Flat Roof Joist Sizes

The difference in the joist sizes applies to the width of the boards which determines the height of the ceiling.

Flat roof joist sizes.

As you may imagine there is a fair amount of weight involved here in terms of boards felt etc. So any timber joists that are used need to be strong enough to support this weight. Usually 0 75 is ok for a flat roof though if you live in an area of higher snowfall go for 1 0. Not more than 0 5 more than 0 5 but not more than 0 75 more than 0 75 but not more than 1.

Dead load per square metre in kn m supported by joists. Permissible clear span metres. Centre to centre spacing of joists in mm joist size width depth mm 400 450 600 400 450 600 400 450 600. This is the distance between joist centres.

Sizes of joists needed for flat roof construction. For pitched roofs flat roofs and ceiling joists. Old asbestos roofs are often replaced with flat roofs. 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.

The range of joist sizes used in homes varies though between 2 by 4 inches and 2 by 12 inches. 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. 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 47 x 220 5 04 4 85 4 41 75 x 170 4 50.