About Rafter Length Calculation
The rafter length calculator helps you determine the correct rafter size for your roof construction. It uses basic geometry and roof pitch angles to find the accurate length from the wall plate to the roof ridge, including any overhang.
Rafter Length Formula
To calculate the rafter length for a gable roof:
Rafter Length = √[(½ × Span)² + (Rise)²]
where Rise = (½ × Span) × tan(Pitch Angle)
Then add overhang length if required.
Example Calculation
Suppose you have a 20 ft span, 30° roof pitch, and 1.5 ft overhang:
- Half Span = 10 ft
- Rise = 10 × tan(30°) = 5.77 ft
- Rafter Length (core) = √(10² + 5.77²) = 11.55 ft
- Total Rafter Length = 11.55 + 1.5 = 13.05 ft
Roof Pitch Reference Table
| Pitch (°) | Rise : Run | Tangent Value |
|---|---|---|
| 15° | 1 : 3.73 | 0.268 |
| 30° | 1 : 1.73 | 0.577 |
| 45° | 1 : 1 | 1.000 |
| 60° | 1 : 0.58 | 1.732 |
Key Tips for Accurate Rafter Cutting
- Use a framing square or rafter angle tool to mark cuts.
- Include seat and plumb cuts in total measurement.
- Account for ridge thickness if rafters meet at center.
- Check building codes for minimum slope and structural requirements.
Advantages of Correct Rafter Sizing
- Ensures stable and symmetrical roof framing.
- Prevents sagging or misalignment over time.
- Improves load distribution and roofing accuracy.
- Reduces material waste during construction.