ABC Tools
Roof Pitch & Rafter Calculator illustration

Roof Pitch & Rafter Calculator

Converts a roof pitch between rise-per-12, degrees and percent, and computes the common rafter length (with overhang), plumb and seat cut angles, the hip/valley rafter length, the ridge height above the wall plate and how many rafters the roof needs.

Read the full guide to this tool

6/12 ≈ 26.6° is a common comfortable pitch.
Outside of one wall plate to outside of the other.
Each rafter run shortens by half of this; 0 for a ridge-less (butted) pair.
Horizontal eave projection past the wall.
406 mm = 16 in on centre; 610 mm = 24 in.
Along the ridge, to count the rafters needed.

Notes

  • Rafter length is the line length along the top edge: run ÷ cos θ. The plumb cut is marked at the pitch angle from square, the seat (level) cut at 90° minus it.
  • Hip and valley rafters run diagonally, so their horizontal run is √2 × the common run — the framing square trick of using the rise against 17 instead of 12.
  • The ridge height is geometric (to the rafter's top edge line); cutting a bird's mouth seat drops the rafter by the seat depth, so set heights from a test rafter.

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