How this rafter length calculator works
A common rafter is the hypotenuse over half the building. The calculator runs the pitch multiplier over the half-span, converts your level overhang to its slope length, and adds them:
rafter = run × multiplier + (overhang ÷ 12) × multiplier · to ridge centerline
Worked example: rafter length for a 24 ft span
A 24 ft building at 6:12 has a 12 ft run. Times the 1.118 multiplier that's 13′-5″ plate to ridge; a 12 in level overhang adds 1′-1 7/16″ along the slope, for a cut length of 14′-6 7/16″ — out of a 16 ft 2× board, plumb-cut at 26.6°.
Rafter lengths by span and pitch
| Span | 4:12 | 6:12 | 8:12 |
|---|---|---|---|
| 16 ft | 9′-5 13/16″ (10) | 10′-3/4″ (12) | 10′-9 13/16″ (12) |
| 20 ft | 11′-7 1/8″ (12) | 12′-3 9/16″ (14) | 13′-2 5/8″ (14) |
| 24 ft | 13′-8 7/16″ (14) | 14′-6 7/16″ (16) | 15′-7 1/2″ (16) |
| 28 ft | 15′-9 3/4″ (16) | 16′-9 1/4″ (18) | 18′-5/16″ (20) |
| 32 ft | 17′-11 1/16″ (18) | 19′-1/16″ (20) | 20′-5 3/16″ (—) |
Rafter FAQ
How long a rafter do I need for a 24 ft wide building?
At a 6:12 pitch with a 12 in overhang, each rafter cuts 14 ft 6 7/16 in along the slope — buy 16 ft 2× stock. At 4:12 the same span needs 13 ft 8 7/16 in; at 8:12, 15 ft 7 1/2 in. The calculator names the shortest standard board for any combination.
Does rafter length include the overhang?
Here, yes — the result is plate to ridge centerline plus the overhang converted to its slope length. Enter 0 overhang for the bare plate-to-ridge number. Remember the printed length runs to the ridge centerline: deduct half the ridge thickness (¾ in for 2× stock) at the plumb cut.
What angle do I cut the rafter ends?
The plumb cut equals the pitch angle — 26.6° for a 6:12 roof. Set your saw to it for the ridge cut and the tail cut; the birdsmouth uses the same angle for its plumb face and the complement for the seat.
What size lumber should rafters be?
That is a span-table question, not a length question: IRC R802.4 sizes rafters by species, grade, spacing and snow load. This tool gives you the cut length and board length; confirm the depth (2×6, 2×8, 2×10…) against the tables or your plans.
How do I lay out the birdsmouth?
Mark the plumb line where the rafter crosses the outside of the wall plate, then the level seat cut from it — the seat should bear fully on the plate, and the remaining rafter depth above the notch should be at least two-thirds of the board. Cut one rafter, test it both ends, then use it as the pattern.
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