Rafter span chart and calculator

How to read a rafter span chart

A rafter span chart gives the longest level distance a rafter may run between supports for one species and grade, size, spacing, roof load and deflection limit. Each IRC table (R802.4.1(1) to (8)) fixes the load and the ceiling, then lists spans by species, size and spacing for 10 and 20 psf dead load^[1][2]. To use one:

1. Measure the horizontal span: from the inside face of the wall plate to the ridge board, level, not along the slope^[1].
2. Pick the load: 20 psf roof live load where snow is light, or the ground snow load your building department publishes.
3. Pick the ceiling: a drywall ceiling fixed to the rafters (a vaulted ceiling) tightens deflection to L/240; otherwise L/180.
4. Pick the dead load: 10 psf for asphalt shingles, 20 psf for tile, slate or a heavy assembly.
5. Read the span for your species, grade, size and spacing; it must be at least your horizontal span.

Rafter span calculator: the method behind the tables

Each rafter span is the shortest of three limits, worked out with the National Design Specification design values, and it reproduces the IRC tables to the inch^[3][4]:

• Bending on the snow or roof live load plus the dead load, with the repetitive-member factor 1.15 and a load-duration factor: 1.25 for a 20 psf roof live load and 1.15 for snow^[3][6].
• Deflection on the snow or roof live load alone, limited to span ÷ 180, or span ÷ 240 with a ceiling attached.
• Shear on the full load; it does not govern sawn rafters at these spans.
• Loads act on the horizontal projection, and the snow tables put the full ground snow load on the rafter. Design values are the NDS Supplement's for No. 2 and better^[5].

The calculator matches every IRC entry it was checked against, including whole rows from Tables R802.4.1(1) and (3). AWC's free span calculator uses the same equations^[7].

How snow load and dead load change the span

Snow load shortens a rafter span more than anything else: a No. 2 Douglas fir-larch 2x8 at 16 in drops from 18 ft 5 in under a 20 psf roof live load to 10 ft 10 in under 70 psf of ground snow.

The ceiling choice changes little for 2x8 and larger rafters, because bending governs them. It matters for small, flexible rafters under light loads: a No. 2 southern pine 2x6 at 16 in spans 13 ft 6 in without a ceiling and 13 ft 5 in with one.

Rafter size calculator: picking the size

The right rafter size is the smallest one whose span is at least your horizontal span at the spacing you want. Work it out in four steps:

1. Horizontal span = the level distance from the inside face of the wall to the face of the ridge board. A 26 ft wide gable with 2x6 walls and a 1-1/2 in ridge board gives 13 ft − 5-1/2 in − 3/4 in = 12 ft 5-3/4 in.
2. At 30 psf ground snow in No. 2 Douglas fir-larch at 16 in, a 2x6 spans 12 ft 1 in, too short; a 2x8 spans 15 ft 4 in.
3. Choose the 2x8, or keep 2x6 rafters at 12 in centres if they reach (14 ft 0 in).
4. Get the length to buy from the rafter calculator, which adds the pitch, overhang and plumb cuts.

Enter your span in the tool above and it names the smallest size that works. For trusses instead of rafters, see the roof truss calculator; for the pitch itself, the roof pitch calculator.

Rafter ties and raised ceiling joists

The span tables assume ceiling joists or rafter ties at the top of the wall, where they stop the walls spreading. Ties raised higher up the rafters, as in a partly vaulted room, shorten the allowed span by a factor from IRC Table R802.4.1(9): 0.67 when the ties sit a third of the way up the roof height, rising to 1.00 when they sit 1/7.5 of the height or less above the plate^[1]. A ridge beam that carries the rafters, rather than a ridge board, removes the thrust and is designed separately.

Common rafter span mistakes

• Measuring along the slope. Tables use the horizontal span; the sloped length is longer and is only for buying lumber.
• Using the 20 psf table where it snows. A 30 psf ground snow load cuts a 2x8's span by about 3 ft.
• Old southern pine tables. Southern pine design values were lowered in June 2013, so older charts overstate southern pine spans^[5].
• Forgetting the dead load of tile or slate. 20 psf instead of 10 psf takes 2 to 3 ft off a 2x8 or 2x10.
• Spacing the sheathing cannot span. Rafters at 24 in need at least 7/16 in (24/16) roof sheathing; see the plywood calculator.