How to Calculate Rafters
Rise: Run x Pitch / 12
Rafter length: sqrt(Run^2 + Rise^2)
Birdsmouth seat cut: Rafter width / (Pitch / 12) x cos(angle)
HAP: Rafter width - (Seat cut x sin(angle))
Key Terms
- Run: Horizontal distance from wall plate to ridge (half building width)
- Rise: Vertical height from plate to ridge
- HAP: Height Above Plate - the vertical dimension of rafter above the wall plate after the birdsmouth cut
- Birdsmouth: The notch cut where the rafter meets the wall plate