A common rafter runs from the top of the wall to the ridge. Its length is the hypotenuse of a right triangle whose legs are the run (half the building span) and the rise (run × pitch ÷ 12). Add the overhang, also measured along the slope, and you have the board length to cut.
Formula
Run = span ÷ 2. Rafter multiplier = √(1 + (pitch ÷ 12)²). Rafter length = (run + overhang) × multiplier. Ridge rise = run × pitch ÷ 12.
Worked example
A 24 ft wide garage with a 6:12 roof and 12" overhangs: run = 12 ft, multiplier = 1.118, rafter = 13 × 1.118 ≈ 14.53 ft, about 14' 6-7/16". Buy 16 ft stock. The ridge sits 6 ft above the top plates (less the ridge-board deduction).
Cuts to make
- Plumb cut at the ridge, at the roof angle (26.57° for 6:12) — set on a framing square using 6 and 12.
- Birdsmouth where the rafter sits on the wall plate: a seat cut no deeper than one-third of the rafter depth.
- Tail cut at the overhang, usually plumb for fascia.
Lay out one pattern rafter, test-fit it on both sides of the building, then use it as a template for the rest.
Watch: how it is done
Video by seejanedrill on YouTube. More in our video library.
Frequently asked questions
What is the ridge-board deduction?
Because rafters butt against a ridge board, shorten each one by half the ridge thickness measured perpendicular to the plumb cut — 3/4" for a 1-1/2" ridge board.
How deep can a birdsmouth be?
A common rule is no deeper than one-third of the rafter depth, so the rafter keeps enough wood above the seat to carry the load.
Is this calculator suitable for hip rafters?
No. Hip and valley rafters run at 45° in plan and use a different multiplier. This tool covers common rafters only.
Results are planning estimates. For structural work, confirm requirements with your local building authority and a qualified professional. See our disclaimer.