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A birdsmouth is the right-angled notch that seats a rafter on the wall plate: a level seat cut as long as the bearing, and a plumb heel cut against the outside of the wall. The heel cut is the seat length × rise ÷ 12, the notch removes seat × sin(pitch angle) from the rafter's depth measured square to its edge, and the height above plate is the rafter depth measured plumb minus the heel cut. The common framing rule keeps the notch within one third of the rafter depth, and the IRC (R802.6) requires at least 1 1/2 in of bearing. A 2x6 rafter at 6/12 on a 3.5 in seat has a 1.75 in heel cut, a 1.57 in notch (28.5% of its depth) and 4.40 in of height above plate.
A birdsmouth is a level seat cut that bears on the wall plate and a plumb heel cut against the outside of the wall. The heel cut is the seat length × rise ÷ 12; the notch removes seat × sin(angle) from the rafter measured square to its edge; the height above plate is the plumb depth of the rafter minus the heel cut.
| Pitch | Heel cut | 2x6 | 2x8 | 2x10 | 2x12 |
|---|---|---|---|---|---|
| 4/12 | 1.17 in | 20% | 15% | 12% | 10% |
| 5/12 | 1.46 in | 24% | 19% | 15% | 12% |
| 6/12 | 1.75 in | 28% | 22% | 17% | 14% |
| 7/12 | 2.04 in | 32% | 24% | 19% | 16% |
| 8/12 | 2.33 in | 35% | 27% | 21% | 17% |
| 9/12 | 2.63 in | 38% | 29% | 23% | 19% |
| 10/12 | 2.92 in | 41% | 31% | 24% | 20% |
| 12/12 | 3.50 in | 45% | 34% | 27% | 22% |
Shares above 33% break the common one-third rule — shorten the seat or use a deeper rafter. The IRC (R802.6) requires at least 1 1/2 in of bearing at rafter ends.
The seat cut is horizontal and as long as the bearing on the wall plate — 3.5 in on a 2x4 wall. The heel (plumb) cut is the seat length × rise ÷ 12. On a 6/12 roof a 3.5 in seat needs a 1.75 in heel cut. The notch depth measured square to the rafter edge is the seat × sin(pitch angle), 1.57 in in that example.
The common framing rule is no more than one third of the rafter depth, measured square to the bottom edge. A 2x6 (5.5 in) allows about 1.83 in; at 6/12 that caps the seat at about 4.1 in. Engineered rafters and local codes can be stricter, so check the span table or the engineer's notes.
The vertical distance from the top of the wall plate to the top edge of the rafter at the outside of the wall. It equals the rafter depth measured plumb (depth ÷ cos of the pitch angle) minus the heel cut. Keeping HAP identical on every rafter keeps the roof plane flat.
The International Residential Code (R802.6) requires at least 1 1/2 in of bearing on wood or metal for rafter ends. A full-width seat on the top plate — 3.5 in on a 2x4 wall, 5.5 in on a 2x6 wall — is the usual target when the one-third depth rule allows it.
Rarely. A 3.5 in seat at 6/12 removes 1.57 in, which is 45% of a 3.5 in deep 2x4 — well past the one-third rule. Shorten the seat (a 2.6 in seat keeps it to one third at 6/12) or move up to a deeper rafter.
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