Calculate the length of the common rafters for a simple gable roof. Enter the span between the outside of the walls, the roof pitch and the horizontal overhang at the eaves.
This is the theoretical length measured along the top of the rafter. When cutting, deduct half the thickness of the ridge board from the run, and buy lumber in the next standard length up.
Formula
Rafter length = (Span ÷ 2 + Overhang) × √(1 + (Pitch ÷ 12)²)
Rise = Span ÷ 2 × Pitch ÷ 12.
Worked example
A 24 ft span with a 6/12 pitch and a 1 ft overhang: run = 12 ft, so the rafter is (12 + 1) × 1.118 = 14.53 ft (about 14 ft 6.4 in). The ridge sits 6 ft above the wall plates — buy 16 ft boards.
Frequently asked questions
What is rafter run?
The horizontal distance a rafter covers — half the building span for a symmetrical gable roof, not counting the overhang.
What size should rafters be?
That depends on span, spacing, roof load and wood species. Use your local building code’s span tables or ask an engineer.
Last updated · Calculations checked by automated tests· Editorial policy