Rafter span chart
Pick the wood, grade, size, spacing and roof load. You get the maximum horizontal rafter span from the American Wood Council's 2024 tables, the length along the slope at your pitch, and a chart for every size.
Inputs
Result
| Size | 12 in | 16 in | 19.2 in | 24 in |
|---|
Every grade
Maximum horizontal span for each grade of the species, size and spacing above.
| Grade | E, million psi | Fb used, psi | Span | Limited by |
|---|
Spans are worked from the American Wood Council's Span Tables for Joists and Rafters (2024) and Design Values for Joists and Rafters (2024), using the equations in the tables' own commentary. We checked those equations against every span in the printed tables and they match. Spans assume three or more members at no more than 24 in on centre, sheathed and nailed on top, single span, and stop at 26 ft as the tables do. Your local code and the authority having jurisdiction govern.
How the span is worked out
Rafter spans are measured level, from the wall plate to the ridge, with the roof load on that level projection. The rafter must be strong enough (bending, Fb) and stiff enough (deflection, E); the shorter of the two spans governs.
Rafters get a load-duration increase: the AWC design values list Fb for 2-month snow loading and for 7-day loading. The calculator uses the 7-day value for a 20 psf roof live load with no snow, and the snow value for snow loads, as the AWC commentary describes.
A 2x8 Douglas Fir-Larch No. 2 rafter at 16 in on centre under 20 psf roof live load and 10 psf dead load with a ceiling attached spans 18 ft 5 in horizontally. At a 6/12 pitch the rafter is 1.118 times that along the slope, before the overhang.
Ceiling or no ceiling
Where drywall is fastened to the underside of the rafters, as in a cathedral ceiling, deflection is held to span/240 to keep it from cracking. Where there is no ceiling attached, span/180 is allowed. A cathedral ceiling also needs a structural ridge beam rather than a ridge board.
For the length of each rafter, overhang and cuts, use the roof pitch calculator. Snow loads are set locally from the ground snow load; ask the building department for the roof snow load to use.
Questions
How far can a 2x6 rafter span?
With a 20 psf roof live load, 10 psf dead load and a ceiling attached, Douglas Fir-Larch No. 2 at 16 in on centre spans 14 ft 1 in horizontally. Under 30 psf snow it drops to 12 ft 1 in.
How far can a 2x8 rafter span?
18 ft 5 in horizontally for Douglas Fir-Larch No. 2 at 16 in on centre under a 20 psf roof live load with a ceiling attached. Under 40 psf snow, 13 ft 8 in.
How far can a 2x10 rafter span?
22 ft 6 in horizontally for Douglas Fir-Larch No. 2 at 16 in on centre under a 20 psf roof live load and 10 psf dead load, ceiling attached.
Is rafter span measured horizontally or along the slope?
Horizontally. Span tables give the level distance from the wall to the ridge; multiply by the pitch factor for the length along the slope.
What is L/240 and L/180 for rafters?
The allowed sag under live load: span divided by 240 where a ceiling is attached to the rafters, span divided by 180 where it is not.
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