How do I calculate roof pitch?
Roof pitch is the rise in inches for every 12 inches of horizontal run. Measure the rise over any run, then scale it to 12.
Pitch = rise ÷ run × 12, written as x/12. Angle = arctan(rise ÷ run).
The quickest way to measure: hold a level flat against the underside of a rafter or on the roof surface, mark 12 inches along the level, and measure straight down (or up) to the roof. If that distance is 6 inches, the roof is a 6/12 pitch. That's 26.6°, with a pitch factor of 1.118 and 13.42 in of rafter per foot of run.
You can measure over any run, as long as rise and run are in the same unit. A rise of 3 ft over a run of 4 ft is 3 ÷ 4 × 12 = 9/12, or 36.9°.
Common roof pitches
| Pitch | Angle | Pitch factor | Rafter per foot of run |
|---|---|---|---|
| 1/12 | 4.8° | 1.003 | 12.04 in |
| 2/12 | 9.5° | 1.014 | 12.17 in |
| 3/12 | 14.0° | 1.031 | 12.37 in |
| 4/12 | 18.4° | 1.054 | 12.65 in |
| 5/12 | 22.6° | 1.083 | 13.00 in |
| 6/12 | 26.6° | 1.118 | 13.42 in |
| 7/12 | 30.3° | 1.158 | 13.89 in |
| 8/12 | 33.7° | 1.202 | 14.42 in |
| 9/12 | 36.9° | 1.250 | 15.00 in |
| 10/12 | 39.8° | 1.302 | 15.62 in |
| 12/12 | 45.0° | 1.414 | 16.97 in |
The pitch factor converts flat footprint area into roof area. Multiply your footprint by it, or use the roofing calculator, which does it for you. Rafter length per foot of run times the run gives the rafter's sloped length before overhang and the ridge board cut.
What pitch means for roofing
Under the IRC, asphalt shingles need a slope of at least 2/12, and roofs from 2/12 up to 4/12 need double underlayment. Below 2/12, use a low-slope system such as a membrane or a metal panel rated for that slope. Most houses fall between 4/12 and 9/12. Many roofers add a steep charge from about 7/12 or 8/12, and roofs above that need roof jacks and fall protection. Check the product's installation instructions and your local code.
Lean-to and shed roof pitch calculator
A lean-to or shed roof slopes one way, from a high wall to a low wall. The rise is the difference in wall height and the run is the horizontal distance between the walls.
For a lean-to that's 10 ft deep with the back wall 2 ft higher than the front: 2 ÷ 10 × 12 = 2.4/12, or 11.3°. The sloped length is 10.2 ft before overhangs. That's a low pitch: shingles are allowed but need double underlayment, so many shed roofs use metal or roll roofing instead.
Roof pitch from an angle
If you have the angle from a phone app or a plan, switch to "Angle in degrees". The calculator converts it with rise per 12 = 12 × tan(angle). A 30° roof is about 6.93/12, so close to a 7/12.
Roof pitch calculator app
This page works on a phone, so you can use it as a roof pitch calculator app on site. Measure the rise over a 12 inch level, type it in, and read the pitch, angle and rafter length per foot. Phone angle apps work too, but check them against a level, since a small error in degrees shifts the pitch.
Questions
What is a 6/12 roof pitch in degrees?
A 6/12 pitch is 26.6 degrees. It rises 6 inches for every 12 inches of run and adds about 12% to the footprint area.
What is the minimum pitch for asphalt shingles?
2/12 under the IRC. Between 2/12 and 4/12, shingles need double underlayment.
How do I measure roof pitch without getting on the roof?
From a ladder at the gable end, hold a level against the underside of the rake or a rafter tail, mark 12 inches, and measure straight up to the roof line. A pitch gauge or a phone angle app also works; convert the angle here.
What is a pitch factor?
The number you multiply footprint area by to get roof surface area. It equals the square root of 1 plus (pitch ÷ 12) squared.