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Roof pitch and rafter calculator

For a simple pitched roof: the pitch or the rise, the rafter length including the overhang, the stock length to buy, how many rafters, and the roof area.

Free, and nothing you type leaves your device. Check the figures before you order.

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Worked example

A garage roof with a 5 m span at 35°, a 50 mm ridge board, 300 mm overhang, 8 m long with rafters at 400 mm centres:

3.39 m rafter length, with the overhang

  • Pitch 35.0°, rise 1.75 m
  • 3.02 m from the wall plate to the ridge board
  • Buy 3.6 m lengths
  • 21 rafters a side at 400 mm centres, 42 in all
  • 54.20 m² of roof slope, both sides, with the overhang

How the sum works

Half the span, less half the ridge board, is the level run of each rafter. The rafter length is the run divided by the cosine of the pitch; the overhang is added the same way. The rise is half the span times the tangent of the pitch, or, if you know the rise, the pitch is worked back from it.

The stock length is the next common timber length (2.4 m to 7.2 m in 0.6 m steps) with at least 50 mm to spare for the cuts.

What it doesn't do

It is for a simple duo-pitch roof. Rafter sizes, spans, bracing and the fixings come from the span tables, the engineer or the truss designer, and building control checks them. It doesn't size timbers.

Guides that go with it

How do I work out roof pitch and roof area?Work out roof pitch from rise and run, the rafter length by Pythagoras, and roof area as plan area divided by cos(pitch), with overhangs, hips and valleys.How do I do a quantity take-off from drawings?How to take off quantities from drawings for a small job: scale check, set order, deductions, waste and pricing, with a worked garden room example.What U-values does an extension need in England?The maximum U-values for new walls, roofs, floors, windows and doors in an extension in England, from Approved Document L Volume 1, and who checks them.

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