Rafter span calculator: how far can a rafter span?
18 ft 5 in
maximum span measured level, 2 × 8 at 16 in on center, IRC Table R802.4.1(1) · for Douglas fir-larch #2, no snow, ceiling not attached, 10 psf dead load
Run your own numbers
Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
18 ft 5 in
maximum span measured level, 2 × 8 at 16 in on center, IRC Table R802.4.1(1)
Source: International Code Council, 2021 International Residential Code, Tables R802.4.1(1) to R802.4.1(9), read October 17, 2026. Read the source
A 2 × 8 Douglas fir-larch No. 2 rafter 16 inches on center spans 18 ft 5 in level with a 20 psf roof live load and no ceiling attached (IRC 2021 Table R802.4.1(1)); 15 ft 4 in with 30 psf of ground snow.
A worked example
A 2 × 10 Hem-fir No. 2 rafter at 16 in on center under 30 psf of ground snow, ceiling joists at the bottom but collar-height rafter ties 1.5 ft above the plates, with the ridge 7 ft up needs 13 ft 9 in. Table R802.4.1(3) gives 18 ft 2 in. The ties sit 1.5 ÷ 7 = 0.21 of the way up, past the 1/5 row of Table R802.4.1(9), so the 1/4 row applies: 18 ft 2 in × 0.76 is 13 ft 9.7 in, which rounds down to 13 ft 9 in.
How far can a 2x6 rafter span?
At 16 inches on center with no snow, 10 psf dead load and the ceiling not attached, from 10 ft 2 in for Southern pine No. 3 to 16 ft 4 in for Douglas fir-larch select structural, and 14 ft 7 in for Douglas fir-larch No. 2, by IRC 2021 Table R802.4.1(1). With 30 psf of ground snow the same No. 2 rafter drops to 12 ft 1 in.
How is rafter span measured?
Level, along the horizontal projection of the rafter, as IRC R802.4.1 says, not along the slope. On a 24 ft wide gable with the ridge in the middle, each rafter spans about 12 ft whatever the pitch; the sloped board is longer than its span.
What else it is called
A rafter span calculator is also searched as a rafter span chart, roof rafter span, 2x6 rafter span or 2x8 rafter span, and for sheds and lean-to roofs. The span is the level distance from the wall to the ridge, not the length of the board: for the cut length use the rafter length calculator.
The IRC rafter span tables, no snow
| Ceiling · spacing · species and grade | 2 × 4, 10 psf dead | 2 × 6, 10 psf dead | 2 × 8, 10 psf dead | 2 × 10, 10 psf dead | 2 × 12, 10 psf dead | 2 × 4, 20 psf dead | 2 × 6, 20 psf dead | 2 × 8, 20 psf dead | 2 × 10, 20 psf dead | 2 × 12, 20 psf dead |
|---|---|---|---|---|---|---|---|---|---|---|
| ceiling not attached · 12 in · Douglas fir-larch SS | 11-6 | 18-0 | 23-9 | Note b | Note b | 11-6 | 18-0 | 23-9 | Note b | Note b |
