Framing tool
Rafter Length Calculator
Enter the building span and roof pitch to get common rafter length, ridge height above the plate, and the board length you need including overhang.
Common rafter length equals the horizontal run multiplied by the pitch multiplier. On a 24 ft span at 6/12, the run is 12 ft and the rafter is 13.42 ft, or 13 ft 5 in, plus overhang.
Calculate rafter length
Run is half the span for a symmetrical gable roof.
Common rafter
15 ft 8 in
run + overhang, along the slope
How rafter length is calculated
A common rafter is the hypotenuse of a right triangle whose base is the run and whose height is the rise. Because pitch is expressed per 12 inches, the shortcut is run × √(1 + (rise/12)²), the same pitch multiplier used for roof area.
The run is measured from the outside of the wall to the centre of the ridge. On a symmetrical gable that is half the span, minus half the ridge board thickness if you are cutting precisely. The overhang extends along the same slope, so it is multiplied too. A 2 ft horizontal overhang adds 2.24 ft of board at 6/12, not 2 ft.
Hip and valley rafters
Hip and valley rafters run diagonally, so they use a 17 unit run instead of 12, because the diagonal of a 12 × 12 square is 16.97. Their length is the run multiplied by √(1 + (rise/17)²) × √2, which for a 6/12 roof over a 12 ft run gives about 17.4 ft.
Always cut one rafter, test-fit it in place, and use it as the pattern for the rest. A calculation error repeated forty times is an expensive afternoon.