Rafter Calculator

Common Questions

How do I calculate rafter length?

Use the Pythagorean theorem: rafter length = √(run² + rise²), then subtract half the ridge-board thickness (projected along the slope) for the plumb cut at the ridge. For a 12′ run at a 6/12 pitch, run = 12, rise = 6 (= 12 × 6 ÷ 12), rafter length ≈ 13.36′ ≈ 13′ 4-1/4″.

What does "6/12 pitch" mean?

6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run — a 26.57° slope angle. The denominator is always 12 by convention, so a "6 pitch" and "6/12 pitch" mean the same thing.

What pitch should I use for my roof?

3/12 to 4/12 = low-slope (will not shed snow well, needs membrane roofing). 5/12 to 8/12 = standard residential, works with shingles. 8/12 to 12/12 = steep, sheds snow and water aggressively, costs more in materials. 12/12 is exactly 45°.

Why does the calculator deduct half the ridge board thickness?

Common rafters meet at the ridge board, which is typically 1.5″ thick (or 45 mm metric). Each rafter's plumb cut sits against half of that thickness — so the rafter's actual length is shorter than the bare hypotenuse by that amount, projected along the rafter's slope.

Does this include the rafter tail / overhang?

No. This calculates from the wall plate to the ridge only. Add your overhang length separately (typically 12–24″ for eaves). The full board length you order = ridge length + overhang + cut waste.