| ceiling not attached · 12 in · Douglas fir-larch #1 | 11-1 | 17-4 | 22-5 | Note b | Note b | 10-6 | 15-4 | 19-5 | 23-9 | Note b |
| ceiling not attached · 12 in · Douglas fir-larch #2 | 10-10 | 16-10 | 21-4 | 26-0 | Note b | 10-0 | 14-7 | 18-5 | 22-6 | 26-0 |
| ceiling not attached · 12 in · Douglas fir-larch #3 | 8-9 | 12-10 | 16-3 | 19-10 | 23-0 | 7-7 | 11-1 | 14-1 | 17-2 | 19-11 |
| ceiling not attached · 12 in · Hem-fir SS | 10-10 | 17-0 | 22-5 | Note b | Note b | 10-10 | 17-0 | 22-5 | Note b | Note b |
| ceiling not attached · 12 in · Hem-fir #1 | 10-7 | 16-8 | 22-0 | Note b | Note b | 10-4 | 15-2 | 19-2 | 23-5 | Note b |
| ceiling not attached · 12 in · Hem-fir #2 | 10-1 | 15-11 | 20-8 | 25-3 | Note b | 9-8 | 14-2 | 17-11 | 21-11 | 25-5 |
| ceiling not attached · 12 in · Hem-fir #3 | 8-7 | 12-6 | 15-10 | 19-5 | 22-6 | 7-5 | 10-10 | 13-9 | 16-9 | 19-6 |
| ceiling not attached · 12 in · Southern pine SS | 11-3 | 17-8 | 23-4 | Note b | Note b | 11-3 | 17-8 | 23-4 | Note b | Note b |
| ceiling not attached · 12 in · Southern pine #1 | 10-10 | 17-0 | 22-5 | Note b | Note b | 10-6 | 15-8 | 19-10 | 23-2 | Note b |
| ceiling not attached · 12 in · Southern pine #2 | 10-4 | 15-7 | 19-8 | 23-5 | Note b | 9-0 | 13-6 | 17-1 | 20-3 | 23-10 |
| ceiling not attached · 12 in · Southern pine #3 | 8-0 | 11-9 | 14-10 | 18-0 | 21-4 | 6-11 | 10-2 | 12-10 | 15-7 | 18-6 |
| ceiling not attached · 12 in · Spruce-pine-fir SS | 10-7 | 16-8 | 21-11 | Note b | Note b | 10-7 | 16-8 | 21-9 | Note b | Note b |
| ceiling not attached · 12 in · Spruce-pine-fir #1 | 10-4 | 16-3 | 21-0 | 25-8 | Note b | 9-10 | 14-4 | 18-2 | 22-3 | 25-9 |
| ceiling not attached · 12 in · Spruce-pine-fir #2 | 10-4 | 16-3 | 21-0 | 25-8 | Note b | 9-10 | 14-4 | 18-2 | 22-3 | 25-9 |
| ceiling not attached · 12 in · Spruce-pine-fir #3 | 8-7 | 12-6 | 15-10 | 19-5 | 22-6 | 7-5 | 10-10 | 13-9 | 16-9 | 19-6 |
| ceiling not attached · 16 in · Douglas fir-larch SS | 10-5 | 16-4 | 21-7 | Note b | Note b | 10-5 | 16-3 | 20-7 | 25-2 | Note b |
| ceiling not attached · 16 in · Douglas fir-larch #1 | 10-0 | 15-4 | 19-5 | 23-9 | Note b | 9-1 | 13-3 | 16-10 | 20-7 | 23-10 |
| ceiling not attached · 16 in · Douglas fir-larch #2 | 9-10 | 14-7 | 18-5 | 22-6 | 26-0 | 8-7 | 12-7 | 16-0 | 19-6 | 22-7 |
| ceiling not attached · 16 in · Douglas fir-larch #3 | 7-7 | 11-1 | 14-1 | 17-2 | 19-11 | 6-7 | 9-8 | 12-12 | 14-11 | 17-3 |
| ceiling not attached · 16 in · Hem-fir SS | 9-10 | 15-6 | 20-5 | Note b | Note b | 9-10 | 15-6 | 19-11 | 24-4 | Note b |
