How deep can a birdsmouth cut be?
A birdsmouth should remove no more than a third of the rafter's depth, measured square to its edge. On a 2x8 at 6/12 that allows a seat cut of 5 3/8" — the seat, heel and height above plate for every common pitch and rafter size.
No more than a third of the rafter's depth: on a 2x8 at 6/12, a seat cut of 5 3/8" at most.
A 3 1/2" seat on a 2x4 wall takes 1 9/16" out of the 2x8 square to its edge, cuts a 1 3/4" heel, and leaves 6 3/8" of rafter standing above the plate. A full 5 1/2" seat on a 2x6 wall would take 2.46" against a limit of 2.42" — over it, so either bear on part of the plate or step up to a 2x10.
How that is worked out
- The seat is a level cut as wide as the bearing, usually the top plate. The heel is the plumb cut at its outer end, and it is the seat times the pitch: 3 1/2" × 6/12 = 1 3/4".
- What the notch removes, measured square to the rafter's edge, is the seat times the sine of the roof angle: 3.5 × sin 26.6° = 1.57". Held to a third of a 7 1/4" 2x8, that is 2.42", so the widest seat is 2.42 ÷ sin 26.6° = 5.40".
- The height above plate — HAP, the rafter left over the outside of the wall, measured plumb — is the rafter's plumb depth less the heel: 6 3/8" here. Every rafter on the roof has to share it, so lay out one and copy it.
Widest seat inside the one-third limit, and the cut a 3 1/2" seat makes
| Pitch | Max seat, 2x6 | Max seat, 2x8 | Max seat, 2x10 | Max seat, 2x12 | Heel (3 1/2" seat) | HAP on a 2x8 |
|---|---|---|---|---|---|---|
| 3:12 | 7 1/2" | 9 15/16" | 12 11/16" | 15 7/16" | 7/8" | 6 5/8" |
| 4:12 | 5 3/4" | 7 5/8" | 9 3/4" | 11 13/16" | 1 3/16" | 6 1/2" |
| 5:12 | 4 3/4" | 6 1/4" | 8" | 9 3/4" | 1 7/16" | 6 3/8" |
| 6:12 | 4 1/16" | 5 3/8" | 6 7/8" | 8 3/8" | 1 3/4" | 6 3/8" |
| 7:12 | 3 5/8" | 4 3/4" | 6 1/16" | 7 7/16" | 2 1/16" | 6 3/8" |
| 8:12 | 3 1/4" | 4 5/16" | 5 1/2" | 6 3/4" | 2 5/16" | 6 3/8" |
| 9:12 | 3" | 4" | 5 1/8" | 6 1/4" | 2 5/8" | 6 7/16" |
| 10:12 | 2 13/16" | 3 3/4" | 4 13/16" | 5 13/16" | 2 15/16" | 6 1/2" |
| 12:12 | 2 9/16" | 3 3/8" | 4 5/16" | 5 1/4" | 3 1/2" | 6 3/4" |
Worth knowing
- The one-third figure is the conventional limit, and it is what the rafter calculator checks. The IRC's own rule for notches at the end of a bending member is a quarter of its depth (R802.7.1); a birdsmouth sitting over bearing is usually held to a third instead. 2021 IRC, defaults rather than law — your inspector decides which applies.
- The shallower the pitch, the wider the seat can be, because a level cut through a nearly flat rafter barely goes into it. Steep roofs tighten fast: at 12/12 a 2x8 allows only 3 3/8", short of even a 2x4 wall's plate.
- A seat does not have to cover the whole plate. Bearing on most of it and letting the inside edge overhang the wall is normal practice where the full seat would take too much — check the bearing length your plans call for.
- This is the geometry of the cut. Whether that rafter spans the roof under your snow load is a span-table question.
Sources: 2021 International Residential Code, International Code Council.
Work out your own numbers
- Rafters
Common, hip and jack rafters for any pitch, with the birdsmouth laid out and the cut angles alongside.
- Roofing calculator
Squares, bundles, starter strip, ridge cap and underlayment from a building footprint and a pitch.
Related questions
- How deep should a birdsmouth cut be?
- Deep enough for the seat to bear on the plate, and no deeper than a third of the rafter's depth measured square to its edge. On a 2x8 at 6/12, a 3 1/2" seat for a 2x4 wall removes 1 9/16" — comfortably inside the 2.42" limit.
- What is HAP on a rafter?
- Height above plate: the depth of rafter left over the outside edge of the wall, measured plumb. It sets the height of the whole roof, so every rafter has to match. A 2x8 at 6/12 with a 3 1/2" seat leaves 6 3/8".
- How do you lay out a birdsmouth cut?
- Set a speed square to the pitch and draw the plumb line where the rafter meets the outside of the wall. Measure the seat width along a level line from it, and the plumb line becomes the heel. At 6/12 a 3 1/2" seat gives a 1 3/4" heel. Cut one, test it on the wall, then use it as the pattern for the rest.
- Can a birdsmouth be cut into a 2x6 wall's full plate?
- Not on a 2x8 at 6/12: a 5 1/2" seat would remove 2.46", past the 2.42" a third allows. Bear on 5 3/8" of the plate, or use a 2x10, which allows 6 7/8".