Setting out, explained

CutRoof / The setting-out notebook

How a common rafter is set out

A 4800 mm span at 35 degrees: ridge deduction, true length, tail, rise reference and the two cut angles.

For a symmetrical roof with a 4800 mm span, a 32 mm ridge board and a 35 degree pitch, the common rafter measures 2910 mm from the ridge plumb cut to the birdsmouth plumb line. A 450 mm horizontal overhang takes the total to 3460 mm.

A CutRoof guide for carpenters and apprentices. CutRoof shows the working and declared conventions beside its cutting schedules.

Original guide retained; corrected 1 October 2026 by CutRoof editorial. Geometry checked independently; no professional site review is claimed.

Measure the horizontal run

Measure the span across the outside faces of the two wall plates. Check the plates for square and level and measure the span at both ends. This example assumes equal pitches and straight, level plates.

Half the span is 2400 mm. The ridge board occupies 32 mm across the centre line, so deduct half its thickness in plan:

Run = 4800 ÷ 2 − 32 ÷ 2 = 2384 mm

That deduction is already in CutRoof's default common-rafter length. Do not take it off again at the bench.

Calculate the length between the plumb lines

True length follows the rafter's top edge, from the ridge plumb cut to the plumb line through the plate's outside face at the birdsmouth:

True length = 2384 ÷ cos 35° = 2910 mm

Keep full precision during calculation; round displayed lengths to the nearest millimetre here. The unrounded length is about 2910.326 mm.

Add the tail along the slope

The overhang is horizontal, from the plate's outside face to the tail plumb cut. Convert that 450 mm to a sloping tail length:

Tail = 450 ÷ cos 35° = 549 mm

Overall = (2384 + 450) ÷ cos 35° = 3460 mm

The unrounded total is about 3459.675 mm. Adding the separately rounded 2910 and 549 gives 3459; calculate the total before rounding. This is a finished top-edge length, with no allowance for saw kerfs or trimming stock.

Keep rise separate from plate-to-ridge height

The setting-out rise is the climb over the undeducted half span:

Rise = 2400 × tan 35° = 1680 mm

It measures the climb from the rafter's top-edge level at the plate face to the theoretical apex. The top edge already stands above the plate by its height above plate (HAP). A physical height from the plate top therefore also needs HAP. A ridge board's top has a further adjustment for its thickness. Use the separately labelled height figures in CutRoof; 1680 mm alone is not a plate-to-board height.

State the reference for both cuts

With a sliding bevel's stock against the rafter's edge, the plumb cut is 55 degrees and the seat cut is 35 degrees. These are CutRoof's default readings. Measured from a line squared across the timber, they become plumb 35 and seat 55. Read the convention before setting the bevel or saw.

Mark the ridge, birdsmouth and tail plumb lines from the same top edge. Check the birdsmouth and HAP against the specified detail and fit one pattern rafter before copying the set. Timber sizing and structural detailing need their own design.

Check this span, pitch, ridge and overhang in the free rafter calculator, then compare the working with your CutRoof schedule. The ridge-deduction, HAP and cut-angle guides explain each reference in more detail.

CutRoof shows the workings and declared conventions behind its cutting schedules. Check them against the dimensions and specified detail for your job.

Try the rafter calculator

More on setting out