| ceiling not attached · 16 in · Hem-fir #1 | 9-8 | 15-2 | 19-2 | 23-5 | Note b | 9-0 | 13-1 | 16-7 | 20-4 | 23-7 |
| ceiling not attached · 16 in · Hem-fir #2 | 9-2 | 14-2 | 17-11 | 21-11 | 25-5 | 8-5 | 12-3 | 15-6 | 18-11 | 22-0 |
| ceiling not attached · 16 in · Hem-fir #3 | 7-5 | 10-10 | 13-9 | 16-9 | 19-6 | 6-5 | 9-5 | 11-11 | 14-6 | 16-10 |
| ceiling not attached · 16 in · Southern pine SS | 10-3 | 16-1 | 21-2 | Note b | Note b | 10-3 | 16-1 | 21-2 | 25-7 | Note b |
| ceiling not attached · 16 in · Southern pine #1 | 9-10 | 15-6 | 19-10 | 23-2 | Note b | 9-1 | 13-7 | 17-2 | 20-1 | 23-10 |
| ceiling not attached · 16 in · Southern pine #2 | 9-0 | 13-6 | 17-1 | 20-3 | 23-10 | 7-9 | 11-8 | 14-9 | 17-6 | 20-8 |
| ceiling not attached · 16 in · Southern pine #3 | 6-11 | 10-2 | 12-10 | 15-7 | 18-6 | 6-0 | 8-10 | 11-2 | 13-6 | 16-0 |
| ceiling not attached · 16 in · Spruce-pine-fir SS | 9-8 | 15-2 | 19-11 | 25-5 | Note b | 9-8 | 14-10 | 18-10 | 23-0 | Note b |
| ceiling not attached · 16 in · Spruce-pine-fir #1 | 9-5 | 14-4 | 18-2 | 22-3 | 25-9 | 8-6 | 12-5 | 15-9 | 19-3 | 22-4 |
| ceiling not attached · 16 in · Spruce-pine-fir #2 | 9-5 | 14-4 | 18-2 | 22-3 | 25-9 | 8-6 | 12-5 | 15-9 | 19-3 | 22-4 |
| ceiling not attached · 16 in · Spruce-pine-fir #3 | 7-5 | 10-10 | 13-9 | 16-9 | 19-6 | 6-5 | 9-5 | 11-11 | 14-6 | 16-10 |
| ceiling not attached · 19.2 in · Douglas fir-larch SS | 9-10 | 15-5 | 20-4 | 25-11 | Note b | 9-10 | 14-10 | 18-10 | 23-0 | Note b |
| ceiling not attached · 19.2 in · Douglas fir-larch #1 | 9-5 | 14-0 | 17-9 | 21-8 | 25-2 | 8-4 | 12-2 | 15-4 | 18-9 | 21-9 |
| ceiling not attached · 19.2 in · Douglas fir-larch #2 | 9-1 | 13-3 | 16-10 | 20-7 | 23-10 | 7-10 | 11-6 | 14-7 | 17-10 | 20-8 |
| ceiling not attached · 19.2 in · Douglas fir-larch #3 | 6-11 | 10-2 | 12-10 | 15-8 | 18-3 | 6-0 | 8-9 | 11-2 | 12-7 | 15-9 |
| ceiling not attached · 19.2 in · Hem-fir SS | 9-3 | 14-7 | 19-2 | 24-6 | Note b | 9-3 | 14-4 | 18-2 | 22-3 | 25-9 |
| ceiling not attached · 19.2 in · Hem-fir #1 | 9-1 | 13-10 | 17-6 | 21-5 | 24-10 | 8-2 | 12-0 | 15-2 | 18-6 | 21-6 |
| ceiling not attached · 19.2 in · Hem-fir #2 | 8-8 | 12-11 | 16-4 | 20-0 | 23-2 | 7-8 | 11-2 | 14-2 | 17-4 | 20-1 |
| ceiling not attached · 19.2 in · Hem-fir #3 | 6-9 | 9-11 | 12-7 | 15-4 | 17-9 | 5-10 | 8-7 | 10-10 | 13-3 | 15-5 |
| ceiling not attached · 19.2 in · Southern pine SS | 9-8 | 15-2 | 19-11 | 25-5 | Note b | 9-8 | 15-2 | 19-7 | 23-4 | Note b |
