calcs.homes
calcs.homes
Enter the building width, roof pitch, and overhang and this rafter length calculator returns the common rafter line length, the tail (overhang) length, and the total board length to order.
Estimate only — add length for the plumb cut, bird's-mouth seat, and tail cut when ordering lumber.
The horizontal run of a common rafter is half the building width minus half the ridge board thickness. Multiply that run by pitch÷12 for the rise, then take the hypotenuse — √(run² + rise²) — for the line length from the ridge to the outside wall. The overhang tail is the horizontal overhang times the same slope factor. Adding the two gives the full board length.
This is the theoretical length to the measuring points. In the shop you still add material for the plumb cut at the ridge, the bird's-mouth seat cut over the wall plate, and a square or decorative tail cut, so order rafters a foot longer than the number here and cut to fit.
This is the line length — measured along the rafter from the long point at the ridge to the point where it crosses the outside face of the wall. Layout lines are struck from there.
Add roughly 8–12 inches per rafter so you have material for the plumb cut, the bird's mouth, and the tail. Buying the next standard length up almost always covers it.
Yes — each rafter run is shortened by half the ridge thickness so the two rafters plus the ridge span the full width. For a 1.5 in ridge that is 3/4 in off each run.