Rafter Length Calculator
Works out common rafter length from the span and pitch, including the overhang and both cut angles, plus how many rafters the roof needs.
From the ridge to the end of the overhang, measured along the rafter.
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The formula
run = span ÷ 2 − ridge thickness ÷ 2 rafter = run × slope factor + overhang × slope factor plumb cut = pitch angle, seat cut = 90° − pitch angle
A rafter is the hypotenuse of a triangle whose base is the run and whose height is the rise, so the slope factor converts one into the other directly. The two cut angles are complementary: the plumb cut matches the pitch, and the seat cut is what is left of the right angle.
What to watch out for
Three things people get wrong on rafters:
- Half the ridge comes off the run. The rafter stops at the face of the ridge board, not the centre line. On a 45 mm ridge that is 22.5 mm per rafter — small, but it shows at the apex.
- The tail is measured along the slope. A 300 mm horizontal overhang is 335 mm of rafter at 6/12, not 300.
- Cut one and test it. Make a pattern rafter, offer it up, check both cuts, then use it to mark the rest.
These are common rafters. Hip and valley rafters run diagonally and are longer — their slope factor is based on a 17-unit run rather than 12, and they are a separate calculation.
Frequently Asked Questions
How do I calculate rafter length?
Take half the span, subtract half the ridge thickness, and multiply by the slope factor for your pitch. Then add the overhang multiplied by the same factor. An 8 m span at 6/12 with a 300 mm overhang gives a rafter of about 4.78 m.
What angle do I cut a rafter at?
The plumb cut at the ridge equals the pitch angle — 26.57° for a 6/12 roof. The seat cut at the wall plate is the complement, 63.43°. Most mitre saws are set from the plumb cut.
How many rafters do I need?
Divide the roof length by the spacing and add one, then double it for both slopes. A 10 m roof at 400 mm centres needs 26 per slope, so 52 in total.