| ceiling not attached · 19.2 in · Southern pine #1 | 9-3 | 14-3 | 18-1 | 21-2 | 25-2 | 8-4 | 12-4 | 15-8 | 18-4 | 21-9 |
| ceiling not attached · 19.2 in · Southern pine #2 | 8-2 | 12-3 | 15-7 | 18-6 | 21-9 | 7-1 | 10-8 | 13-6 | 16-0 | 18-10 |
| ceiling not attached · 19.2 in · Southern pine #3 | 6-4 | 9-4 | 11-9 | 14-3 | 16-10 | 5-6 | 8-1 | 10-2 | 12-4 | 14-7 |
| ceiling not attached · 19.2 in · Spruce-pine-fir SS | 9-1 | 14-3 | 18-9 | 23-11 | Note b | 9-1 | 13-7 | 17-2 | 21-0 | 24-4 |
| ceiling not attached · 19.2 in · Spruce-pine-fir #1 | 8-10 | 13-1 | 16-7 | 20-3 | 23-6 | 7-9 | 11-4 | 14-4 | 17-7 | 20-4 |
| ceiling not attached · 19.2 in · Spruce-pine-fir #2 | 8-10 | 13-1 | 16-7 | 20-3 | 23-6 | 7-9 | 11-4 | 14-4 | 17-7 | 20-4 |
| ceiling not attached · 19.2 in · Spruce-pine-fir #3 | 6-9 | 9-11 | 12-7 | 15-4 | 17-9 | 5-10 | 8-7 | 10-10 | 13-3 | 15-5 |
| ceiling not attached · 24 in · Douglas fir-larch SS | 9-1 | 14-4 | 18-10 | 23-9 | Note b | 9-1 | 13-3 | 16-10 | 20-7 | 23-10 |
| ceiling not attached · 24 in · Douglas fir-larch #1 | 8-7 | 12-6 | 15-10 | 19-5 | 22-6 | 7-5 | 10-10 | 13-9 | 16-9 | 19-6 |
| ceiling not attached · 24 in · Douglas fir-larch #2 | 8-2 | 11-11 | 15-1 | 18-5 | 21-4 | 7-0 | 10-4 | 13-0 | 15-11 | 18-6 |
| ceiling not attached · 24 in · Douglas fir-larch #3 | 6-2 | 9-1 | 11-6 | 14-1 | 16-3 | 5-4 | 7-10 | 10-0 | 12-2 | 14-1 |
| ceiling not attached · 24 in · Hem-fir SS | 8-7 | 13-6 | 17-10 | 22-9 | Note b | 8-7 | 12-10 | 16-3 | 19-10 | 23-0 |
| ceiling not attached · 24 in · Hem-fir #1 | 8-5 | 12-4 | 15-8 | 19-2 | 22-2 | 7-4 | 10-9 | 13-7 | 16-7 | 19-3 |
| ceiling not attached · 24 in · Hem-fir #2 | 7-11 | 11-7 | 14-8 | 17-10 | 20-9 | 6-10 | 10-0 | 12-8 | 15-6 | 17-11 |
| ceiling not attached · 24 in · Hem-fir #3 | 6-1 | 8-10 | 11-3 | 13-8 | 15-11 | 5-3 | 7-8 | 9-9 | 11-10 | 13-9 |
| ceiling not attached · 24 in · Southern pine SS | 8-11 | 14-1 | 18-6 | 23-8 | Note b | 8-11 | 13-10 | 17-6 | 20-10 | 24-8 |
| ceiling not attached · 24 in · Southern pine #1 | 8-7 | 12-9 | 16-2 | 18-11 | 22-6 | 7-5 | 11-1 | 14-0 | 16-5 | 19-6 |
| ceiling not attached · 24 in · Southern pine #2 | 7-4 | 11-0 | 13-11 | 16-6 | 19-6 | 6-4 | 9-6 | 12-1 | 14-4 | 16-10 |
| ceiling not attached · 24 in · Southern pine #3 | 5-8 | 8-4 | 10-6 | 12-9 | 15-1 | 4-11 | 7-3 | 9-1 | 11-0 | 13-1 |
| ceiling not attached · 24 in · Spruce-pine-fir SS | 8-5 | 13-3 | 17-5 | 21-8 | 25-2 | 8-4 | 12-2 | 15-4 | 18-9 | 21-9 |
| ceiling not attached · 24 in · Spruce-pine-fir #1 | 8-0 | 11-9 | 14-10 | 18-2 | 21-0 | 6-11 | 10-2 | 12-10 | 15-8 | 18-3 |
