How to calculate rafter length?
Answer
Square the horizontal run and the rise. Add the values. Calculate the square root.
Add the sloped tail length when the rafter extends beyond the wall.
| Pitch | Rise | Line length | Multiplier |
|---|---|---|---|
| 3/12 | 3 ft | 12.369 ft | 1.0308 |
| 4/12 | 4 ft | 12.649 ft | 1.0541 |
| 6/12 | 6 ft | 13.416 ft | 1.1180 |
| 8/12 | 8 ft | 14.422 ft | 1.2019 |
| 12/12 | 12 ft | 16.971 ft | 1.4142 |
The table gives lengths from the wall plate to the ridge. Add the sloped tail length for an overhang.
Rafter length formula
Rafter line length = square root of (run squared + rise squared).
Multiply the run by the pitch multiplier in the table.
Gable roof run
- Divide the building span by two.
- Divide the ridge thickness by two.
- Subtract half the ridge thickness from half the building span.
Overhang length
Measure the overhang horizontally from the outside wall. Multiply this run by the pitch multiplier.
Add the result to the rafter line length. The rafter calculator calculates the overhang length.
Example
A 12 ft run at 6/12 has a 13.416 ft line length. A 12 in overhang adds 1.118 ft.
Important information
- Measure the run horizontally.
- Include the ridge-board correction for a gable roof.
- Use the measured lumber size for the birdsmouth.