Rafter Span Tables
Calculating Maximum Rafter Spans
Rafter span tables - Use these tables to determine lengths, sizes and spacing of rafters based on a variety of factors such as, species, load, grade, and spacing. Visit the roof framing page for more information on cutting roof rafters, and visit the roof pitch calculator for determining rafter lengths based on rise and run.
The span tables on this page cover the following two design scenarios:
- Rafters with ceiling not attached to rafters, live load = 20 psf, dead load = 20 psf, deflection limit L/180.
- Rafters with ceiling not attached to rafters, ground snow load = 50 psf, dead load = 20 psf, deflection limit L/180.
The first span table is for roofs where the ceiling is not attached to the rafters (with no snow load), a live load of 20 psf, a dead load of 20 psf and a deflection limit of L/180. The second span table is the same as the first, except that it assumes a ground snow load of 50 psf.
When calculating the maximum span of a rafter, use the horizontal distance between two vertical supports. Note also that you can break up the span of a rafter by adding a purlin and bracing to the underside. The braces need to be supported by a bearing wall, as shown in the diagram above.
Example: In the rafter span table below, the highlighted cell (13-0), indicates that 2" x 8" Douglas Fir rafter, with a grade of #2, spaced 24" apart, can have a maximum span of 13 feet - 0 inches (13-0) if designing for a live load of 20 psf, and dead load of 20 psf.
Rafter Span Tables
| Nominal Size | Spaced (o.c.) | Species/Grade | |||
| S. Pine | Doug. Fir | Hem-fir | S.P.F. | ||
| #2 grade | #2 grade | #2 grade | #2 grade | ||
| 2" x 4" | 12 | 9-0 | 10-0 | 9-8 | 9-10 |
| 16 | 7-9 | 8-7 | 8-5 | 8-6 | |
| 24 | 6-4 | 7-0 | 6-10 | 6-11 | |
| 2" x 6" | 12 | 13-6 | 14-7 | 14-2 | 14-4 |
| 16 | 11-8 | 12-7 | 12-3 | 12-5 | |
| 24 | 9-6 | 10-4 | 10-0 | 10-2 | |
| 2" x 8" | 12 | 17-1 | 18-5 | 17-11 | 18-2 |
| 16 | 14-9 | 16-0 | 15-6 | 15-9 | |
| 24 | 12-1 | 13-0 | 12-8 | 12-10 | |
| 2" x 10" | 12 | 20-3 | 22-6 | 21-11 | 22-3 |
| 16 | 17-6 | 19-6 | 18-11 | 19-3 | |
| 24 | 14-4 | 15-11 | 15-6 | 15-8 | |
| 2" x 12" | 12 | 23-10 | 26-0 | 25-5 | 25-9 |
| 16 | 20-8 | 22-7 | 22-0 | 22-4 | |
| 24 | 16-10 | 18-6 | 17-11 | 18-3 | |
Note: Snow load can be very specific to the actual location of a structure. Consult your local building code authority to determine the snow load in your area. The following span table uses a moderate snow load of 50 psf, but yours could be more or less.
| Nominal Size | Spaced (o.c.) | Species/Grade | |||
| S. Pine | Doug. Fir | Hem-fir | S.P.F. | ||
| #2 grade | #2 grade | #2 grade | #2 grade | ||
| 2" x 4" | 12 | 9-0 | 10-0 | 9-8 | 9-10 |
| 16 | 7-9 | 8-7 | 8-5 | 8-6 | |
| 24 | 6-4 | 7-0 | 6-10 | 6-11 | |
| 2" x 6" | 12 | 13-6 | 14-7 | 14-2 | 14-4 |
| 16 | 11-8 | 12-7 | 12-3 | 12-5 | |
| 24 | 9-6 | 10-4 | 10-0 | 10-2 | |
| 2" x 8" | 12 | 17-1 | 18-5 | 17-11 | 18-2 |
| 16 | 14-9 | 16-0 | 15-6 | 15-9 | |
| 24 | 12-1 | 13-0 | 12-8 | 12-10 | |
| 2" x 10" | 12 | 20-3 | 22-6 | 21-11 | 22-3 |
| 16 | 17-6 | 19-6 | 18-11 | 19-3 | |
| 24 | 14-4 | 15-11 | 15-6 | 15-8 | |
| 2" x 12" | 12 | 23-10 | 26-0 | 25-5 | 25-9 |
| 16 | 20-8 | 22-7 | 22-0 | 22-4 | |
| 24 | 16-10 | 18-6 | 17-11 | 18-3 | |
The above spans are based on American Softwood Lumber standard sizes.
To calculate maximum rafter spans using different design criteria (load, snow load, spacing, grade, etc.), for these common lumber species, see the International Residential Code (IRC).
To calculate maximum spans of species not shown above, use the Span Calculator or the Span Tables for Joists and Rafters on the American Wood Council website.
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