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.

2002 bs 5268 7 5. For pitched roofs flat roofs and ceiling joists. Calculations to bs 5268 2. Flat roof joists.

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. Here the number is 1166 66 inches. These span tables apply only to flat roof and roof with a slope of up to 10 degrees. Flat roofs timber sizes and construction.

Flat roof with no permanent access and altitude not exceeding 100m 0 75 kn m udl 0 9 kn point load most areas exceeding 100m but not exceeding 200m excluding parts of scotland pennines north east england refer to bs 6399 3 1 kn m udl 0 9 kn point load. Here the number is 941 86 inches. Supporting imposed load. 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.

There are several variables to consider when determining the proper rafter size. 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. 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. This sizing guide for roof rafters will help determine what you need for your small project.

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. Roof rise this should be calculated from the base of the roof to the highest point of the roof gable the peak. Structural calculations for timber joists timber beams glulam beams flitch beams using the latest design codes. 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.

Surveyors span tables for designing roof rafters.