| ceiling not attached · 24 in · Spruce-pine-fir #2 | 8-0 | 11-9 | 14-10 | 18-2 | 21-0 | 6-11 | 10-2 | 12-10 | 15-8 | 18-3 |
| ceiling not attached · 24 in · Spruce-pine-fir #3 | 6-1 | 8-10 | 11-3 | 13-8 | 15-11 | 5-3 | 7-8 | 9-9 | 11-10 | 13-9 |
| ceiling attached · 12 in · Douglas fir-larch SS | 10-5 | 16-4 | 21-7 | Note b | Note b | 10-5 | 16-4 | 21-7 | Note b | Note b |
| ceiling attached · 12 in · Douglas fir-larch #1 | 10-0 | 15-9 | 20-10 | Note b | Note b | 10-0 | 15-4 | 19-5 | 23-9 | Note b |
| ceiling attached · 12 in · Douglas fir-larch #2 | 9-10 | 15-6 | 20-5 | 26-0 | Note b | 9-10 | 14-7 | 18-5 | 22-6 | 26-0 |
| ceiling attached · 12 in · Douglas fir-larch #3 | 8-9 | 12-10 | 16-3 | 19-10 | 23-0 | 7-7 | 11-1 | 14-1 | 17-2 | 19-11 |
| ceiling attached · 12 in · Hem-fir SS | 9-10 | 15-6 | 20-5 | Note b | Note b | 9-10 | 15-6 | 20-5 | Note b | Note b |
| ceiling attached · 12 in · Hem-fir #1 | 9-8 | 15-2 | 19-11 | 25-5 | Note b | 9-8 | 15-2 | 19-2 | 23-5 | Note b |
| ceiling attached · 12 in · Hem-fir #2 | 9-2 | 14-5 | 19-0 | 24-3 | Note b | 9-2 | 14-2 | 17-11 | 21-11 | 25-5 |
| ceiling attached · 12 in · Hem-fir #3 | 8-7 | 12-6 | 15-10 | 19-5 | 22-6 | 7-5 | 10-10 | 13-9 | 16-9 | 19-6 |
| ceiling attached · 12 in · Southern pine SS | 10-3 | 16-1 | 21-2 | Note b | Note b | 10-3 | 16-1 | 21-2 | Note b | Note b |
| ceiling attached · 12 in · Southern pine #1 | 9-10 | 15-6 | 20-5 | Note b | Note b | 9-10 | 15-6 | 19-10 | 23-2 | Note b |
| ceiling attached · 12 in · Southern pine #2 | 9-5 | 14-9 | 19-6 | 23-5 | Note b | 9-0 | 13-6 | 17-1 | 20-3 | 23-10 |
| ceiling attached · 12 in · Southern pine #3 | 8-0 | 11-9 | 14-10 | 18-0 | 21-4 | 6-11 | 10-2 | 12-10 | 15-7 | 18-6 |
| ceiling attached · 12 in · Spruce-pine-fir SS | 9-8 | 15-2 | 19-11 | 25-5 | Note b | 9-8 | 15-2 | 19-11 | 25-5 | Note b |
| ceiling attached · 12 in · Spruce-pine-fir #1 | 9-5 | 14-9 | 19-6 | 24-10 | Note b | 9-5 | 14-4 | 18-2 | 22-3 | 25-9 |
| ceiling attached · 12 in · Spruce-pine-fir #2 | 9-5 | 14-9 | 19-6 | 24-10 | Note b | 9-5 | 14-4 | 18-2 | 22-3 | 25-9 |
| ceiling attached · 12 in · Spruce-pine-fir #3 | 8-7 | 12-6 | 15-10 | 19-5 | 22-6 | 7-5 | 10-10 | 13-9 | 16-9 | 19-6 |
| ceiling attached · 16 in · Douglas fir-larch SS | 9-6 | 14-11 | 19-7 | 25-0 | Note b | 9-6 | 14-11 | 19-7 | 25-0 | Note b |
| ceiling attached · 16 in · Douglas fir-larch #1 | 9-1 | 14-4 | 18-11 | 23-9 | Note b | 9-1 | 13-3 | 16-10 | 20-7 | 23-10 |
| ceiling attached · 16 in · Douglas fir-larch #2 | 8-11 | 14-1 | 18-5 | 22-6 | 26-0 | 8-7 | 12-7 | 16-0 | 19-6 | 22-7 |
