Rafter Span Calculator
Find the maximum horizontal span of a rafter for your species, size, spacing, roof load and ceiling, and check a planned span.
Douglas fir-larch No. 2 rafters at 16 in on center, 30 psf ground snow, 10 psf dead load and an attached ceiling (L/240) span about: 2×6 12′1″, 2×8 15′4″, 2×10 18′9″ and 2×12 21′8″, measured horizontally. Heavier snow or roofing shortens the span.
Rafter span chart: Douglas fir-larch No. 2, 10 psf dead load, ceiling attached (L/240)
| Rafter | 20 psf roof live | 30 psf snow | 50 psf snow | 70 psf snow |
|---|---|---|---|---|
| 2×4 at 12 in | 9′10″ | 8′7″ | 7′3″ | 6′6″ |
| 2×4 at 16 in | 8′11″ | 7′10″ | 6′7″ | 5′10″ |
| 2×4 at 24 in | 7′10″ | 6′9″ | 5′6″ | 4′9″ |
| 2×6 at 12 in | 15′6″ | 13′6″ | 11′5″ | 9′10″ |
| 2×6 at 16 in | 14′1″ | 12′1″ | 9′10″ | 8′7″ |
| 2×6 at 24 in | 11′11″ | 9′10″ | 8′1″ | 7′0″ |
| 2×8 at 12 in | 20′5″ | 17′8″ | 14′5″ | 12′6″ |
| 2×8 at 16 in | 18′5″ | 15′4″ | 12′6″ | 10′10″ |
| 2×8 at 24 in | 15′1″ | 12′6″ | 10′3″ | 8′10″ |
| 2×10 at 12 in | 26′0″ | 21′7″ | 17′8″ | 15′3″ |
| 2×10 at 16 in | 22′6″ | 18′9″ | 15′3″ | 13′3″ |
| 2×10 at 24 in | 18′5″ | 15′3″ | 12′6″ | 10′10″ |
| 2×12 at 12 in | 30′2″ | 25′1″ | 20′5″ | 17′9″ |
| 2×12 at 16 in | 26′1″ | 21′8″ | 17′9″ | 15′4″ |
| 2×12 at 24 in | 21′4″ | 17′9″ | 14′6″ | 12′6″ |
Spans are horizontal projections. To get the rafter's length along the slope, multiply by the slope factor from the roof pitch calculator.
Frequently asked questions
How far can a 2×6 rafter span?
About 12′1″ horizontally for Douglas fir-larch No. 2 at 16 in on center with 30 psf snow, a 10 psf roof and an attached ceiling.
How far can a 2×8 rafter span?
About 15′4″ horizontally under the same loads.
Is rafter span measured horizontally?
Yes. Code span tables use the horizontal distance between supports, not the length along the slope.
What snow load should I use?
Use your local ground snow load from the building department; it ranges from 0 in the South to 70 psf or more in mountain areas.
What does "ceiling attached" change?
A ceiling (drywall) attached to the rafters must not crack, so the rafter must be stiffer: L/240 instead of L/180.
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