Rafter Span Calculator
Find the maximum horizontal roof rafter span from the IRC R802.4.1 tables. Pick the roof or ground snow load, species, grade, size, and spacing to get the max span by size and the minimum rafter for your roof. Free, no sign-up.
What to calculate next
Tools commonly used alongside this calculation
Ceiling Joist Span Calculator
Find the maximum ceiling joist span from the IRC R802.5.1 tables. Pick the attic use, species, grade, size, and spacing to get the max span by size and the minimum joist for your ceiling. Free, no sign-up.
Joist Span Calculator
Find the maximum allowable floor or deck joist span from the IRC span tables. Pick species, grade, size, and spacing to get the code max span by joist size and check it against the span you need. Free, no sign-up.
Deck Beam Span Calculator
Size a built-up deck beam from the IRC R507.5 table. Pick the species and the deck joist span it carries to get the maximum span for double and triple 2× beams and the minimum size for your post spacing. Free, no sign-up.
Explanation
A rafter span is how far a sloped roof rafter can run — measured as the horizontal projection, not the length along the slope — between the wall plate and the ridge. The allowable span depends on the rafter size, the lumber species and grade, the spacing, and the roof load, which in most of the country is driven by ground snow. The International Residential Code (IRC) gives prescriptive rafter spans in Table R802.4.1. This calculator reads that table: choose the roof load, species, grade, and spacing, and it returns the maximum horizontal span for each rafter size and the smallest size that reaches your ridge.
How roof load and the R802.4.1 tables work
The IRC publishes a separate rafter table for each roof load — a 20 psf roof live load where there is no snow, and ground snow loads of 30, 50, and 70 psf. Heavier snow means shorter spans. These spans are for rafters where the ceiling is not attached to the underside (a deflection limit of L/180); rafters that also carry a finished ceiling use the stiffer L/240 tables. The example below is Douglas Fir-Larch No. 2 at 16″ on-center, showing how the same rafter shrinks as snow load grows.
| DF-L No. 2, 16″ o.c. | 2×6 | 2×8 | 2×10 | 2×12 |
|---|---|---|---|---|
| No snow | 14'-7" | 18'-5" | 22'-6" | 26'-0" |
| 30 psf snow | 12'-1" | 15'-4" | 18'-9" | 21'-8" |
| 50 psf snow | 9'-10" | 12'-6" | 15'-3" | 17'-9" |
| 70 psf snow | 8'-7" | 10'-10" | 13'-3" | 15'-4" |
Example: IRC Table R802.4.1, Douglas Fir-Larch No. 2, ceiling not attached, 16″ o.c., 10 psf dead load. Horizontal spans.
Rafters and the rest of the frame
Rafters are one of three framing members the IRC tabulates the same way. For floors, use the joist span calculator; for an exterior deck, the deck beam span calculator sizes the beam under the deck joists. All three read prescriptive IRC span tables for sawn dimension lumber so you can size a member without running the stress and deflection math by hand.
Notes and limitations
These spans are the horizontal projection and assume rafter ties or ceiling joists at the bottom of the rafters; where ties sit higher in the attic, the IRC requires the reduction factors in Table R802.4.1(9). This tool covers the 10 psf dead load, ceiling-not-attached case for the four standard roof loads. Heavier dead loads (tile, multiple layers), the ceiling-attached L/240 tables, the 2×4 size, and ceiling-joist spans are outside this version — use the full IRC tables or the AWC span tables for those.
This calculator reads the published IRC R802.4.1 rafter tables and flags the smallest rafter that covers your horizontal span. It does not size the ridge beam, collar ties, or rafter ties, check uplift or wind, or replace a stamped structural plan. Confirm the governing code edition, the ground snow load for your area, and any local amendments with the authority having jurisdiction.