Rafter Length Calculator

Line length, and where the measurement actually starts

Rafter geometry is calculated between two theoretical points, and neither of them is where you put a saw.

The line length runs from the centreline of the ridge to the outside face of the wall plate, measured along the slope. It is pure trigonometry: the hypotenuse of a triangle whose legs are the run and the rise. Everything else is an adjustment to it.

The rafter does not reach the ridge centreline, because a ridge board occupies that space. Half its thickness comes off, measured along the slope rather than horizontally — which is why the shortening is slightly more than half the board thickness.

At the other end, the seat cut sits on the plate and the tail extends beyond it. The plate end is where the line length terminates; the tail is additional.

Skipping the ridge shortening makes every rafter long by the same amount, and the error does not cancel. Both rafters of a pair are long, so the pair either will not close at the ridge or forces the walls outward. It is one of the most common framing errors on a first roof.

Common, hip and valley rafters are different calculations

This calculator handles common rafters — the ones running perpendicular from plate to ridge. Hip and valley rafters run diagonally and use different arithmetic.

A common rafter rises its pitch over 12 inches of run, because it travels straight up the slope. A hip or valley rafter travels diagonally across the plan, so for every 12 inches it advances along the wall it also moves 12 inches perpendicular — a diagonal distance of about 17 inches.

That is why hip and valley layout uses a unit run of 17 rather than 12. An 8/12 common rafter has a slope factor of 1.202; the hip on the same roof works to 8 over 17, giving a much shallower apparent pitch and a longer rafter.

Jack rafters — the shortened commons that die into a hip or valley — have the same slope factor as the commons, but each is a different length, decreasing by a fixed common difference determined by the spacing and the hip run.

Framing squares have hip, valley and jack tables stamped on the blade for exactly this reason. If your roof has hips, the numbers on this page apply only to the common rafters within it.

Cutting from the calculation

Turning a length into cuts takes three angles, and two of them are the same number.

The plumb cut at the ridge is set to the pitch angle from vertical — 26.6 degrees for a 6/12 roof. The seat cut, where the rafter bears on the plate, is its complement. Both are marked from the same framing square setting: the pitch on the tongue against 12 on the blade.

The birdsmouth is the notch formed by the seat cut and its vertical face. Its depth matters: leaving less than about two thirds of the rafter depth above the notch weakens it in the same way a stringer notch does, and many jurisdictions limit it explicitly.

The tail cut can be plumb, square or level depending on the soffit detail, and it does not affect structure.

The reliable method is to cut one rafter, offer it up in place, confirm it fits at both ends, and only then use it as a pattern for the rest. A calculation gets you to a test piece, not to a stack of finished rafters.