Timber Roof Purlin Span Tables

1 refer to general notes for information that is relevant for all span tables.

Timber roof purlin span tables.

All the figures are based on roofing tiles or slates laid on timber laths over sarking felt. Surveyors span tables for designing roof rafters. Spacing should be interpreted as the centre to centre distance between adjacent parallel members. Wind classification n1 n2.

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. 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. Pergola rafter spans for 600 mm and 900 mm spacing. The following tables give details of the allowable spans and spacing for some of the more common timber sizes used in traditional roofing construction.

3 where rafters are equal to or exceed four times their breath blocking should be used to reduce distortion. For pitched roofs flat roofs and ceiling joists. 2 for design parameters refer to figure 7 26. Maximum clear span of purlins in metres for roof pitches of between 30 and 45 degrees.

Required size for a given span.