Flat Roof Timber Joist Sizes

All the figures are based on the normal loading conditions.

Flat roof timber joist sizes.

For pitched roofs flat roofs and ceiling joists. Subject ceilings and flat roofs timber sizes and construction details 09 issued 01 04 13 rev page 1 of 2 the following tables give details of the allowable spans and spacing for some of the more common timber sizes used in ceiling and flat roof construction. 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. A cavity closer should be provided to the top of the walls to prevent fire spread and to stop damp air entering the roof space.

1 2 timber strength class and joist size automatically calculate sizes suitable for supporting the loads below c16 cost effective for most applications c24 stronger more demanding c14 c18 c22 c27 c30 c35 c40. 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. Timber to be covered these span tables do not apply to timber which is fully exposed to the elements. These span tables apply only to flat roof and roof with a slope of up to 10 degrees.

Where a new flat roof abuts an existing external wall the joists can be hung on metal joist hangers which are fixed to a horizontal timber plate wall bolted to the wall.