| ceiling attached · 16 in · Douglas fir-larch #3 | 7-7 | 11-1 | 14-1 | 17-2 | 19-11 | 6-7 | 9-8 | 12-2 | 14-11 | 17-3 |
| ceiling attached · 16 in · Hem-fir SS | 8-11 | 14-1 | 18-6 | 23-8 | Note b | 8-11 | 14-1 | 18-6 | 23-8 | Note b |
| ceiling attached · 16 in · Hem-fir #1 | 8-9 | 13-9 | 18-1 | 23-1 | Note b | 8-9 | 13-1 | 16-7 | 20-4 | 23-7 |
| ceiling attached · 16 in · Hem-fir #2 | 8-4 | 13-1 | 17-3 | 21-11 | 25-5 | 8-4 | 12-3 | 15-6 | 18-11 | 22-0 |
| ceiling attached · 16 in · Hem-fir #3 | 7-5 | 10-10 | 13-9 | 16-9 | 19-6 | 6-5 | 9-5 | 11-11 | 14-6 | 16-10 |
| ceiling attached · 16 in · Southern pine SS | 9-4 | 14-7 | 19-3 | 24-7 | Note b | 9-4 | 14-7 | 19-3 | 24-7 | Note b |
| ceiling attached · 16 in · Southern pine #1 | 8-11 | 14-1 | 18-6 | 23-2 | Note b | 8-11 | 13-7 | 17-2 | 20-1 | 23-10 |
| ceiling attached · 16 in · Southern pine #2 | 8-7 | 13-5 | 17-1 | 20-3 | 23-10 | 7-9 | 11-8 | 14-9 | 17-6 | 20-8 |
| ceiling attached · 16 in · Southern pine #3 | 6-11 | 10-2 | 12-10 | 15-7 | 18-6 | 6-0 | 8-10 | 11-2 | 13-6 | 16-0 |
| ceiling attached · 16 in · Spruce-pine-fir SS | 8-9 | 13-9 | 18-1 | 23-1 | Note b | 8-9 | 13-9 | 18-1 | 23-0 | Note b |
| ceiling attached · 16 in · Spruce-pine-fir #1 | 8-7 | 13-5 | 17-9 | 22-3 | 25-9 | 8-6 | 12-5 | 15-9 | 19-3 | 22-4 |
| ceiling attached · 16 in · Spruce-pine-fir #2 | 8-7 | 13-5 | 17-9 | 22-3 | 25-9 | 8-6 | 12-5 | 15-9 | 19-3 | 22-4 |
| ceiling attached · 16 in · Spruce-pine-fir #3 | 7-5 | 10-10 | 13-9 | 16-9 | 19-6 | 6-5 | 9-5 | 11-11 | 14-6 | 16-10 |
| ceiling attached · 19.2 in · Douglas fir-larch SS | 8-11 | 14-0 | 18-5 | 23-7 | Note b | 8-11 | 14-0 | 18-5 | 23-0 | Note b |
| ceiling attached · 19.2 in · Douglas fir-larch #1 | 8-7 | 13-6 | 17-9 | 21-8 | 25-2 | 8-4 | 12-2 | 15-4 | 18-9 | 21-9 |
| ceiling attached · 19.2 in · Douglas fir-larch #2 | 8-5 | 13-3 | 16-10 | 20-7 | 23-10 | 7-10 | 11-6 | 14-7 | 17-10 | 20-8 |
| ceiling attached · 19.2 in · Douglas fir-larch #3 | 6-11 | 10-2 | 12-10 | 15-8 | 18-3 | 6-0 | 8-9 | 11-2 | 13-7 | 15-9 |
| ceiling attached · 19.2 in · Hem-fir SS | 8-5 | 13-3 | 17-5 | 22-3 | Note b | 8-5 | 13-3 | 17-5 | 22-3 | 25-9 |
| ceiling attached · 19.2 in · Hem-fir #1 | 8-3 | 12-11 | 17-1 | 21-5 | 24-10 | 8-2 | 12-0 | 15-2 | 18-6 | 21-6 |
| ceiling attached · 19.2 in · Hem-fir #2 | 7-10 | 12-4 | 16-3 | 20-0 | 23-2 | 7-8 | 11-2 | 14-2 | 17-4 | 20-1 |
| ceiling attached · 19.2 in · Hem-fir #3 | 6-9 | 9-11 | 12-7 | 15-4 | 17-9 | 5-10 | 8-7 | 10-10 | 13-3 | 15-5 |
