How do you use a roof pitch angle finder?
Short answer. A roof pitch angle finder, whether a magnetic protractor, a digital angle gauge, or a phone inclinometer app, reads the roof's slope in degrees when you set it on the roof surface or a rafter. Convert the reading to pitch by multiplying 12 by the tangent of the angle, or use a chart: about 26.6 degrees is a 6/12 pitch.
Types of angle finders
- Magnetic or dial protractor: a simple gauge with a swinging needle. Inexpensive and needs no batteries.
- Digital angle gauge: shows the angle to a tenth of a degree on a screen; some read pitch directly.
- Pitch gauge: a roofing tool marked directly in rise per 12, so no conversion is needed.
- Speed square: carpenters use the pitch scale on a rafter square with a level.
- Phone inclinometer apps: handy, but phone cases and edges can throw off the reading by a degree or two.
How to take a good reading
- Measure on a rafter in the attic when possible; it is safer and flatter than shingles.
- On the roof surface, lay a straight board across several courses first, then set the gauge on the board, so shingle bumps do not tilt it.
- Take readings in two or three places and average them.
- Check each roof section separately; porches and dormers often differ.
Quick conversion chart
- 14.0 degrees = 3/12
- 18.4 degrees = 4/12
- 22.6 degrees = 5/12
- 26.6 degrees = 6/12
- 30.3 degrees = 7/12
- 33.7 degrees = 8/12
- 36.9 degrees = 9/12
- 45 degrees = 12/12
Readings between values usually round to the nearest whole or half pitch. For how pitch affects roof area and price, see how roofers measure a roof.