Rafter length calculator
Span, pitch and overhang in; common rafter length, the board to buy, the rise and the plumb cut angle out. Pitch as x/12 or in degrees.
= 6.00/12 · 26.6°
- Rafter to ridge (line length)
- 13' 4 1/8"
- Tail (overhang along slope)
- 1' 1 3/8"
- Run
- 11' 11 1/4"
- Rise to ridge
- 5' 11 5/8"
- Plumb cut angle
- 26.6°
- Slope multiplier
- 1.118
Lengths are along the top edge of the rafter. Check against a framing square and your local code before cutting.
Common rafter lengths
Total rafter with a 12" overhang and a 1½" ridge board, and the board to buy.
| Span | 4/12 | 6/12 | 8/12 | 10/12 | 12/12 |
|---|---|---|---|---|---|
| 16 ft | 9' 5"10 ft board | 9' 11 7/8"10 ft board | 10' 8 7/8"12 ft board | 11' 7 5/8"12 ft board | 12' 7 5/8"14 ft board |
| 20 ft | 11' 6 3/8"12 ft board | 12' 2 3/4"14 ft board | 13' 1 3/4"14 ft board | 14' 2 7/8"16 ft board | 15' 5 5/8"16 ft board |
| 24 ft | 13' 7 5/8"14 ft board | 14' 5 5/8"16 ft board | 15' 6 5/8"16 ft board | 16' 10 1/8"18 ft board | 18' 3 1/2"20 ft board |
| 28 ft | 15' 9"16 ft board | 16' 8 3/8"18 ft board | 17' 11 3/8"18 ft board | 19' 5 3/8"20 ft board | 21' 1 1/2"22 ft board |
| 32 ft | 17' 10 1/4"18 ft board | 18' 11 1/4"20 ft board | 20' 4 1/4"22 ft board | 22' 0 5/8"24 ft board | 23' 11 1/2"24 ft board |
Not sure of the pitch? See each one drawn to scale with the roof pitch calculator, or measure the whole roof from an address with the satellite roof measurement tool.
Rafter questions
How do you calculate rafter length?
Take the run (half the building span, less half the ridge board) and multiply it by the slope multiplier for the pitch, √(1 + (rise ÷ 12)²). A 24 ft span at 6/12 with a 1½" ridge has a run of 11' 11 1/4"; × 1.118 gives a line length of 13' 4 1/8". Add the overhang times the same multiplier for the tail.
What length board do I buy?
The next even length at or above the total rafter (line length plus tail): lumber is sold in 2 ft steps. A 14' 6" rafter needs a 16 ft board. The calculator shows it.
What is the plumb cut angle?
The angle of the vertical cut at the ridge and the birdsmouth, equal to the roof angle: 26.6° at 6/12, 33.7° at 8/12, 45° at 12/12. On a framing square, use 12 on the body and the rise on the tongue.
Is the overhang measured level or along the rafter?
Level, out from the outside of the wall, the way it appears on plans. The calculator turns it into the length along the slope for you.
Do I need to know the pitch in degrees?
No. Switch the pitch to x/12 or degrees, whichever you have. 15° is about 3.2/12, 30° about 6.9/12, 45° exactly 12/12.
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