Rafter length calculator
Common rafter on a gable roof, in millimetres, under UK conventions. Every figure comes with the sum that produced it.
The figures
How they were worked out
This page does the common rafter on a gable. The app does hips, jack rafters, crown rafters and the ridge as well, on gable, hipped and lean-to roofs, and prints the lot as an A4 cut schedule you can take up the roof.
The conventions these figures assume
Where a carpenter makes a choice, this page picks the UK default and says so. The app lets you change any of them and prints which it used.
- The run stops half a ridge thickness short of the centre line. Why your length already includes it.
- The birdsmouth leaves seven tenths of the rafter depth, with the seat capped at the plate width. That is Goss's limit, which is the stricter of the two rules in use. How deep the notch goes.
- Bevels are stated against the edge of the timber, the way the reckoner tables state them. Reading cuts off a square.
- Height above plate is measured to the rafter's top edge at the plate's outside face. Why HAP is the number to check.
Is it right?
The arithmetic on this page is checked against CutRoof's own engine rather than written twice and hoped over. The engine exports 211 worked cases spanning 2400 to 11000 mm and 12.5 to 52.5 degrees, and this page has to reproduce every figure in all of them before it ships. The engine itself is written twice by two independent methods and cross-checked over ten thousand roofs.
CutRoof calculates geometry from your dimensions. Structural design and timber sizing are the responsibility of a competent person.