Free rafter calculator · worked by SiteSum’s engine
Rafters, to the sixteenth.
Line length≈ 14' 5-9/16"
- Line length
- ≈ 14' 5-9/16"
- Rise
- 6' 0"
- Angle
- 26.57°
- Overhang (on slope)
- ≈ 1' 1-7/16"
The same engine, in your pocket
On site, with no signal.
This page runs the math from SiteSum, a feet-inch calculator for iPhone with ten builders like this one. It works offline, prints every answer to a paper tape and exports the job as a PDF. Pro is $24.99, once.
How it’s worked
- Slope factor = √(1 + (pitch ÷ 12)²), the length of rafter per inch of run.
- Line length = (run + overhang − half the ridge) × slope factor.
- Rise = run × pitch ÷ 12, plate to the ridge centre.
- Angle = the angle whose tangent is pitch ÷ 12. A 6/12 roof is 26.57°.
Give the pitch in degrees instead and it’s turned into x/12 first. Answers are given to the nearest sixteenth; ≈ means the exact length fell between marks.
What is the run?
The level distance from the outside of the wall plate to the centre of the ridge. On a gable roof with the ridge in the middle, it’s half the span.
Why take off half the ridge board?
The run is measured to the ridge’s centre, but the rafter stops at the ridge’s face. Half the ridge thickness comes off each side, measured level, which the calculator turns into length along the slope.
Is the overhang measured level or along the rafter?
Type it level, out from the wall, the way it shows on plans. The result gives it back along the slope too, so you can mark the tail.
Does the line length include the birdsmouth or the cuts?
It’s the length along the rafter’s line, from the tail to the ridge face. Mark the plumb cuts with a speed square set to the pitch or the angle shown, and choose a board long enough to take them.
More free calculators
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- All ten builders in the app: cut lists, stud layout, concrete, balusters and more.