Flat Roof Joist Sizing

This is the distance between joist centres.

Flat roof joist sizing. Dead load per square metre in kn m supported by joists. The difference in the joist sizes applies to the width of the boards which determines the height of the ceiling. 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 lengths and sizes of joists vary depending on the species and quality of the lumber used.

3 stud construction standard utility. Maximum clear span of flat roof joists in metres for sc3 timber spacing between joists size. 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. L 720 l 600 l 480 l 360 l 240 l 180.

Spacing in 12 16 19 2 24. Centre to centre spacing of joists in mm joist size width depth mm 400 450 600 400 450 600 400 450 600. 2x4 2x6 2x8 2x10 2x12. Permissible clear span metres.

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. Floor joists ceiling joists rafters snow load rafters roof live load deflection limit. Weather you re building a shed roof or adding a dormer to your second story it is important to choose the proper size rafter. 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.

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. Ceilings and roofs excluding trussed rafter roofs for dwellings or ask your lbc surveyor for advice. 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. The joists will normally be placed at 400mm centres but no more than 600mm centres. There are several variables to consider when determining the proper rafter size. The sizing and spacing will depend upon the loads imposed on them and the required span.

For example you can span a 1 grade no knots southern yellow pine a greater distance than a 2 grade with knots southern yellow pine.