11 Best Of What Size Roof Joists Do I Need Best Resolution Image. The range of joist sizes used in homes varies though between 2-by-4 inches and 2-by-12 inches. This is based upon the span of the rafter horizontal span not total rafter length live and dead loads.
You need to measure the complete span of your floor joists together with the distance between them the centres and the size of the timbers. Depth to width ratio should not be greater than 6. To size joists and rafters use the Maximum Span Tables for southern yellow pine.
Depth mm 400 450 600 400 450.
Depth to width ratio should not be greater than 6. FLOOR JOISTS FLAT ROOF JOISTS Assumptions at 400mm Centers - Supporting permanent load excluding self-weight of joists up to 50kgm2. 13 rows MAXIMUM CLEAR SPAN OF FLAT ROOF JOISTS IN METRES FOR SC3 TIMBER Spacing between joists Size. A 2x8 up to 12 feet.