Timber Roof Joist Sizes

Maximum clear span of flat roof joists in metres for sc3 timber spacing between joists size of roof joist 400mm 450mm 600mm 47 x 97 1 84 1 81 1 74 47 x 120 2 43 2 39 2 27 47 x 145 3 08 3 02 2 87 47 x 170 3 75 3 67 3 42 47 x 195 4 41 4 31 3 92 47 x 220 5 04 4 85 4 41 75 x 170 4 50 4 38 3 99.

Timber roof joist sizes.

Where the tables do not apply or where there are concentrated loads floor joists should be designed by an engineer in accordance with technical requirement r5. These tables can help calculate whether floor joists are strong enough to support themselves and the load they carry without the help of any walls underneath. For pitched roofs flat roofs and ceiling joists. Timber floor joists shall be adequate for the spans and loads and be correctly installed.

Bs 5268 7 1 was published before bs 6399 1. And the row for your timber size to find the permissible clear span in metres. Usually 400 450 or 600mm but any value can be used up to 610mm max usually 400 450 or 600mm but any value can be used up to 610mm max 1 2 timber strength class and joist size. All the figures are based on roofing tiles or slates laid on timber laths over sarking felt.

Solid timber joist sizes are provided in the bs 8103 3 span tables. 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. 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. Rafter and joist span faq.

These span tables apply only to flat roof and roof with a slope of up to 10 degrees. Surveyors and structural engineers use data from tables below to help calculate the size of timbers necessary to give adequate support to timber floors. Timber to be covered these span tables do not apply to timber which is fully exposed to the elements. Surveyors span tables for designing roof rafters.

Pitched roof timber sizes. Greater than 2400 mm and 3 6 kn load per metre width of floor for spans less than 2400 mm to ensure that very small joist sizes do not result from the calculations for smaller 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. 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.

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