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

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. These span tables apply only to flat roof and roof with a slope of up to 10 degrees. Supporting imposed load. 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.

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

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. Flat roofs timber sizes and construction. Automatically calculate suitable rafter sizes enter a specific rafter size. 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.

For pitched roofs flat roofs and ceiling joists. Calculations to bs 5268 2. 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.

Structural calculations for timber joists timber beams glulam beams flitch beams using the latest design codes.