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Rafter Length Calculator

Calculate rafter length from span, pitch, and overhang. Get rafter run, rise, diagonal length, and birdsmouth cut dimensions for roof framing.

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Inputs that matter
Building Span, Pitch (rise/12), Overhang
Output to expect
Rafter Length Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Rafter Run

Half the clear span from wall plate to ridge, minus half the ridge board thickness (typically 3/4″ to 1-1/2″).

Run = (Span − Ridge Board) ÷ 2

Rafter Rise

Vertical height from plate to ridge for the given pitch. A 6/12 pitch on 12 ft run gives 6 ft rise.

Rise = Run × (Pitch Rise ÷ 12)

Rafter Length

Hypotenuse of the right triangle plus overhang measured along the slope. Add tail length for eave projection.

Length = √(Run² + Rise²) + Overhang Extension

Updated: July 2026

Example Scenarios

Run = 11.92 ft, rise = 3.97 ft, rafter body ≈ 12.56 ft plus overhang tail ≈ 1.06 ft for 12″ eave.

Smaller span with steeper pitch: run 8 ft, rise 4 ft, rafter length ≈ 8.94 ft before overhang. Use for cutting list on 10 ft or 12 ft lumber.

Ridge beam designs may use full run without ridge board subtraction — adjust span input per engineered plan.

FAQ

Yes for conventional ridge board framing — each rafter shortens by half the ridge thickness. Engineered ridge beams may use full run to beam centerline per plan.

Overhang is horizontal eave projection from the wall plate. Convert to slope length along the rafter using pitch factor for the tail cut.

Round rafter length up to next standard length (10, 12, 14, 16 ft). Long rafters may need splices or engineered trusses instead.

Calculated length is to the theoretical plate line. Birdsmouth seat cut reduces effective bearing depth — lay out cuts on a scrap before production cuts.

This calculator gives common rafter length. Hip and valley rafters require adjusted run from corner geometry — use dedicated hip rafter formulas or tables.

About Rafter Length Calculator

Determine common rafter length for stick-framed roofs before cutting lumber. Enter building span, pitch, and overhang to get rafter run, rise, and true diagonal length including tail for fascia overhang.