What a birdsmouth cut is
A birdsmouth is the notch cut into a rafter where it sits on the top plate of a wall. It has two faces:
- the seat cut, horizontal, which bears on the top of the plate;
- the heel cut, vertical (plumb), which bears against the outside face of the plate.
The notch gives the rafter a flat bearing so that the roof load goes straight down into the wall and the rafter cannot slide off.
The geometry
For a roof angle θ, a seat cut of length s and a rafter depth d:
- heel cut height h = s × tan θ, or s × rise ÷ 12 for a pitch given as rise per 12;
- notch depth measured square to the rafter edge = s × sin θ;
- plumb depth of the rafter = d ÷ cos θ;
- height above plate (HAP) = plumb depth − heel cut.
The HAP is the distance, measured plumb, from the top of the wall plate to the top edge of the rafter. Keeping it the same on every rafter keeps the roof plane flat, so carpenters mark it on a pattern rafter and copy it.
Example
A 6/12 roof with 2×8 rafters (actual depth 7.25 in) on a 2×4 wall (plate 3.5 in wide):
- roof angle = arctan(6 ÷ 12) ≈ 26.6°;
- heel cut = 3.5 × 6 ÷ 12 = 1.75 in;
- notch depth ≈ 3.5 × sin 26.6° ≈ 1.57 in, about 22% of the rafter depth;
- HAP ≈ 7.25 ÷ cos 26.6° − 1.75 ≈ 6.36 in.
How deep can the notch go?
A common carpentry rule limits the notch to one third of the rafter depth, so that enough wood remains above the cut to carry the load. Some building codes are stricter for notches at the ends of members, and engineered rafters often have their own rules. Check your local code and any drawings from the engineer or truss supplier.
If the seat would be too deep, you can let the seat cut bear on only part of the plate width, as long as the bearing length required by code is kept, typically at least 1.5 inches.
Layout tips
- Use a speed square or framing square set to the pitch to draw the plumb and level lines.
- Cut the first rafter, test it on both walls, then use it as a pattern.
- Do not over-cut the corner with the circular saw: finish with a handsaw so the rafter is not weakened.
- Fasten each rafter to the plate with hurricane ties or the nailing schedule required by code.