Flat Roof Joist Spacing

Permissible clear span metres.

Flat roof joist spacing.

Joist spacing of 24 inches is allowed for spans between 16 to 20 feet using 2 inch by 10 inch lumber of these three grades. Centre to centre spacing of joists in mm joist size width depth mm 400 450 600 400 450 600 400 450 600. Spacing in exterior exposure. The sizing and spacing will depend upon the loads imposed on them and the required span.

Douglas fir maximum horizontal roof rafter span for lumber grade selected structural and no. Not more than 0 5 more than 0 5 but not more than 0 75 more than 0 75 but not more than 1. 2 are indicated below. 1 psf lb f ft 2 47 88 n m 2.

Usually 400 450 or 600mm but any value can be used up to 610mm max 1 2 timber strength class and joist size automatically calculate sizes suitable for supporting the loads below. Keep in mind that these are minimums for the spacing indicated you can select larger sizes to handle excessive loads or for appearance. If the roof is not completely square you can decrease the space between joists but never increase the space. Wherever possible joists should span the shortest distance.

The joists will normally be placed at 400mm centres but no more than 600mm centres. This is the distance between joist centres. Common joists sizes are 200 x 50mm 175 x 50mm and 150 x 50mm. 1 ft 0 3048 m.

Live load is weight of furniture wind snow and more. Dead load weight of structure and fixed loads 15 lbs ft 2. Of course beefier sizes also increase your lumber bill. In other words you would space a roof joist 15 inches apart from another roof joist but never 17 inches apart.

Common spacing for trusses on this type of building is often between 2 and 4 feet but trusses built with advanced designs may allow spacing as great as 8 or 12 feet. A roof joist is always spaced no more than 16 inches apart from another roof joist. Variations in ceiling construction nonparallel ceiling joist designs require additional support construction in the ceiling. For example in the rafter span table below the highlighted cell in the 40 live load table indicates that 2 x 8 southern yellow pine joists that have a grade of 2 that are spaced 24 apart can have a maximum span of 10 feet 3 inches 10 3 if designing for a live load of 40 lbs ft 2.

Live load psf dead load psf span calculator for wood joists and rafters available for the iphone. So the space between rafters can be greater without reducing the maximum span.