How to calculate roof pitch?
Answer
Divide the vertical rise by the horizontal run. Multiply the result by 12 to get the rise for a 12-unit run.
A 6 ft rise with a 12 ft run gives a 6/12 pitch and a 26.57-degree angle.
| Pitch | Angle | Slope | Rafter multiplier |
|---|---|---|---|
| 3/12 | 14.04 degrees | 25% | 1.0308 |
| 4/12 | 18.43 degrees | 33.33% | 1.0541 |
| 5/12 | 22.62 degrees | 41.67% | 1.0833 |
| 6/12 | 26.57 degrees | 50% | 1.1180 |
| 8/12 | 33.69 degrees | 66.67% | 1.2019 |
| 12/12 | 45.00 degrees | 100% | 1.4142 |
Multiply the horizontal run by the rafter multiplier to get the rafter line length.
Measure the rise and run
- Measure the horizontal run from the wall plate to the ridge centerline.
- Measure the vertical rise across the same horizontal run.
- Divide the rise by the run.
- Multiply by 12 to get the rise for a 12-unit run.
Calculate the roof angle
Divide the rise by the run. Calculate the inverse tangent of this value.
The result is the roof angle in degrees. A 4/12 pitch gives 18.43 degrees.
Use the pitch
The pitch controls rafter length, plumb cuts, seat cuts, and roof-material coverage.
Use the rafter calculator with rise, degrees, or percent.
Important information
- Use the same unit for rise and run.
- Measure the structural roof line.
- Use the approved plans for structural dimensions.