Flat Roof Joist 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 joist sizes. Old asbestos roofs are often replaced with flat roofs. Permissible clear span metres. 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. Not more than 0 5 more than 0 5 but not more than 0 75 more than 0 75 but not more than 1.

The range of joist sizes used in homes varies though between 2 by 4 inches and 2 by 12 inches. So any timber joists that are used need to be strong enough to support this weight. 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 difference in the joist sizes applies to the width of the boards which determines the height of the ceiling.

Dead load per square metre in kn m supported by joists. A 2 by 4 inch joist for instance hangs into the room by 4 inches which may leave the ceiling high while a 2 by 12 inch joist in the same position hangs 12 inches into the room which decreases the ceiling height by 8 inches. Sizes of joists needed for flat roof construction. Surveyors span tables for designing roof rafters.

Usually 400 450 or 600mm but any value can be used up to 610mm max usually 400 450 or 600mm but any value can be used up to 610mm max 1 2 timber strength class and joist size. As you may imagine there is a fair amount of weight involved here in terms of boards felt etc. Usually 0 75 is ok for a flat roof though if you live in an area of higher snowfall go for 1 0.