Timber Roof Purlin Span Tables

2 for design parameters refer to figure 7 26.

Timber roof purlin span tables.

Required size for a given span. 3 where rafters are equal to or exceed four times their breath blocking should be used to reduce distortion. 1 refer to general notes for information that is relevant for all span tables. Pergola rafter spans for 600 mm and 900 mm spacing.

Maximum clear span of purlins in metres for roof pitches of between 30 and 45 degrees. As per as1684 and as4055 spanman adopts a nett pressure coefficient applied along the full length of the purlin of cpn 2 0 for non cyclonic zones n1 to n6 and cpn 2 5 for cyclonic zones c1 to c4. For pitched roofs flat roofs and ceiling joists. Span tables for flat roof joists to bs 5268 7 2 imposed loads of 0 75 kn m for maintenance and snow load are applicable where there is no permanent access no fixed ladder or staircase and most areas where the altitude does not exceed 100 metres refer to bs 6399 3.

Surveyors and structural engineers utilise data from tables below to help calculate the correct size strength and centres of roof timbers for the required spans and loadings. All the figures are based on roofing tiles or slates laid on timber laths over sarking felt. Purlins near roof edges or changes of roof slope are subject to higher wind forces than interior purlins. The following tables give details of the allowable spans and spacing for some of the more common timber sizes used in traditional roofing construction.

Roof load width rlw and floor load width flw can be determined from as1684 or from the images below.