Rafter Length Calculator
Rafter length is a hypotenuse, not a subtraction. Enter your span and pitch to get the true slant length, how many rafters you need at 16 inches on center, the ridge board length, and the birdsmouth cuts.
Results
Enter your measurements above and press Calculate.
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Questions people ask
How do I calculate rafter length?
Rafter length = the hypotenuse of half the span and the rise. Rise = half span × pitch ÷ 12. For a 16 foot span at 6:12, half span is 8 feet, rise is 4 feet, so the rafter is 8.94 feet.
What is a birdsmouth cut?
It is the L-shaped notch where the rafter sits on the wall plate. The vertical leg is the seat cut, the diagonal leg is the plumb bevel. The classic 47.5 degree rule is a memory aid, not a formula — angle the saw to half the roof pitch.
How many rafters do I need?
At 16 inches on center, one rafter every 16 inches plus one at each end. For a 16 foot span that is 13 rafters. They go on both slopes, so double it, then add the ridge board separately.
How long should the tail overhang be?
Between 6 and 12 inches is typical for a shingle roof; up to 24 inches for exposed or high-wind areas. Longer overhangs look good but need a stronger rafter tail to resist droop.
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