Rafter Length Calculator
Enter the building span and roof pitch to get the length of a common rafter from ridge to wall, the extra length for the overhang, the rise and the roof angle.
Result
- Rafter length in feet and inches
- 13 ft 4.16 in
- Overhang length along the slope
- 13.42 in
- Total length in feet and inches
- 14 ft 5.57 in
- Run
- 143.25 in
- Rise
- 71.63 in
- Roof angle
- 26.57°
Lengths are measured along the rafter line from the ridge cut to the outside of the wall plate. Rafter sizing and spacing must follow your local span tables.
Formula
- Run = (span − ridge thickness) ÷ 2
- Rafter length = run × √(1 + (pitch ÷ 12)²)
- Rise = run × pitch ÷ 12
- Angle = arctan(pitch ÷ 12)
How the Rafter Length Calculator works
A common rafter is the hypotenuse of a right triangle whose base is the run and whose height is the rise. The run is half the building span, less half the thickness of the ridge board.
For a pitch of 6 in 12, each foot of run adds 13.416 inches of rafter. The overhang is extended along the same slope, so a 12 inch horizontal overhang adds 13.42 inches of rafter tail.
Worked example
For a 24 ft span, 6 in 12 pitch and 1.5 in ridge: run = (288 − 1.5) ÷ 2 = 143.25 in. Rafter length = 143.25 × 1.1180 = 160.16 in, or 13 ft 4.16 in. A 12 in overhang adds 13.42 in, for a total of 173.57 in.
Frequently asked questions
How do I calculate rafter length from pitch?
Multiply the run by the square root of 1 plus the pitch divided by 12, squared. For a 6 in 12 pitch that multiplier is 1.118.
What angle is a 6 in 12 roof pitch?
A 6 in 12 pitch is 26.57 degrees. A 4 in 12 pitch is 18.43 degrees and a 12 in 12 pitch is 45 degrees.
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