Section 6 Roofs

6.1 This section gives guidance for three situations:

a. roofs exposed to precipitation from the outside (see paragraphs 6.3 to 6.9);

b. the risk of interstitial condensation in roofs (see paragraphs 6.10 to 6.13);

c. the risk of condensation or mould growth on the internal surface of roofs (see paragraph 6.14).

6.2 Roofs should:

a. resist the penetration of precipitation to the inside of the building; and

b. not be damaged by precipitation and not carry precipitation to any part of the building which would be damaged by it;

c. be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial condensation.

ROOFS (RESISTANCE TO MOISTURE FROM THE OUTSIDE)

6.3 Roofing can be designed to protect a building from precipitation either by holding the precipitation at the face of the roof or by stopping it from penetrating beyond the back of the roofing system.

6.4 Any roof will meet the requirement if:

a. it is jointless or has sealed joints, and is impervious to moisture (so that moisture will not enter the roofing system); or

b. it has overlapping dry joints, is impervious or weather resisting, and is backed by a material which will direct precipitation which enters the roof towards the outer face (as with roofing felt).

6.5 Some materials can deteriorate rapidly without special care and they should only be used as the weather-resisting part of a roof if certain conditions are met (see Approved Document supporting Regulation 7, Materials and workmanship133). The weather-resisting part of a roofing system does not include paint nor does it include any coating, surfacing or rendering which will not itself provide all the weather resistance.

Technical solution

6.6 Roofing systems may be:

a. impervious including metal, plastic and bituminous products; or

b. weather resisting including natural stone or slate, cement based products, fired clay and wood; or

c. moisture resisting including bituminous and plastic products lapped at the joints, if used as a sheet material, and permeable to water vapour unless there is a ventilated space directly behind the material; or

d. jointless materials and sealed joints, which would allow for structural and thermal movement.

6.7 Dry joints between roofing sheets should be designed so that precipitation will not pass through them, or the system should be designed so that precipitation which enters the joints will be drained away without penetrating beyond the back of the roofing system.
Note: Whether dry joints are suitable will depend on the design of the joint or the design of the roofing system and the severity of the exposure to wind and rain.

6.8 Each sheet, tile and section of roof should be fixed in an appropriate manner. Guidance as to appropriate fixing methods is given in BS 8000-6:1990134.

Alternative approach

6.9 The requirement can also be met by following the relevant recommendations of:

a. BS CP 143135 for sheet roof and wall coverings made from the following materials:

Part 1:1958 Corrugated and troughed aluminium
Part 5:1964 Zinc
Part 10:1973 Galvanized corrugated steel Part 12:1970 (1988) Copper
Part 15:1973 (1986) Aluminium
Part 16:1974 Semi-rigid asbestos bitumen sheets.
Recommendations for lead are included in BS 6915:2001;

b. BS 8219:2001;

c. BS 8200:1985;

d. MCRMA Technical Paper 6;

e. MCRMA Technical Paper 9.

These documents describe the materials and contain design considerations including recommendations for fixing.

ROOFS (RESISTANCE TO DAMAGE FROM INTERSTITIAL CONDENSATION)

6.10 A roof will meet the requirement if it is designed and constructed in accordance with Clause 8.4 of BS 5250:2002 and BS EN ISO 13788:2002. Further guidance is given in the BRE Report BR 262.

6.11 The requirement will be met by the ventilation of cold deck roofs, i.e. those roofs where the moisture from the building can permeate the insulation. For the purposes of health and safety it may not always be necessary to provide ventilation to small roofs such as those over porches and bay windows. Although a part of a roof which has a pitch of 70° or more is to be insulated as though it were a wall, the provisions in this document apply to roofs of any pitch.

6.12 To avoid excessive moisture transfer to roof voids gaps and penetrations for pipes and electrical wiring should be filled and sealed; this is particularly important in areas of high humidity, e.g. bathrooms and kitchens. An effective draught seal should be provided to loft hatches to reduce inflow of warm air and moisture.

6.13 Because of the high internal temperatures and humidities, there is a particular risk of interstitial condensation in the roofs of swimming pools and other buildings in which high levels of moisture are generated; specialist advice should be sought when these are being designed.

ROOFS (RESISTANCE TO SURFACE CONDENSATION AND MOULD GROWTH)

6.14 A roof will meet the requirement if:

a. it is designed and constructed so that the thermal transmittance (U-value) does not exceed 0.35W/m2K at any point; and

b. the junctions between elements and the details of openings, such as windows, are designed to Accredited Construction Details, or follow the guidance of BRE IP17/01 or MCRMA Paper 14 for profiled metal roofing.

Section 6: Roofs