How to Calculate Rafter Length
Rafter Length = (Half Span ÷ cos(pitch angle)) + Overhang Tail
Ridge Height = Half Span × tan(pitch angle)
Birdsmouth Seat Cut = Rafter Width × cos(pitch angle)
The rafter runs from the ridge board at the peak down to the wall plate, plus the eave overhang beyond the wall. The pitch angle determines how long the rafter needs to be for a given building span.
Key Rafter Components
- Common Rafter: Runs from the ridge to the wall plate. The most basic rafter type.
- Birdsmouth Cut: The notch cut into the rafter where it sits on the wall plate. Provides a flat bearing surface.
- Seat Cut: The horizontal portion of the birdsmouth that rests on the wall plate.
- Plumb Cut: The vertical cut at the birdsmouth and at the ridge end of the rafter.
- Tail: The portion of the rafter extending beyond the wall to form the eave overhang.