed to carry ventilating air should have a diameter of at least 100mm. Ventilation openings should incorporate suitable grilles which prevent the entry of vermin to the sub-floor but do not resist the air flow unduly. If floor levels need to be nearer to the ground to provide level access sub-floor ventilation can be provided through offset (periscope) ventilators.
Diagram 5 Suspended timber floor-construction
Diagram 6 Suspended floor – preventing water collection
c. Damp-proof courses of impervious sheet material, engineering brick or slates in cement mortar or other material which will prevent the passage of moisture. Guidance for choice of materials is given in BS 5628:Part 3:2001.
d. In shrinkable clay soils, the depth of the air space may need to be increased to allow for heave.
4.15 In areas such as kitchens, utility rooms and bathrooms where water may be spilled, any board used as a flooring, irrespective of the storey, should be moisture resistant. In the case of chipboard it should be of one of the grades with improved moisture resistance specified in BS 7331:1990 or BS EN 312 Part 5:1997. It should be laid, fixed and jointed in the manner recommended by the manufacturer. To demonstrate compliance the identification marks should be facing upwards. Any softwood boarding should be at least 20mm thick and from a durable species or treated with a suitable preservative.
Alternative approach
4.16 The requirement can also be met (see paragraph 4.14 above) by following the relevant recommendations of Clause 11 of BS CP 102:1973.
Suspended concrete ground floors (moisture from the ground)
4.17 Any suspended floor of in situ or precast concrete, including beam and block floors, next to the ground will meet the requirement if it will adequately prevent the passage of moisture to the upper surface and if the reinforcement is protected against moisture.
Technical solution
4.18 One solution for a suspended concrete floor could be:
a. in situ concrete at least 100mm thick (but thicker if the structural design requires) containing at least 300kg of cement for each m3 of concrete; or
b. precast concrete construction with or without infilling slabs; and
c. reinforcing steel protected by concrete cover of at least 40mm if the concrete is in situ and at least the thickness required for a moderate exposure if the concrete is precast.
4.19 A suspended concrete floor will meet the requirements if it incorporates:
a. a damp-proof membrane (if the ground below the floor has been excavated below the lowest level of the surrounding ground and will not be effectively drained); and
b. a ventilated air space. This should measure at least 150mm clear from the ground to the underside of the floor (or insulation if provided). Two opposing external walls should have ventilation openings placed so that the ventilating air will have a free path between opposite sides and to all parts of the floor void. The openings should be not less than either 1500mm2/m run of external wall or 500mm2/m2 of floor area, whichever gives the greater opening area. Any pipes needed to carry ventilating air should have a diameter of at least 100mm. Ventilation openings should incorporate suitable grilles which prevent the entry of vermin to the sub-floor but do not resist the air flow unduly.
4.20 In localities where flooding is likely, consideration may be given to including means of inspecting and clearing out the sub-floor voids beneath suspended floors. For guidance, see the ODPM publication on preparing for floods.
Diagram 7 Typical floors exposed from below
GROUND FLOORS AND FLOORS EXPOSED FROM BELOW (RESISTANCE TO DAMAGE FROM INTERSTITIAL CONDENSATION)
4.21 A ground floor or floor exposed from below, i.e. above an open parking space or passageway, as shown in Diagram 7, will meet the requirement if it is designed and constructed in accordance with Clause 8.5 and Appendix D of BS 5250:2002, BS EN ISO 13788:2002 and BR 262.
FLOORS (RESISTANCE TO SURFACE CONDENSATION AND MOULD GROWTH)
4.22 A floor will meet the requirement if:
a. a ground floor is designed and constructed so that the thermal transmittance (U-value) does not exceed 0.7W/m2K at any point; and
b. in the case of all floors, the junctions between elements are designed to Accredited Construction Details, or follow the guidance of BRE IP17/01.
