Roof pitch explained
What 4/12 or 6/12 means, how to convert pitch to degrees, which pitches are walkable and how pitch changes roof area.
Roof pitch is how steep a roof is, written as rise over a 12-inch run. A 6/12 roof rises 6 inches for every 12 inches it runs horizontally.
Pitch, degrees and the multiplier
To convert pitch to degrees, take the arctangent of rise ÷ 12: a 6/12 roof is 26.6°. The multiplier (slope factor) is √(1 + (rise ÷ 12)²) — the number you multiply a flat footprint by to get the real roof surface.
| Pitch | Degrees | Multiplier | Extra area vs. flat |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | +1.4% |
| 3/12 | 14.0° | 1.031 | +3.1% |
| 4/12 | 18.4° | 1.054 | +5.4% |
| 5/12 | 22.6° | 1.083 | +8.3% |
| 6/12 | 26.6° | 1.118 | +11.8% |
| 7/12 | 30.3° | 1.158 | +15.8% |
| 8/12 | 33.7° | 1.202 | +20.2% |
| 10/12 | 39.8° | 1.302 | +30.2% |
| 12/12 | 45.0° | 1.414 | +41.4% |
Low slope, walkable and steep
- Low slope (up to 2/12): nearly flat. Usually needs membrane roofing rather than shingles.
- Walkable (3/12 to 6/12): the most common residential range. Installers can work without special equipment.
- Steep (7/12 and up): requires roof jacks and harnesses, so installation costs more per square. Many roofers add a steep charge.
Asphalt shingles generally need a slope of at least 2/12, and between 2/12 and 4/12 most codes and manufacturers require double underlayment. Check your local code and the shingle manufacturer’s instructions.
How to find your roof’s pitch
From a ladder, hold a level horizontally against the roof, measure 12 inches along it, and measure the vertical distance down to the roof. Or let a Horus report measure the pitch of every plane from aerial data. Convert any rise and run with the roof pitch calculator.