The COMMON scale on a speed square gives the plumb cut for common rafters; the HIP/VAL scale gives the shallower hip angle. This tool shows which mark to pivot to and the equivalent saw settings in degrees.
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| Pitch | Angle | Slope % | Multiplier | Hip / valley | Rafter per 12" run |
|---|---|---|---|---|---|
| 0.25/12 | 1.19° | 2.08% | 1.0002 | 1.4144 | 12.00" |
| 0.5/12 | 2.39° | 4.17% | 1.0009 | 1.4148 | 12.01" |
| 1/12 | 4.76° | 8.33% | 1.0035 | 1.4167 | 12.04" |
| 1.5/12 | 7.13° | 12.50% | 1.0078 | 1.4197 | 12.09" |
| 2/12 | 9.46° | 16.67% | 1.0138 | 1.4240 | 12.17" |
| 2.5/12 | 11.77° | 20.83% | 1.0215 | 1.4295 | 12.26" |
| 3/12 | 14.04° | 25.00% | 1.0308 | 1.4361 | 12.37" |
| 3.5/12 | 16.26° | 29.17% | 1.0417 | 1.4440 | 12.50" |
| 4/12 | 18.43° | 33.33% | 1.0541 | 1.4530 | 12.65" |
| 4.5/12 | 20.56° | 37.50% | 1.0680 | 1.4631 | 12.82" |
| 5/12 | 22.62° | 41.67% | 1.0833 | 1.4743 | 13.00" |
| 5.5/12 | 24.62° | 45.83% | 1.1000 | 1.4866 | 13.20" |
| 6/12 | 26.57° | 50.00% | 1.1180 | 1.5000 | 13.42" |
| 6.5/12 | 28.44° | 54.17% | 1.1373 | 1.5144 | 13.65" |
| 7/12 | 30.26° | 58.33% | 1.1577 | 1.5298 | 13.89" |
| 7.5/12 | 32.01° | 62.50% | 1.1792 | 1.5462 | 14.15" |
| 8/12 | 33.69° | 66.67% | 1.2019 | 1.5635 | 14.42" |
| 8.5/12 | 35.31° | 70.83% | 1.2255 | 1.5817 | 14.71" |
| 9/12 | 36.87° | 75.00% | 1.2500 | 1.6008 | 15.00" |
| 9.5/12 | 38.37° | 79.17% | 1.2754 | 1.6207 | 15.31" |
| 10/12 | 39.81° | 83.33% | 1.3017 | 1.6415 | 15.62" |
| 11/12 | 42.51° | 91.67% | 1.3566 | 1.6853 | 16.28" |
| 12/12 | 45.00° | 100.00% | 1.4142 | 1.7321 | 16.97" |
| 13/12 | 47.29° | 108.33% | 1.4743 | 1.7815 | 17.69" |
| 14/12 | 49.40° | 116.67% | 1.5366 | 1.8333 | 18.44" |
| 15/12 | 51.34° | 125.00% | 1.6008 | 1.8875 | 19.21" |
| 16/12 | 53.13° | 133.33% | 1.6667 | 1.9437 | 20.00" |
| 18/12 | 56.31° | 150.00% | 1.8028 | 2.0616 | 21.63" |
| 20/12 | 59.04° | 166.67% | 1.9437 | 2.1858 | 23.32" |
| 24/12 | 63.43° | 200.00% | 2.2361 | 2.4495 | 26.83" |
How do I use a speed square to mark a rafter?
Hold the fence tight against the top edge of the rafter, pivot the square on the corner until your pitch number on the COMMON scale lines up with that same edge, then scribe along the square. That line is the plumb cut.
Why are there two scales?
Because a hip or valley rafter at the same roof pitch sits at a shallower angle than a common rafter. A 6/12 reads just under 27 degrees on the common scale and just under 20 on the hip/valley scale.
