Flat Roof Joist Sizing

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.

Flat roof joist sizing. L 720 l 600 l 480 l 360 l 240 l 180. The sizing and spacing will depend upon the loads imposed on them and the required span. 3 stud construction standard utility. 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. Rafter and joist span faq. There are several variables to consider when determining the proper rafter size. The difference in the joist sizes applies to the width of the boards which determines the height of the ceiling.

Centre to centre spacing of joists in mm joist size width depth mm 400 450 600 400 450 600 400 450 600. The lengths and sizes of joists vary depending on the species and quality of the lumber used. 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. Floor joists ceiling joists rafters snow load rafters roof live load deflection limit.

Spacing in 12 16 19 2 24. Permissible clear span metres. 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. Dead load per square metre in kn m supported by joists.

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. 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. Wherever possible joists should span the shortest distance. 2x4 2x6 2x8 2x10 2x12.

Common joists sizes are 200 x 50mm 175 x 50mm and 150 x 50mm. This sizing guide for roof rafters will help determine what you need for your small project. This is the distance between joist centres. Weather you re building a shed roof or adding a dormer to your second story it is important to choose the proper size rafter.

Ceilings and roofs excluding trussed rafter roofs for dwellings or ask your lbc surveyor for advice.