BS 5268-7.5:1990 recommends a calculation basis for the permissible clear span for rafters for roofs with a slope from15° to 45°. The recommendations apply to rafters at a maximum spacing of 610 mm centre-to-centre, this being the maximum spacing for which the “load-sharing” assumption may be adopted as described in BS 5268-2. The method of calculation makes no allowance for any contribution of other parts of the roof to the load resistance of the rafters although it is assumed that the tiling battens are capable of providing lateral load distribution and lateral support. It does not cover the design of rafters taking account of a structural contribution by sheet material supporting the roofing where such action can be provided by adequate design of its attachments as in a stressed skin panel roof. The uniform and concentrated loads of BS 6399-1, are considered. Provision is made for a uniformly distributed snow load derived from BS 6399-3.
BS 5268-7.5:1990 is applicable to the species and grades of timber given in BS 5268-2.