A birdsmouth cut is a notch cut into the underside of a rafter where it rests on the wall top plate, creating a flat seat that provides full, stable bearing contact between the rafter and the wall beneath it, rather than the rafter resting at an angle on just its edge.
This cut, named for its resemblance to an open bird’s beak, is one of the most fundamental and frequently made cuts in traditional rafter framing, executed at each rafter’s connection to the wall structure.
Getting the birdsmouth cut’s dimensions correct matters significantly, since cutting too deep can dangerously weaken the rafter at exactly its point of maximum bearing stress, while cutting too shallow prevents the rafter from seating properly and fully on the wall plate.
What Does a Birdsmouth Cut Involve?
- A notch matching the wall plate’s width and angle
- A seat cut providing flat, stable bearing
- A heel cut aligning with the rafter’s outer edge
- Depth limited by code to preserve adequate remaining rafter strength
Why Is a Birdsmouth Cut Important?
A properly executed birdsmouth cut provides secure, full-contact bearing between the rafter and wall, distributing the rafter’s load evenly rather than concentrating stress at a single point of edge contact the way an uncut rafter resting at an angle would experience. Building codes specifically limit how deep this notch can be cut, since removing too much material at this critical bearing point can significantly reduce the rafter’s remaining structural capacity right where it needs strength most.
An improperly cut birdsmouth, whether too deep, poorly angled, or inconsistently executed across multiple rafters, can create real structural weaknesses or an uneven roof surface, making this detail one worth getting right through careful measurement rather than rushing through it as a routine cut.
Birdsmouth Cut vs. Rafter Tail
A birdsmouth cut is the notch where a rafter meets the wall plate for bearing support. A rafter tail is the portion of the rafter extending beyond that bearing point, out past the wall, creating the roof’s overhang and eave.