Flat Roof Timber Sizes

Weather you re building a shed roof or adding a dormer to your second story it is important to choose the proper size rafter.

Flat roof timber sizes.

Timber to be covered these span tables do not apply to timber which is fully exposed to the elements. Roof rise this should be calculated from the base of the roof to the highest point of the roof gable the peak. Flat roofs timber sizes and construction. Supporting imposed load.

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. 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. Here the number is 1166 66 inches. This sizing guide for roof rafters will help determine what you need for your small project.

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. 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. Structural calculations for timber joists timber beams glulam beams flitch beams using the latest design codes. 2002 bs 5268 7 5.

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. Automatically calculate suitable rafter sizes enter a specific rafter size. Here the number is 941 86 inches. For pitched roofs flat roofs and ceiling joists.

Calculations to bs 5268 2. 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. 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. Surveyors span tables for designing roof rafters.

There are several variables to consider when determining the proper rafter size.