OneTaskBy Tim King · Updated
Rafter Length Calculator
Rafter Length From Span and Roof Pitch
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How the Rafter Length Calculator works
This rafter length calculator works out how long the common rafters on a gable roof need to be. Enter the span of the building from outside wall to outside wall, the roof pitch in degrees or as x:12, and the horizontal overhang past the wall, and it returns the rafter length to the wall and with the overhang, the rise at the ridge and the plumb and seat cut angles.
On a symmetrical gable roof each rafter covers half the span, called the run. The rafter is the sloping side of a right-angled triangle whose base is the run and whose height is the rise. The overhang continues along the same slope past the wall to form the eaves.
How the rafter length calculator works
The formula is: run = span ÷ 2; rise = run × tan(pitch); rafter length = run ÷ cos(pitch); with overhang = (run + overhang) ÷ cos(pitch). For example, an 8 m span at 30° has a 4 m run, a 2.31 m rise and a rafter of 4.62 m, or 5.14 m with a 450 mm overhang. A second example in feet: a 24 ft span at 6:12 has a 12 ft run, a 6 ft rise and a 13.42 ft rafter, 15.09 ft with an 18 in overhang.
This is the theoretical line length to the centre of the ridge. Deduct half the ridge board thickness, measured square to the plumb cut, and allow extra for trimming. The plumb cut at the top equals the pitch angle from square; the seat cut on the birdsmouth is 90° minus the pitch.
Rafter size (depth and spacing) depends on span, load and timber grade, and is set by span tables in building regulations; this calculator gives length only. To see how many rafters to buy, divide the roof length by the spacing (commonly 400 or 600 mm) and add one, then double it for the two slopes.
Common rafter length without overhang by span and roof pitch (metres)
| Span | 15° | 22.5° | 30° | 35° | 40° | 45° |
|---|---|---|---|---|---|---|
| 3.0 m | 1.553 | 1.624 | 1.732 | 1.831 | 1.958 | 2.121 |
| 3.5 m | 1.812 | 1.894 | 2.021 | 2.136 | 2.284 | 2.475 |
| 4.0 m | 2.071 | 2.165 | 2.309 | 2.442 | 2.611 | 2.828 |
| 4.5 m | 2.329 | 2.435 | 2.598 | 2.747 | 2.937 | 3.182 |
| 5.0 m | 2.588 | 2.706 | 2.887 | 3.052 | 3.264 | 3.536 |
| 5.5 m | 2.847 | 2.977 | 3.175 | 3.357 | 3.590 | 3.889 |
| 6.0 m | 3.106 | 3.247 | 3.464 | 3.662 | 3.916 | 4.243 |
| 6.5 m | 3.365 | 3.518 | 3.753 | 3.968 | 4.243 | 4.596 |
| 7.0 m | 3.623 | 3.788 | 4.041 | 4.273 | 4.569 | 4.950 |
| 7.5 m | 3.882 | 4.059 | 4.330 | 4.578 | 4.895 | 5.303 |
| 8.0 m | 4.141 | 4.330 | 4.619 | 4.883 | 5.222 | 5.657 |
| 8.5 m | 4.400 | 4.600 | 4.907 | 5.188 | 5.548 | 6.010 |
| 9.0 m | 4.659 | 4.871 | 5.196 | 5.493 | 5.874 | 6.364 |
| 9.5 m | 4.918 | 5.141 | 5.485 | 5.799 | 6.201 | 6.718 |
| 10.0 m | 5.176 | 5.412 | 5.774 | 6.104 | 6.527 | 7.071 |
| 10.5 m | 5.435 | 5.683 | 6.062 | 6.409 | 6.853 | 7.425 |
| 11.0 m | 5.694 | 5.953 | 6.351 | 6.714 | 7.180 | 7.778 |
| 11.5 m | 5.953 | 6.224 | 6.640 | 7.019 | 7.506 | 8.132 |
| 12.0 m | 6.212 | 6.494 | 6.928 | 7.325 | 7.832 | 8.485 |
| 12.5 m | 6.470 | 6.765 | 7.217 | 7.630 | 8.159 | 8.839 |
| 13.0 m | 6.729 | 7.036 | 7.506 | 7.935 | 8.485 | 9.192 |
FAQ
How do you calculate rafter length?
Halve the span to get the run, then divide the run by the cosine of the pitch angle. For a 10 m span at 35°, the run is 5 m and the rafter is 5 ÷ cos 35° = 6.10 m, before any overhang, ridge deduction or trimming allowance.
How much overhang should rafters have?
Eaves overhangs of 300–600 mm (12–24 in) measured horizontally are common. A bigger overhang shades walls and windows and keeps rain off them, but it catches more wind, so very deep eaves may need extra fixings. Match the neighbouring houses if planning rules require it.
What is a birdsmouth cut?
It is a notch cut into the underside of a rafter so it sits flat on the wall plate. It has a level seat cut and a vertical plumb cut. Do not cut deeper than about a third of the rafter depth, or the rafter is weakened at the point where it carries the most load.
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