Calculate common rafter length from run and rise in feet, inches or meters, with an optional overhang, plus roof pitch and length per foot of run.
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Common Rafter Length Formula
The common rafter length is found with the Pythagorean theorem. The run and rise form the two square sides of a right triangle, and the common rafter length is the sloped side.
- CRL = common rafter length
- Run = horizontal distance covered by the rafter
- Rise = vertical height from the top of the wall plate to the ridge line
Choose the value to solve for. If you enter the run and rise in the line-length mode, the calculator finds the common rafter line length. If you enter the rafter length and rise, it finds the run. If you enter the rafter length and run, it finds the rise. The values can be entered in inches, feet, centimeters, or meters, but the formula requires all measurements to be in the same unit before calculation.
Common Roof Pitch Length Factors
The table below shows the approximate rafter length for each 12 inches of horizontal run. This is useful when you know the roof pitch.
| Roof Pitch | Rise per 12 in Run | Rafter Length per 12 in Run | Length Factor |
|---|---|---|---|
| 3/12 | 3 in | 12.37 in | 1.031 |
| 4/12 | 4 in | 12.65 in | 1.054 |
| 6/12 | 6 in | 13.42 in | 1.118 |
| 8/12 | 8 in | 14.42 in | 1.202 |
| 12/12 | 12 in | 16.97 in | 1.414 |
Measurement Notes for Common Rafters
| Measurement | What It Means |
|---|---|
| Run | Horizontal distance from the outside wall line to the center ridge line. For a simple gable roof, this is usually half the building span. |
| Rise | Vertical height from the wall plate level to the ridge level. |
| Common rafter length | The sloped distance from the wall line to the ridge line before adding overhang, ridge adjustment, or cutting allowance. |
Example Calculations
Example 1: Find common rafter length
You have a roof run of 12 ft and a rise of 6 ft.
The common rafter length is approximately 13.4164 ft.
Example 2: Find rise
You have a common rafter length of 15 ft and a run of 12 ft.
The roof rise is 9 ft.
FAQ
Does common rafter length include the roof overhang?
No. This calculation gives the sloped length based on the run and rise you enter. If your run stops at the outside wall line, the result does not include overhang. An overhang that follows the same roof slope adds both horizontal run and the corresponding vertical change: its sloped length is horizontal overhang × √(1 + (rise/run)2), for positive run. Add this sloped length separately; do not increase run while holding rise unchanged. Ridge deductions and cutting allowances are separate.
Is the run the same as the building width?
Usually not. For a simple gable roof, the run is typically half the building width because the rafter runs from one wall to the center ridge. For example, a 24 ft wide building usually has a 12 ft run.
Why must the rafter length be longer than the run or rise?
The common rafter length is the hypotenuse of a right triangle, so it is at least as long as either leg; equality gives degenerate geometry with a zero other leg. If the rafter length you enter is shorter than the run or rise, the triangle is not possible and the missing value cannot be calculated correctly.
