How to measure roof pitch
Roof pitch in the US is written as rise over run, with the run fixed at 12 inches: a 6/12 roof rises 6 inches for every 12 inches it runs horizontally. There are three easy ways to find it:
- With a level. In the attic, hold a level horizontally against the underside of a rafter, mark 12 in along the level, and measure straight up from that mark to the rafter. On the roof, rest one end of the level on the shingles and do the same. A 24 in level works too — enter 24 as the run.
- From an angle. An angle finder or a phone app placed on the roof or a rafter gives degrees. Choose Roof angle and enter it.
- From the building. On a symmetrical gable, measure the height from the top of the wall to the ridge and the width of the building, and use Total rise and building span. The rafter run is half the span.
Roof pitch formulas
Angle = arctan(rise ÷ run)
Slope % = rise ÷ run × 100
Pitch multiplier = √(rise² + run²) ÷ run
Rafter length = (horizontal run + overhang) × pitch multiplier
The pitch multiplier (also called the rafter or slope factor) converts any horizontal distance into the matching distance along the roof surface. It's how a roofer turns a building's footprint into roof area: footprint including overhangs × multiplier. The roofing calculator does that step and turns it into bundles and squares.
Worked example: a 6/12 gable
Measuring 6 in of rise over 12 in of run gives 6 ÷ 12 = 0.5, so the pitch is 6/12. The angle is arctan(0.5) = 26.57°, the slope is 50%, and the multiplier is √(6² + 12²) ÷ 12 = 1.1180.
On a 24 ft wide building, each rafter runs 12 ft horizontally, so the rafter is 12 × 1.1180 = 13.42 ft from wall to ridge. With a 1 ft overhang it's 13 × 1.1180 = 14.53 ft (about 14 ft 6⅜ in) — a 16 ft board leaves room for the cuts.
Worked example: measured with a 24 in level
If the roof rises 9 in over a 24 in level, the pitch is 9 ÷ 24 × 12 = 4.5/12, an angle of 20.56°. Between 4/12 and 5/12, it's still within the normal range for asphalt shingles.
Rafter lengths here are theoretical line lengths. They don't include the plumb cut at the ridge, the birdsmouth seat or the deduction for half the ridge board's thickness, so lay those out on the board itself.
Common roof pitches
| Pitch | Angle | Slope | Multiplier |
|---|---|---|---|
| 2/12 | 9.46° | 16.7% | 1.0138 |
| 3/12 | 14.04° | 25.0% | 1.0308 |
| 4/12 | 18.43° | 33.3% | 1.0541 |
| 5/12 | 22.62° | 41.7% | 1.0833 |
| 6/12 | 26.57° | 50.0% | 1.1180 |
| 7/12 | 30.26° | 58.3% | 1.1577 |
| 8/12 | 33.69° | 66.7% | 1.2019 |
| 9/12 | 36.87° | 75.0% | 1.2500 |
| 10/12 | 39.81° | 83.3% | 1.3017 |
| 11/12 | 42.51° | 91.7% | 1.3566 |
| 12/12 | 45.00° | 100% | 1.4142 |
Conventional house roofs commonly fall between about 4/12 and 9/12. A 12/12 roof is a 45° roof: it rises one foot for every foot of run.
Minimum pitch by roofing material
The International Residential Code (IRC, chapter 9) sets minimum slopes for each roof covering. Manufacturers' installation instructions can be stricter, and your local code governs, so check both before you choose a material.
| Roof covering | Minimum slope |
|---|---|
| Asphalt shingles | 2/12; from 2/12 up to 4/12, a double layer of underlayment is required |
| Clay and concrete tile | 2½/12, with extra underlayment below 4/12 |
| Metal shingles | 3/12 |
| Slate | 4/12 |
| Standing-seam metal panels | ¼/12 |
| Membranes (EPDM, TPO, modified bitumen) | ¼/12 for drainage |
Walkability and safety
OSHA treats any roof steeper than 4/12 as a "steep roof", where workers 6 ft or more above a lower level need fall protection. As a practical guide, roofs up to about 4/12 are easy to walk; 5/12 and 6/12 can be walked carefully in dry weather with soft-soled shoes; from 7/12 (about 30°) roofers generally use roof jacks, toe boards and harnesses; and 9/12 and steeper is hard to stand on at all. Dew, frost, dust and loose granules make any pitch slippery. If you're not equipped for the pitch you've measured, get a pro — and for stair or deck slopes, use the stair calculator instead.
Frequently asked questions
What is a 6/12 roof pitch in degrees?
A 6/12 pitch is 26.57°. It rises 6 inches per 12 inches of run, a 50% slope, with a pitch multiplier of 1.118.
How do I convert degrees to roof pitch?
Take the tangent of the angle and multiply by 12. For 30°, tan(30°) = 0.577, and 0.577 × 12 = 6.93, so the pitch is about 7/12.
What is the minimum pitch for asphalt shingles?
Under the IRC, 2/12. Between 2/12 and 4/12 a double layer of underlayment is required; 4/12 and steeper is the standard range. Always follow the shingle manufacturer's instructions and your local code.
How do I calculate rafter length from pitch?
Multiply the horizontal run (including any overhang) by the pitch multiplier. For a 6/12 roof with a 12 ft run and 1 ft overhang: 13 × 1.118 = 14.53 ft, before ridge and seat cuts.
What roof pitch is walkable?
Roofs up to about 4/12 are easy to walk, and 5/12–6/12 can be walked with care when dry. From about 7/12, roofers use roof jacks and fall protection. OSHA classes anything steeper than 4/12 as a steep roof.