How do you calculate roof pitch, and what tools help?
Short answer. Calculate roof pitch by dividing the rise by the run and multiplying by 12: pitch = (rise ÷ run) × 12, giving inches of rise per foot. If a roof rises 7 feet over a 14 foot run, the pitch is (7 ÷ 14) × 12 = 6, or 6/12. Levels, tape measures, pitch gauges, angle apps and aerial reports all help gather the numbers.
The calculation with any measurements
Rise and run do not have to be 12 inches; just keep the units the same.
- Rise 9 feet over a run of 13.5 feet: 9 ÷ 13.5 = 0.667, × 12 = 8. Pitch is 8/12.
- Rise 20 inches over a 48 inch run: 20 ÷ 48 = 0.417, × 12 = 5. Pitch is 5/12.
- Rise 3.2 metres over 6 metres: 0.533 × 12 = 6.4, about 6.5/12.
Getting rise and run on a real house
For a symmetrical gable, run is half the building's width at the wall line. Rise is the height from the top of the wall plate to the ridge line, which you can measure in the attic from the ceiling joists up to the ridge.
Tools that help
- Torpedo or 2 foot level and tape: the classic 12 inch method from the attic.
- Speed square: reads pitch directly on its common scale.
- Bubble pitch gauge or magnetic angle finder: sits on the roof or rafter and reads x/12 or degrees.
- Phone level or angle apps: handy, but check calibration.
- Aerial measurement reports: list every facet's pitch and area; see how roofers measure a roof.
Rounding
Roofers round to the nearest whole pitch for estimating. If you get 7.6, check again; framed roofs are usually built to whole numbers. More background in roof anatomy explained.