Flat Roof Joist Sizes

So any timber joists that are used need to be strong enough to support this weight.

Flat roof joist sizes.

Permissible clear span metres. For pitched roofs flat roofs and ceiling joists. The range of joist sizes used in homes varies though between 2 by 4 inches and 2 by 12 inches. Not more than 0 5 more than 0 5 but not more than 0 75 more than 0 75 but not more than 1.

As you may imagine there is a fair amount of weight involved here in terms of boards felt etc. 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. Centre to centre spacing of joists in mm joist size width depth mm 400 450 600 400 450 600 400 450 600. 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. Usually 0 75 is ok for a flat roof though if you live in an area of higher snowfall go for 1 0. Old asbestos roofs are often replaced with flat roofs. This is the distance between joist centres.

Surveyors span tables for designing roof rafters. Dead load per square metre in kn m supported by joists. 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.