| ceiling attached · 19.2 in · Southern pine SS | 8-9 | 13-9 | 18-2 | 23-1 | Note b | 8-9 | 13-9 | 18-2 | 23-1 | Note b |
| ceiling attached · 19.2 in · Southern pine #1 | 8-5 | 13-3 | 17-5 | 21-2 | 25-2 | 8-4 | 12-4 | 15-8 | 18-4 | 21-9 |
| ceiling attached · 19.2 in · Southern pine #2 | 8-1 | 12-3 | 15-7 | 18-6 | 21-9 | 7-1 | 10-8 | 13-6 | 16-0 | 18-10 |
| ceiling attached · 19.2 in · Southern pine #3 | 6-4 | 9-4 | 11-9 | 14-3 | 16-10 | 5-6 | 8-1 | 10-2 | 12-4 | 14-7 |
| ceiling attached · 19.2 in · Spruce-pine-fir SS | 8-3 | 12-11 | 17-1 | 21-9 | Note b | 8-3 | 12-11 | 17-1 | 21-0 | 24-4 |
| ceiling attached · 19.2 in · Spruce-pine-fir #1 | 8-1 | 12-8 | 16-7 | 20-3 | 23-6 | 7-9 | 11-4 | 14-4 | 17-7 | 20-4 |
| ceiling attached · 19.2 in · Spruce-pine-fir #2 | 8-1 | 12-8 | 16-7 | 20-3 | 23-6 | 7-9 | 11-4 | 14-4 | 17-7 | 20-4 |
| ceiling attached · 19.2 in · Spruce-pine-fir #3 | 6-9 | 9-11 | 12-7 | 15-4 | 17-9 | 5-10 | 8-7 | 10-10 | 13-3 | 15-5 |
| ceiling attached · 24 in · Douglas fir-larch SS | 8-3 | 13-0 | 17-2 | 21-10 | Note b | 8-3 | 13-0 | 16-10 | 20-7 | 23-10 |
| ceiling attached · 24 in · Douglas fir-larch #1 | 8-0 | 12-6 | 15-10 | 19-5 | 22-6 | 7-5 | 10-10 | 13-9 | 16-9 | 19-6 |
| ceiling attached · 24 in · Douglas fir-larch #2 | 7-10 | 11-11 | 15-1 | 18-5 | 21-4 | 7-0 | 10-4 | 13-0 | 15-11 | 18-6 |
| ceiling attached · 24 in · Douglas fir-larch #3 | 6-2 | 9-1 | 11-6 | 14-1 | 16-3 | 5-4 | 7-10 | 10-0 | 12-2 | 14-1 |
| ceiling attached · 24 in · Hem-fir SS | 7-10 | 12-3 | 16-2 | 20-8 | 25-1 | 7-10 | 12-3 | 16-2 | 19-10 | 23-0 |
| ceiling attached · 24 in · Hem-fir #1 | 7-8 | 12-0 | 15-8 | 19-2 | 22-2 | 7-4 | 10-9 | 13-7 | 16-7 | 19-3 |
| ceiling attached · 24 in · Hem-fir #2 | 7-3 | 11-5 | 14-8 | 17-10 | 20-9 | 6-10 | 10-0 | 12-8 | 15-6 | 17-11 |
| ceiling attached · 24 in · Hem-fir #3 | 6-1 | 8-10 | 11-3 | 13-8 | 15-11 | 5-3 | 7-8 | 9-9 | 11-10 | 13-9 |
| ceiling attached · 24 in · Southern pine SS | 8-1 | 12-9 | 16-10 | 21-6 | Note b | 8-1 | 12-9 | 16-10 | 20-10 | 24-8 |
| ceiling attached · 24 in · Southern pine #1 | 7-10 | 12-3 | 16-2 | 18-11 | 22-6 | 7-5 | 11-1 | 14-0 | 16-5 | 19-6 |
| ceiling attached · 24 in · Southern pine #2 | 7-4 | 11-0 | 13-11 | 16-6 | 19-6 | 6-4 | 9-6 | 12-1 | 14-4 | 16-10 |
| ceiling attached · 24 in · Southern pine #3 | 5-8 | 8-4 | 10-6 | 12-9 | 15-1 | 4-11 | 7-3 | 9-1 | 11-0 | 13-1 |
| ceiling attached · 24 in · Spruce-pine-fir SS | 7-8 | 12-0 | 15-10 | 20-2 | 24-7 | 7-8 | 12-0 | 15-4 | 18-9 | 21-9 |
| ceiling attached · 24 in · Spruce-pine-fir #1 | 7-6 | 11-9 | 14-10 | 18-2 | 21-0 | 6-11 | 10-2 | 12-10 | 15-8 | 18-3 |
