Rafter length calculator
Common rafter on a gable roof, in millimetres, under UK conventions. Every figure comes with the sum that produced it.
How do you calculate common rafter length?
For equal roof pitches, divide half the span minus half the ridge thickness by the cosine of the pitch. This measures along the rafter from the ridge plumb cut to the birdsmouth plumb line. Add the horizontal overhang divided by the same cosine to include the tail.
Default example: a 4800 mm span, 35 degree pitch and 32 mm ridge give a run of 2384 mm and a true length of 2910 mm. A 450 mm horizontal overhang takes the overall length to 3460 mm. These lengths are rounded to the nearest millimetre. Follow the complete worked example.
Change the inputs below for your roof. The live results use those inputs; the example above stays fixed.
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 the limit the standard UK roofing tables set, and 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.