How Roof Pitch Works
Pitch is the rise per 12 in of run: how many inches the roof climbs for each foot it goes across. A 6/12 roof rises 6 in for every 12 in.
Pitch = rise ÷ run × 12 · Angle = arctan(rise ÷ run) · Slope factor = √(1 + (rise ÷ run)²)
Worked example. A roof that rises 6 in over 12 in is a 6/12 pitch: 26.6°, a 50% slope, with a slope factor of 1.118. A rafter spanning 12 ft level is about 13.4 ft long along the slope, before cuts and overhang.
Common Pitches
| Pitch | Angle | Slope factor |
|---|---|---|
| 2/12 | 9.5° | 1.014 |
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
How to Measure Pitch Safely
You don’t need to climb onto the roof:
- In the attic: hold a level against the underside of a rafter, mark 12 in along the level, and measure straight up to the rafter.
- At a gable end: a ladder at the gable lets you hold a level and tape against the rake trim.
- With your phone: many level apps measure angle; enter it in degrees.
What Pitch Tells You
- Roof area: the slope factor turns your house’s footprint into roof area. The roofing calculator does this for you.
- Material choice: the Asphalt Roofing Manufacturers Association says asphalt shingles should never go on roofs below 2/12, and need special underlayment from 2/12 up to 4/12.
- Walkability: the steeper the roof, the harder and more dangerous it is to work on. Steep roofs need fall protection.
Frequently Asked Questions
What Is Roof Pitch?
Pitch is how many inches a roof rises for every 12 in it runs horizontally. A 6/12 roof rises 6 in over 12 in, an angle of about 26.6°.How Do I Measure Roof Pitch?
Hold a level horizontally against the roof (or a rafter in the attic), mark 12 in along it, and measure straight up from that mark to the roof. That measurement is the rise per 12.What Is a Slope Factor?
It’s the number you multiply a roof’s footprint by to get its actual sloped area. It equals √(1 + (rise/12)²), or 1.118 for a 6/12 roof.What Pitch Is Too Low for Shingles?
Anything below 2/12 is too low, according to the Asphalt Roofing Manufacturers Association. From 2/12 to under 4/12 shingles need special low-slope underlayment.Sources
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