Choice of material
The choice of roofing material will be determined by factors such as design and service life, complexity, roof size, form, pitch and degree of exposure. The selection and specification of roof finishes must also recognise all statutory requirements under, for example Building Regulations, where thermal and acoustic as well as weather performance is required. Such specification will need to include insulation products.
The selection of the type of specification will largely depend upon the roof product and the design complexity but may also be influenced by other elements such as contract type and procurement. With traditional and small format materials prescription specifications would generally be used.
However, for areas where the proposed roof finish has little impact on the visual appearance it may be preferable to allow the contractor or subcontractor to specify the materials under a descriptive specification.
Factors that will influence roof performance will include: design; pitch; structure; flexibility; scale; exposure; thermal performance; strength; durability and acoustic quality.
If specifying tiled roofs the underlay should not be regarded as the first line of defence. To ensure water doesn't enter between tiles during a heavy deluge of rain, designers should make sure that the manufacturers' minimum pitch value for the tile is greater than the pitch of the roof. To avoid water penetration by wind-driven rain the specifier should check with manufacturers to ensure that the tiles have an adequate headlap.
Specifiers should ask the manufacturer to carry out the fixing specification. They will calculate the fixings required depending on the wind exposure of the roof, which will be determined by factors such as storey height, location, altitude, and whether it's in an aircraft zone. Refer to BS 5534 Code of Practice for Slates and Tiling for more information.
Condensation in a pitched roof should be controlled by effective ventilation. Information on precise methods of avoiding condensation can be found in BS 5520.
Heath and safety
The specification of products needs to recognise the installation processes required with regard to health and safety. There are a number of specialist advisers and regulatory bodies that can help with material selection, such as BRE, British Board of Agrément, and individual bodies such as the Clay Roofing Council, the Concrete Tile Manufacturers' Association, and so on.
Depending upon the design approach, consideration may need to be given in respect of fall risks due to roof material and strength. Where risks are identified, then appropriate safety systems should be specified.
Roofing materials and products should be British Board of Agrément-certified. Products selected should have been tested for:
- fire resistance
- water tightness
- thermal characteristics
The specifier should ensure all installation clauses and conditions are clearly and precisely identified, including all ancillary components, flashings and interface abutments. Reference should be made to product manufacturer's installation recommendations as well as relevant British Standard Codes of Practice and trade association guidelines.
Samples and mock-ups
The specification should ensure that there are suitable clauses calling for both tender and contract samples of each product and relevant site benchmarking. Prototypes and mock-ups may also be appropriate where the complexity of design requires visual explanation or testing to ensure weather resistance at interfaces, penetrations, drainage locations and so on.
Final installation for complex designs should be site tested by appropriate testing methods to ensure weather tightness and completeness of the system. Fire resistance, loading, thermal and acoustic qualities testing should also be specified as appropriate.