Four ways to work out an unknown roof pitch from whatever measurements you can actually take: a level and tape, a metric spirit level, a rafter length and run, or a ridge height and half span.
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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" |
Which measurement method is most accurate?
A one metre or two foot level against a rafter in the attic. The longer the level, the smaller the effect of any error at the measuring end, and a rafter is a true plane where the roof surface may not be.
My readings differ across the roof. Is that a problem?
A variation up to about half an inch in twelve is normal on an older roof. More than that can indicate settlement or a structural issue worth having looked at.
Can I measure pitch from the ground?
Yes, roughly. Measure the ridge height above the wall plate and half the building span, then enter both. It is less accurate than a level on a rafter but needs no ladder.
