Roof Pitch Calculator
Convert roof pitch rise/run factors into angles, grade slopes, and estimate rafter lengths.
Slope Metrics
Roof Pitch
4:12 (Flat/Low)
Slope Angle
18.43°
Rafter & Grade Details
| Rafter Length (without overhang) | 15.81 Feet |
| Slope Grade Percentage | 33.33% |
| Rafter Length per Foot | 12.65 Inches |
Understanding Roof Pitch Calculations
Roof pitch represents the steepness of a roof. It is traditionally written in the US as a ratio of vertical **Rise** inches over a fixed horizontal **Run** of 12 inches (e.g. "4:12" pitch represents 4 inches of vertical rise for every 12 inches of horizontal run).
1. Calculate Angle (Degrees):
The incline angle is the arctangent of the rise-to-run slope ratio:
Angle (θ) = Arctan(Rise / Run) * (180 / π)
2. Solve Grade Percentage:
Grade (%) = (Rise / Run) * 100
3. Solve Rafter Length:
If you specify the actual horizontal run length of the rafter span ($L_h$), the rafter length ($L_r$) is resolved using the Pythagorean theorem:
Total Rise = L_h * (Rise / Run)
Rafter Length = Sqrt( L_h^2 + Total Rise^2 )
Rafter Length = Sqrt( L_h^2 + Total Rise^2 )