Flat Roof Joist Sizing

Weather you re building a shed roof or adding a dormer to your second story it is important to choose the proper size rafter.

Flat roof joist sizing.

Centre to centre spacing of joists in mm joist size width depth mm 400 450 600 400 450 600 400 450 600. Not more than 0 5 more than 0 5 but not more than 0 75 more than 0 75 but not more than 1. L 720 l 600 l 480 l 360 l 240 l 180. Slope completely flat roofs typically present weather proofing problems so the rafters or joists of some flat roofs are laid at a slight angle for drainage.

Floor joists ceiling joists rafters snow load rafters roof live load deflection limit. Common joists sizes are 200 x 50mm 175 x 50mm and 150 x 50mm. Spacing in 12 16 19 2 24. 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.

This sizing guide for roof rafters will help determine what you need for your small project. Ceilings and roofs excluding trussed rafter roofs for dwellings or ask your lbc surveyor for advice. The difference in the joist sizes applies to the width of the boards which determines the height of the ceiling. 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.

Wherever possible joists should span the shortest distance. The range of joist sizes used in homes varies though between 2 by 4 inches and 2 by 12 inches. Rafter and joist span faq. Permissible clear span metres.

The sizing and spacing will depend upon the loads imposed on them and the required span. Maximum clear span of flat roof joists in metres for sc3 timber spacing between joists size. 2x4 2x6 2x8 2x10 2x12. 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.

The joists will normally be placed at 400mm centres but no more than 600mm centres. The lengths and sizes of joists vary depending on the species and quality of the lumber used. The size of rafters for a flat roof depends on the horizontal span of the building the type and grade of timber used and the load on the roof. There are several variables to consider when determining the proper rafter size.

3 stud construction standard utility.