CalcArcade.

Rafter Length Calculator

Enter the building span, the roof pitch as rise per 12 inches of run, and the overhang. You get the rafter length along the slope, the rise at the ridge and the roof angle.

Your numbers

Result

How we calculate it

A common rafter runs from the ridge to the wall line. Its run is half the span, its rise is the run × pitch ÷ 12, and its length is the run × the slope factor √(1 + (pitch ÷ 12)²). An overhang measured level is lengthened by the same slope factor.

For a 24 foot span at a 6 in 12 pitch, the run is 12 feet, the rise is 6 feet, the slope factor is 1.118 and the common rafter is 13.42 feet. A 12 inch overhang adds 1.118 feet, so the total is 14.53 feet (14 ft 6.4 in). The roof angle is 26.6°.

Assumptions and limits

  • The result is the length along the top edge from the ridge line to the end of the overhang. Take off half the thickness of the ridge board, and allow for the birdsmouth cut and some waste.
  • This is for a simple gable roof with equal slopes on both sides. Hip and valley rafters are longer and need other calculations.
  • Check your plans and local code. A structural designer decides the size and spacing of the rafters.

Frequently asked questions

How do I calculate rafter length?

Halve the span to get the run, then multiply by √(1 + (pitch ÷ 12)²). For a 6 in 12 pitch the multiplier is 1.118.

What does a 6 in 12 pitch mean?

The roof rises 6 inches for every 12 inches it runs horizontally. That is a slope of 1 in 2 and an angle of about 26.6 degrees.

Is the overhang measured level or along the rafter?

Most plans give it as a level distance from the wall. This calculator takes it that way and adds the extra length for the slope.

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Worked example (a fixed example using the starting numbers, it does not change as you type)

Inputs: Building span (feet) = 24; Roof pitch (rise in 12 inches of run) = 6; Overhang (inches, measured level) = 12.

  • Total rafter length 14.53 ft
  • Total rafter length (feet and inches) 14 ft 6.4 in
  • Common rafter length 13.42 ft
  • Overhang along the rafter 1.12 ft
  • Run (half the span) 12.00 ft
  • Rise at the ridge 6.00 ft
  • Roof angle 26.6 degrees
  • Slope factor 1.118

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Last reviewed: . Formulas and assumptions are described on our methodology page. For information only; not professional advice.