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Rafter span is the horizontal distance a rafter covers — the run measured on plan — not the sloped length of the board, and that distinction causes more DIY framing errors than any other. On a 6:12 roof the board is about 1.118 times the run, so a 14 foot span means roughly a 15 foot 8 inch rafter before the overhang tail. This chart gives representative maximum spans for 2x4 through 2x12 at 12, 16 and 24 inch on centre spacing as planning figures, then explains everything that moves those numbers: spacing, because 24 inch centres load each rafter about 50% more than 16 inch, lumber species and grade, ground snow load, the dead load of tile or slate versus asphalt, and the deflection limit required when drywall hangs below. Allowable spans are ultimately set by the code edition your jurisdiction has adopted, so verify with local tables or a structural engineer before buying lumber. If your span is too long, the fixes are deeper lumber, tighter spacing, a purlin and struts, an interior bearing wall or designed ridge beam, or engineered lumber and trusses.
Span is how far a rafter can reach between supports for a given size, spacing, species and load. It is set by structural tables in the code, not by rule of thumb, and it is the number that decides whether your rafter size works.
| Building width | 4/12 | 6/12 | 8/12 | 12/12 |
|---|---|---|---|---|
| 20 ft | 10.5 | 11.2 | 12 | 14.1 |
| 24 ft | 12.6 | 13.4 | 14.4 | 17 |
| 28 ft | 14.8 | 15.7 | 16.8 | 19.8 |
| 32 ft | 16.9 | 17.9 | 19.2 | 22.6 |
| 36 ft | 19 | 20.1 | 21.6 | 25.5 |
This is geometry, not a span table. The rafter length tells you how long to cut the timber; whether that size of timber may span it is a code question, answered by the span tables adopted in your jurisdiction.
Use the span tables in the building code adopted in your area, or the tables published by the American Wood Council. Both are keyed to species, grade, size, spacing and load. A span table from a different region or an older code edition is not a safe substitute.
If the roof carries anything unusual — tile, a solar array, a heavy snow zone, or a room below with no ceiling ties — the answer comes from a structural engineer rather than from a table.
It depends on species, grade, spacing and snow load, so it comes from the span table adopted in your jurisdiction rather than from a single number. Depth matters far more than width — a 2x10 spans considerably further than a 2x8.
Yes. Going from 24-inch to 16-inch centres increases the allowable span for the same member, because each rafter carries less load.
The building code adopted in your area, or the tables published by the American Wood Council. Tables from a different region or an older code edition are not a safe substitute.
Whenever the roof carries something unusual — tile, a solar array in a heavy snow zone, a room below with no ceiling ties — or wherever there is a visible sag or a split member.