Common rafter calculator
Calculate rafter length, ridge height, birdsmouth, and cutting angles from building span and roof pitch.
How to Calculate Rafter Length
A common rafter runs from the ridge board down to the wall plate and beyond as the overhang. The total run is half the building span minus half the ridge thickness. The total rise is the run multiplied by the pitch ratio (e.g., 6/12 = 0.5).
The rafter length along the slope equals the run divided by the cosine of the roof angle. Add the overhang length (also measured along the slope) for the total rafter you need to cut.
Birdsmouth Cut
The birdsmouth is a notch cut into the rafter where it sits on the wall plate. It has two parts: the seat cut (horizontal, bearing on the plate) and the plumb cut (vertical, against the wall). The HAP (height above plate) is the remaining rafter depth above the seat cut.
Common Roof Pitches
- 3/12 (14.0°) — Low slope, minimum for shingles
- 4/12 (18.4°) — Common for ranch-style homes
- 6/12 (26.6°) — Standard residential pitch
- 8/12 (33.7°) — Steeper, good for snow shedding
- 12/12 (45.0°) — 45-degree, steep residential