How a common rafter is set out

Run, rise and true length, and where each one comes from on a 4800 span at 35 degrees.

Every common rafter on a roof comes off two numbers: the run and the rise. Get those right and the length, the cuts and the birdsmouth all follow.

The run

The run is the horizontal distance the rafter covers, from the outside face of the wall plate to the point where it stops at the ridge. On a symmetrical roof that starts as half the span.

Take a 4800 span, measured across the outside faces of both wall plates. Half of it is 2400. The ridge board is 32 thick, so half the ridge, 16, comes off the top: the run is 2384. That deduction is the app's UK default and it has its own article.

The rise

The rise is the vertical distance from the top of the wall plate to the apex. At 35 degrees on that span:

H = (S ÷ 2) × tan θ = 2400 × tan 35° = 1680 mm

Note the half span in there, not the deducted run. The apex sits on the centre line of the building whatever the ridge board is made of, so the rise never sees the ridge deduction. Only the length does.

The true length

True length is the sloping distance between the two plumb lines: the plumb cut at the ridge and the plumb line of the birdsmouth. It is the run divided by the cosine of the pitch.

TL = R ÷ cos θ = 2384 ÷ cos 35° = 2910 mm

That is the figure to set out along the top edge of the stick. It does not include the tail.

The tail

Overhang is measured horizontally, from the outside face of the plate to the plumb cut at the tail. With 450 of overhang at 35 degrees:

Tail = O ÷ cos θ = 450 ÷ cos 35° = 549 mm

Add it to the true length and the whole rafter is 3460 long before the birdsmouth is cut in.

The cuts

Two angles do the work. The plumb cut at the ridge is the pitch, 35 degrees. The seat cut is what is left of the right angle, 55 degrees. Both come off the square, and both are set out from the same pair of numbers the run and rise gave you.

On the stick

Mark the true length along the top edge. Square a plumb line down from each end. The birdsmouth sits on the lower plumb line, with its seat running back towards the tail. Cut the tail beyond it at the overhang length, plumb, so the fascia has a flat face to land on.

Every other member on the roof is worked out from this rafter. The hips, the jacks and the ridge all start from the run and rise you set out here, which is why a mistake in the span reads through the whole schedule.

Every figure above is worked out by the same engine that produces CutRoof's cut schedules, and is checked against it on every build.

Try the rafter calculator