Flat Roof Timber Sizes And Spans

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.

Flat roof timber sizes and spans. Notches are not to exceed 0 125 of the depth of the joist and are to be located between 0 07 and 0 25 of the span from the support. 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. Size of joists mm clear span c16 joist m clear span c24 joist m 47 x 95 47 x 120 47 x 145 47 x 170 47 x 195 47 x 220 1 77 2 40 2 89 3 38 3 87 4 36 2 05 2 67 3 22 3 77 4 31 4 85 assumptions at 400mm centers supporting permanent load excluding self weight of joists up to 50kg m2. Surveyors span tables for designing roof rafters.

Supporting imposed load up to 150kg m2. For example you can span a 1 grade no knots southern yellow pine a greater distance than a 2 grade with knots southern yellow pine. The lengths and sizes of joists vary depending on the species and quality of the lumber used. Stress grades and timber sizes combine to determine the spanning ability of load carrying members.

For simple construction such as domestic construction this can be determined from span table supplements in as1684 2 and as1684 3. For pitched roofs flat roofs and ceiling joists. Usually 400 450 or 600mm but any value can be used up to 610mm max 1 2 timber strength class and joist size automatically calculate sizes suitable for supporting the loads below. Timber to be covered these span tables do not apply to timber which is fully exposed to the elements.

Span tables allow users to choose an appropriate size and stress grade to achieve spanning needs. This is the distance between joist centres.