CutRoof / The setting-out notebook

Check a gable cutting schedule before cutting the first rafter

Work through a 6000 mm span at 30 degrees, then check the lengths, quantity and bevel references against one set of inputs.

Check a gable schedule against the measured span, the ridge thickness and the horizontal overhang before marking timber. In this example, a 6000 mm span at 30 degrees gives a 3442 mm common-rafter length between the ridge and birdsmouth plumb lines. A 300 mm horizontal overhang takes its finished top-edge length to 3789 mm.

A CutRoof guide for carpenters and apprentices. CutRoof puts the inputs, declared conventions and workings beside the cutting schedule, so you can follow each figure back to the roof it describes.

Published and checked 9 October 2026 by CutRoof editorial. This is an illustrative geometry example, independently calculated for this guide; it is not a record of a roof fitted on site.

Write down one complete set of inputs

This example uses a symmetrical gable roof with straight, level plates and equal pitches. Check the span at both ends and the plan for square before using those assumptions.

Span: 6000 mm, measured across the outside faces of the wall plates.
Plate length: 7200 mm, measured along the ridge direction.
Pitch: 30 degrees on each slope.
Ridge board thickness: 38 mm.
Horizontal overhang: 300 mm, from the outside plate face to the tail plumb cut.
Rafter centres: 400 mm along the plate.
Example rafter section: 47 mm wide by 147 mm deep; wall plate width: 100 mm.

The section sizes and centres are inputs for the example, not timber-sizing recommendations. Use the specified sections and details for the actual job. The calculation here measures the run to the ridge face, uses CutRoof's seven-tenths birdsmouth setting, states bevels against the rafter edge and rounds displayed lengths to the nearest millimetre.

Keep the plate length and span in separate boxes. The 7200 mm dimension controls this example's rafter count and gable ridge length. The 6000 mm dimension controls the common rafter's horizontal run.

Check the ridge deduction in the working

Half the span is 3000 mm. Half the 38 mm ridge thickness is 19 mm, so the horizontal run to the ridge face is 2981 mm.

Run = 6000 ÷ 2 − 38 ÷ 2 = 2981 mm

The common-rafter length between the two plumb lines follows the top edge:

True length = 2981 ÷ cos 30° = 3442 mm

The unrounded result is 3442.162 mm. That length already includes the ridge deduction. Taking another 19 mm off the timber would apply a second deduction, and would also confuse a horizontal distance with a distance along the slope.

As a check on the convention, the undeducted half-span length is 3464 mm. The 19 mm deduction in plan shortens the unrounded sloping length by 21.939 mm. A calculator measuring to the theoretical centre line will therefore give a different answer from one measuring to the ridge face.

Keep the horizontal overhang separate from the tail

The 300 mm overhang is measured level. Its distance along the rafter is 346 mm:

Tail = 300 ÷ cos 30° = 346 mm

Calculate the finished total at full precision, then round it once:

Overall = (2981 + 300) ÷ cos 30° = 3789 mm

Adding the separately rounded 3442 and 346 gives 3788 mm. The unrounded total is 3788.572 mm, which rounds to 3789 mm. Use the overall figure when marking the finished top edge; choose stock and trimming allowances separately. This total includes neither saw kerfs nor spare timber for fitting.

Check the quantity against the layout

For this exact 7200 mm plate length at 400 mm centres, CutRoof's gable count is:

Bays per side = 7200 ÷ 400 = 18

Positions per side = 18 + 1 = 19

Commons across both slopes = 19 × 2 = 38

There is a position at each end of this modelled run, which is why 18 bays require 19 positions. Compare those positions with the actual setting-out drawing, including its end details and any openings. The app's count does not add trimmers, verge framing or other members outside its model.

The gable ridge-board row uses the full 7200 mm plate length. Subtracting the 6000 mm span from it is a hipped-roof calculation and would give the wrong ridge row for this gable. Confirm the end detail and any cutting or jointing allowances separately.

Read the cut reference before setting the bevel

With the sliding bevel's stock against the rafter edge, this 30 degree roof uses a 60 degree plumb cut and a 30 degree seat cut. From a line squared across the timber, the readings are plumb 30 degrees and seat 60 degrees.

Those are two references for the same cut lines. A saw's scale may use another reference: translate the required line to the tool's own scale rather than copying a schedule number without checking it.

The setting-out rise is 1732 mm, calculated over the undeducted half span. It is the climb to the theoretical apex from the rafter's top-edge level at the plate face. Physical plate-to-ridge heights also depend on the height above plate and the ridge detail. Compare the separately labelled height figures; do not use 1732 mm as a plate-to-board height.

Check the first rafter before copying the set

Use one consistent top-edge reference for the ridge, birdsmouth and tail marks. Check the birdsmouth and height above plate against the specified detail, and check one pattern rafter under the site's fitting arrangements before reproducing it.

Confirm the job inputs and convention labels match the schedule you are using.
Check 3442 mm between the ridge and birdsmouth plumb lines, and 3789 mm overall, for this example.
Check the plumb and seat lines against the declared bevel reference.
Compare the modelled 19 positions on each slope with the drawing's actual layout.

Try these inputs in the free rafter calculator, then follow the common-rafter working in CutRoof and save the job with its conventions. The common-rafter, ridge-deduction, HAP and cut-angle guides linked below explain the individual references when a check needs 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