Rafter Length Calculator
Calculate rafter length, roof truss dimensions, and roofing costs with our easy-to-use rafter length calculator.
Rafter length18 ft 2.3 in18.1913 ft including the overhang
Plate to ridge16 ft 8.41 in16.7006 ft, the body of the rafter
Tail beyond the plate1 ft 5.89 in16 in horizontal becomes 17.889 in along the slope
True run14 ft 11.25 inhalf the span less half the ridge board
Ridge height above the plate7 ft 5.63 in
Plumb cut angle26.565°set the saw to 26.57° for the ridge and tail cuts
Seat cut angle63.435°
Pitch multiplier1.118034
Rafters per slope31at 16 in centres over 40 ft
Rafters in total62both slopes of a gable roof
Linear feet of rafter1,127.86 ft
Roof area1,341.64 sq ftboth slopes, excluding overhangs
The method
rafter = √(run² + rise²), plus the overhang along the slope
Deduct half the ridge board
The rafter runs from the plate to the face of the ridge board, not to the centreline of the building. Half the ridge thickness comes off the run before you apply the multiplier — three-quarters of an inch for a 1½-inch ridge. Skip it and every rafter is long by about the same amount, which pushes the walls out.
The overhang is given horizontally but is cut along the slope, so it too gets multiplied. A 16-inch horizontal overhang on a 6 in 12 roof adds nearly 18 inches of rafter.