Rafter length calculator: how long do my rafters need to be?
14 ft 5 9/16 in
13 ft 4 3/16 in ridge to wall + 1 ft 1 7/16 in of overhang · 6/12, pitch factor 1.118 · plumb cut 26.6° · for a 24 ft span at 6/12 with a 12 in overhang
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Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
14 ft 5 9/16 in
13 ft 4 3/16 in ridge to wall + 1 ft 1 7/16 in of overhang · 6/12, pitch factor 1.118 · plumb cut 26.6°
On a building 24 ft wide with a 6/12 roof, a 12 inch overhang and a 1 1/2 inch ridge board, each rafter is 14 ft 5 9/16 in long: 12 ft of run less 3/4 in, plus the overhang, times the pitch factor 1.118.
A worked example
A garage 27 ft 4 in wide at 7/12 with a 16 in overhang and a 1 1/2 in ridge board needs rafters 17 ft 3 1/2 in long. The run is half of 328 in, 164 in, less 3/4 in for the ridge: 163.25 in. Adding the 16 in overhang gives 179.25 in, and the 7/12 pitch factor is √(1 + (7/12)²) = 1.158, so 207.5 in, 17 ft 3 1/2 in. The plumb cut is 30.3°.
How do you calculate rafter length?
Take half the building span as the run, subtract half the ridge board, add the overhang measured level, and multiply by the pitch factor, √(1 + (rise ÷ 12)²). For a 30 ft span at 4/12 with a 1 1/2 in ridge and an 18 in overhang, that is (180 − 0.75 + 18) in × 1.054 = 207.9 in, 17 ft 3 15/16 in.
How much does the overhang add to a rafter?
The overhang is measured level out from the wall, so it gets the same pitch factor as the rest of the rafter. A 12 in overhang adds 12 5/8 in of rafter at 4/12, 13 7/16 in at 6/12 and 17 in at 12/12. That is why a steeper roof with the same overhang needs longer boards.
What else it is called
A rafter length calculator is also called a rafter calculator or a roof rafter length calculator. For the pitch from a rise and a run, or in degrees and percent, the roof pitch calculator does that; for the shingles over the rafters, the roofing calculator counts the bundles.
Rafter length chart
| Building span | 4/12 | 6/12 | 8/12 | 10/12 | 12/12 |
|---|---|---|---|---|---|
| 12 ft | 7 ft 3 3/4 in | 7 ft 9 1/16 in | 8 ft 4 1/16 in | 9 ft 3/8 in | 9 ft 9 3/4 in |
| 16 ft | 9 ft 5 1/16 in | 9 ft 11 15/16 in | 10 ft 8 7/8 in | 11 ft 7 5/8 in | 12 ft 7 11/16 in |
| 20 ft | 11 ft 6 3/8 in | 12 ft 2 3/4 in | 13 ft 1 3/4 in | 14 ft 2 7/8 in | 15 ft 5 5/8 in |
| 24 ft | 13 ft 7 5/8 in | 14 ft 5 9/16 in | 15 ft 6 9/16 in | 16 ft 10 1/16 in | 18 ft 3 9/16 in |
| 28 ft | 15 ft 8 15/16 in | 16 ft 8 7/16 in | 17 ft 11 7/16 in | 19 ft 5 5/16 in | 21 ft 1 1/2 in |
| 32 ft | 17 ft 10 1/4 in | 18 ft 11 1/4 in | 20 ft 4 1/4 in | 22 ft 9/16 in | 23 ft 11 7/16 in |
| 36 ft | 19 ft 11 9/16 in | 21 ft 2 1/16 in | 22 ft 9 1/8 in | 24 ft 7 13/16 in | 26 ft 9 3/8 in |
Rafter line length with a 12 in overhang and a 1 1/2 in ridge board, to the nearest 1/16 in. Change either one in the calculator.
Where the numbers come from
- Rafter length is geometry: the horizontal run is half the building span less half the ridge board thickness, since the rafters bear on its two faces. The overhang, measured level out from the wall, is added to the run, and the total is multiplied by the pitch factor √(1 + (rise ÷ 12)²), the length of rafter per inch of run.
- The plumb cut angle is the roof angle, atan(rise ÷ 12): 26.6° for 6/12 and 45° for 12/12. Lengths are rounded to the nearest 1/16 in, the finest mark on a tape.
This is the line length laid out along the rafter at its slope, from the ridge plumb cut to the tail; the birdsmouth seat is cut where it crosses the wall line and does not change it. Measure the span from the outside of one wall to the outside of the other, where the rafters sit. Buy the next board length up: the rest is scrap.
By Javier Miralles · Last updated . How this site is made.
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