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Roof Rafter Length Calculator

Enter the roof span and pitch to get rafter length, rise and the pitch angle.

How it is calculated

The tool is for a symmetrical gable roof. Enter the total span (the width between the outer walls) and the pitch as the rise per 12 units of run, so a 6/12 roof is entered as 6. The run is half the span: run = span / 2. Then rise = run × pitch / 12 and the common rafter is the hypotenuse, rafter = √(run² + rise²). The pitch angle is atan(pitch / 12) in degrees. Results come out in whatever unit you entered the span in; the 12 in the pitch is only a ratio. The length is measured along the rafter from the ridge to the outside of the wall plate. Overhang, ridge board thickness and birdsmouth depth are not added, so add the overhang separately.

Worked example

A building with a 24 ft span and a 6/12 pitch. Run = 24 / 2 = 12.00 ft. Rise = 12 × 6 / 12 = 6.00 ft. Rafter = √(12² + 6²) = √180 = 13.42 ft. The pitch angle is atan(6 / 12) = 26.57°. If the roof has a 1 ft horizontal overhang, you would add the length for that extra run to the 13.42 ft yourself before ordering stock.

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Common mistakes

  • Entering the pitch as an angle in degrees (such as 26.57) instead of the rise per 12: the tool treats it as rise per 12 units and gives an extremely steep roof.
  • Entering half the span instead of the full span: the tool halves it itself.
  • Forgetting the overhang, which adds length beyond the wall line that the result does not include.
  • Mixing units, such as a span in metres with the result read as feet.
  • Ordering lumber at the exact calculated length with no allowance for the ridge board, the birdsmouth cut and trimming waste.

About this calculator

A common rafter's length is found from the roof span and pitch: half the span gives the run, the pitch (often written as rise over a 12-inch run, like a 6/12 roof) gives the pitch angle via the arctangent of rise over run, and the rafter length itself equals the run divided by the cosine of that pitch angle — the hypotenuse of the right triangle formed by the run and the rise.

Builders, roofers and framing carpenters use this calculation to order the correct rafter stock length before cutting, to set the birdsmouth and plumb cuts at the right angle, and to check that a chosen pitch delivers enough headroom or the right roofline before construction begins, since getting the pitch angle wrong on paper means costly rework on the roof.

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