| ceiling attached · 24 in · Spruce-pine-fir #2 | 7-6 | 11-9 | 14-10 | 18-2 | 21-0 | 6-11 | 10-2 | 12-10 | 15-8 | 18-3 |
| ceiling attached · 24 in · Spruce-pine-fir #3 | 6-1 | 8-10 | 11-3 | 13-8 | 15-11 | 5-3 | 7-8 | 9-9 | 11-10 | 13-9 |
IRC 2021 Tables R802.4.1(1), ceiling not attached to rafters (L/Δ = 180), and R802.4.1(2), ceiling attached (L/Δ = 240), roof live load 20 psf, as printed: feet-inches measured along the horizontal projection (18-5 is 18 ft 5 in). Note b: Span exceeds 26 feet in length. Table R802.4.1(1) prints 12-12 for a 2 × 8 Douglas fir-larch #3 at 16 in with 20 psf dead load (12 inches is not an inch figure); the calculator reads Table R802.4.1(2) there. Check sources for availability of lumber in lengths greater than 20 feet. With ceiling joists or rafter ties higher than the bottom of the attic, multiply by Table R802.4.1(9), H_C/H_R: 1/7.5 or less → 1.00; 1/6 → 0.90; 1/5 → 0.83; 1/4 → 0.76; 1/3 → 0.67. The ground snow tables, (3) to (8), are in the calculator.
Where the numbers come from
- Spans: IRC 2021 Tables R802.4.1(1) to R802.4.1(8), Rafter Spans for Common Lumber Species, one table per roof load (20 psf live, or 30, 50 or 70 psf ground snow) and ceiling (not attached, L/Δ = 180; attached, L/Δ = 240), measured along the horizontal projection; note b, «Span exceeds 26 feet in length». Table R802.4.1(9) multiplies them by 0.90, 0.83, 0.76 or 0.67 when the ties sit 1/6, 1/5, 1/4 or 1/3 of the way to the ridge. Read October 17, 2026, in the New Jersey edition, which does not amend R802.4.1. Source
- Checked row by row on October 19, 2026, against the 2021 Seattle Residential Code, which prints the model tables without amendment: 511 of the 512 rows match, and the other is a typo in Seattle's PDF. Six cells are printed out of line in both, so in the 2021 IRC itself; the calculator reads the same cell from the next stricter table and says so. Data sheet (PDF)
- Between two rows of Table R802.4.1(9) the calculator takes the higher ratio, the smaller factor, and it rounds an adjusted span down to the whole inch.
The tables cover rafters with ceiling joists or rafter ties at the bottom of the attic, or higher with the Table R802.4.1(9) factor up to a third of the rise. For other species, grades and loads the code refers to the AWC Span Tables for Joists and Rafters.
By Javier Miralles · Last updated . How this site is made.
Other materials
Embed this calculator
Free to use on your own site. Copy the code and keep the credit line.