CutRoof / The setting-out notebook

Check your roof calculator inputs before cutting a rafter

Audit a 4800 mm gable one input at a time, with worked results for half-span entry, a doubled ridge deduction and a missing tail.

Enter the full outside-plate span, let the calculator make its declared ridge deduction, and check whether the result includes the tail. On the example below, the correct run to the ridge face is 2375 mm and the overall rafter length is 3089 mm. Entering half the span in the span field produces a plausible-looking but much shorter 1703 mm overall result.

A CutRoof guide for carpenters and apprentices. CutRoof's free rafter calculator shows each step of the run, length and tail calculation. Use its working alongside your drawing to check what each input means before marking a pattern.

Published and checked 22 October 2026 by CutRoof editorial. These are original illustrative calculations, not measurements from a fitted roof.

Write down the intended inputs first

This example is a symmetrical gable, with equal pitches and a centred ridge. The plates are straight, level and parallel. Record:

Full span: 4800 mm between the outside faces of the wall plates.
Pitch: 30 degrees from horizontal.
Ridge board thickness: 50 mm.
Overhang: 300 mm horizontally from the outside plate face to the chosen tail plumb line.

All dimensions entered in the linked calculator are millimetres. The tail endpoint must match the actual fascia detail; check whether the drawing's dimension ends at the back or front of the fascia. Fitting allowance, saw kerfs and spare stock are separate from these geometry lengths.

The half span is 2400 mm. The centred ridge takes up 25 mm on this side, so:

Run to ridge face = (4800 − 50) ÷ 2 = 2375 mm

True length = 2375 ÷ cos 30° = 2742 mm

Tail along the slope = 300 ÷ cos 30° = 346 mm

Overall = (2375 + 300) ÷ cos 30° = 3089 mm

Keep full precision until the final display rounding. The unrounded true and tail lengths are approximately 2742.414 mm and 346.410 mm. Their sum rounds to 3089 mm; adding the displayed whole-millimetre parts gives 3088 mm. That one-millimetre difference comes from rounding each part first.

Four input checks at 30 degrees: intended inputs give 3089 mm overall; half-span entry gives 1703 mm; a doubled ridge deduction gives 3060 mm; zero overhang gives 2742 mm. — Change one field at a time. Bars compare the overall geometry lengths, using full precision before rounding each label to whole millimetres. They are not drawings of timber.

If the result is much too short, check span versus run

The span field takes 4800, not 2400. A calculator cannot know whether an otherwise valid number describes your full roof or only one side.

Leave the other inputs alone and enter 2400 mm as the span:

Wrong run = (2400 − 50) ÷ 2 = 1175 mm

Wrong overall = (1175 + 300) ÷ cos 30° = 1703 mm

This result belongs to a different, narrower roof. Restore the full 4800 mm span and confirm the working returns to 2375 mm run and 3089 mm overall. If a different tool asks for run, read whether it means undeducted half span or the distance to the ridge face before entering a number.

If the result is slightly short, check for a doubled deduction

In this calculator, the ridge field accepts the full 50 mm board thickness. The working deducts half of it from the half span. Keep the span at its measured 4800 mm.

Reducing the span yourself to 4750 mm and still entering a 50 mm ridge makes the allowance twice:

Wrong run = (4750 − 50) ÷ 2 = 2350 mm

Wrong overall = (2350 + 300) ÷ cos 30° = 3060 mm

Restore the span to 4800 mm. The additional 25 mm horizontal deduction had shortened the sloping rafter by about 28.868 mm before rounding. Taking a horizontal allowance directly off a sloping length would introduce a different error. The ridge-deduction guide explains that conversion.

If the result ends at the plate, check the tail

With the intended span, pitch and ridge, entering zero overhang gives 2742 mm overall. With no tail, overall and true length are equal. Restore the 300 mm horizontal overhang and overall returns to 3089 mm; true length stays at 2742 mm.

A tape reading of 300 mm along the rafter is a different measurement from this example's 300 mm horizontal overhang. At 30 degrees, a 300 mm sloping tail projects only about 260 mm horizontally. Check the dimension arrows, rather than copying the same number into a differently defined field. The calculator-comparison guide illustrates those endpoints.

Check pitch format before changing any dimensions

The free web calculator takes degrees. Enter 30 for this example. In the CutRoof app, pitch can also be supplied as rise:run or total rise; check the resolved angle in the working. Use the pitch-format guide when the drawing gives a ratio or a height. A height measured from the plate to the ridge board needs its references resolved before it can stand in for the setting-out rise.

Change one input at a time when tracing an unexpected answer. Keep the drawing, intended inputs and corrected working together so the next check starts from the same roof.

Transfer the checked result to a pattern

For this example, the linked calculator should show 2375 mm run, 2742 mm true length and 3089 mm overall, with a 346 mm tail in the working. Confirm the ridge face, birdsmouth plumb line and tail endpoint on the first pattern before copying identical rafters.

An input audit establishes geometry for the declared arrangement. Timber sizing, bearing, birdsmouth and HAP details still follow the design. Site fitting also needs the job's access and fall-protection arrangements; see HSE's roof-work guidance.

Enter the four intended inputs in CutRoof's free calculator, check the working, then use the gable cutting-schedule guide to continue to quantities and cut references.

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