Flat Roof Timber Sizes

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.

Flat roof timber sizes.

Here the number is 941 86 inches. Flat roof joists. Weather you re building a shed roof or adding a dormer to your second story it is important to choose the proper size rafter. These span tables apply only to flat roof and roof with a slope of up to 10 degrees.

Automatically calculate suitable rafter sizes enter a specific rafter size. There are several variables to consider when determining the proper rafter size. Supporting imposed load. Flat roofs timber sizes and construction.

This sizing guide for roof rafters will help determine what you need for your small project. For pitched roofs flat roofs and ceiling joists. Roof rise this should be calculated from the base of the roof to the highest point of the roof gable the peak. 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.

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. 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 04 2 75 3 47 4 20 4 93 5 66 floor joists flat roof joists assumptions at 400mm centers supporting permanent load excluding self weight of joists up to 50kg m2. Rafter run outer wall this should be measured starting from the center of the roof span and then extending to the very edge of the exterior wall. Timber to be covered these span tables do not apply to timber which is fully exposed to the elements.

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. Calculations to bs 5268 2. Here the number is 1166 66 inches. 2002 bs 5268 7 5.

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.