Birdsmouth Cut Calculator

Rafter Parameters

Rafter board width
Rafter board depth
Wall plate thickness
Roof slope
Measured level from the outside of the wall; leave blank for none. With an overhang the code checks for a cantilevered rafter apply

Calculation Results

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How to Calculate a Birdsmouth Cut

  1. Seat cut = the width of the wall's top plate (3.5 in. on a 2x4 wall, 5.5 in. on a 2x6 wall), so the rafter bears on the whole plate.
  2. Heel cut = seat × tan(angle): on a 6:12 roof, half the seat (6 ÷ 12).
  3. Notch depth = seat × sin(angle), measured square to the rafter edge. At the bearing it may not exceed 1/4 of the rafter depth (IRC R802.7.1). If it would, the calculator shortens the seat and tells you, as long as at least 1.5 in. of bearing remains (IRC R802.6).
  4. Overhang (horizontal): if you enter one, the rafter is cantilevered past the wall and IRC R802.7.1.1 also applies: at least 3-1/2 in. of rafter must remain above the notch (measured square to the edge), and the cantilever, measured along the rafter from the notch (overhang ÷ cos(angle)), may not exceed 24 in. The calculator checks both and warns if the rafter can't keep 3-1/2 in. or the cantilever is longer than 24 in. With no overhang, only the 1/4 limit applies.
  5. Height above plate = rafter depth ÷ cos(angle) − heel cut. Mark it on one rafter and use the same HAP on all of them so the ridge stays straight.

Why the seat is often shortened: on a 2x6 rafter (5.5 in. deep) at 6:12, a full 3.5 in. seat would make a 1.57 in. notch, more than the 1.38 in. (1/4 of 5.5 in.) the code allows at the bearing, so the seat is cut to 3.07 in.

Mark the cuts with a framing square or speed square set to the pitch: the heel cut is plumb, the seat cut is level.

Birdsmouth Cut Examples

Examples 1-4 have an overhang, the common case; Example 5 has none.

Example 1: 2x6 rafter on a 2x4 wall, 6:12, 12 in. overhang

- Seat 3.07 in. (shortened), heel 1.54 in.
- Notch depth 1.38 in. (25%) · Rafter left above the notch 4.13 in. · HAP 4.61 in.
- Cantilever 13.42 in. along the rafter

Example 2: 2x6 rafter on a 2x4 wall, 4:12, 12 in. overhang

- Seat 3.50 in. (full plate), heel 1.17 in.
- Notch depth 1.11 in. (20%) · Rafter left above the notch 4.39 in. · HAP 4.63 in.
- Cantilever 12.65 in.

Example 3: 2x8 rafter on a 2x6 wall, 8:12, 12 in. overhang

- Seat 3.27 in. (shortened), heel 2.18 in.
- Notch depth 1.81 in. (25%) · Rafter left above the notch 5.44 in. · HAP 6.54 in.
- Cantilever 14.42 in.

Example 4: 2x10 rafter on a 2x6 wall, 30°, 16 in. overhang

- Seat 4.63 in. (shortened), heel 2.67 in.
- Notch depth 2.31 in. (25%) · Rafter left above the notch 6.94 in. · HAP 8.01 in.
- Cantilever 18.48 in.

Example 5: 2x6 rafter on a 2x4 wall, 6:12, no overhang

- Seat 3.07 in. (shortened), heel 1.54 in.
- Notch depth 1.38 in. (25%, the 1/4 limit) · HAP 4.61 in.

On 2x6 and deeper rafters the 1/4 limit is the tighter one, so the overhang doesn't change the seat. Warnings: a 2x4 rafter (3.5 in.) with an overhang can't keep 3-1/2 in. above any notch, so the calculator shows "No birdsmouth: rafter too shallow" instead of a seat; a 24 in. horizontal overhang at 6:12 is a 26.83 in. cantilever, over the 24 in. limit.

How Deep Can a Birdsmouth Be?

  • At the bearing, notches in sawn lumber rafters follow Section R502.8.1: "Notches at the ends of the member shall not exceed one-fourth the depth of the member" (2024 International Residential Code (IRC), Section R802.7.1, which applies R502.8.1).
  • On a rafter with an overhang, Section R802.7.1.1 (Cantilevered portions of rafters) adds: "Notches on cantilevered portions of rafters are permitted provided the dimension of the remaining portion of the rafter is not less than 3 1/2 inches (89 mm) and the length of the cantilever does not exceed 24 inches (610 mm) in accordance with Figure R802.7.1.1." The figure shows D/4 max at the bearing, at least 3-1/2 in. of rafter above the notch, and the cantilever measured along the rafter (2024 International Residential Code (IRC); the same text is in the 2021 IRC).
  • Rafters need at least 1-1/2 in. of bearing on wood or metal (2024 International Residential Code (IRC), Section R802.6).
  • Don't notch the rafter elsewhere along its span and don't over-cut the corner: stop the saw at the line and finish with a handsaw.
  • Engineered rafters (LVL, I-joists) have their own rules from the manufacturer; many don't allow a birdsmouth at all without a bearing block.
  • Your jurisdiction may have adopted a different edition or local amendments; check with your building department.

Work out the rafter length with the rafter length calculator.

Birdsmouth Cut Calculator FAQs

What is a birdsmouth cut?
A notch near the bottom end of a rafter, made of a level seat cut that sits on the wall's top plate and a plumb heel cut that bears against the outside of the plate.
How deep should a birdsmouth cut be?
No deeper than 1/4 of the rafter depth, measured square to the edge (2024 International Residential Code (IRC), Section R802.7.1): about 1-3/8 in. on a 2x6 and 1-13/16 in. on a 2x8. If the rafter has an overhang, at least 3-1/2 in. of rafter must also remain above the notch and the cantilever may not exceed 24 in. (Section R802.7.1.1), so a 2x4 rafter with an overhang can't take a birdsmouth.
How long is the seat cut?
Ideally the width of the top plate (3-1/2 in. on a 2x4 wall, 5-1/2 in. on a 2x6 wall). It is shortened when the notch would be deeper than 1/4 of the rafter depth (a 2x6 at 6:12 gets a 3.07 in. seat) or, with an overhang, would leave less than 3-1/2 in. of rafter; keep at least 1-1/2 in. of bearing (IRC R802.6).
How do I calculate the heel cut?
Heel cut = seat × rise ÷ 12. A 3-1/2 in. seat on a 4:12 roof gives a 1-3/16 in. heel.
What is height above plate (HAP)?
The depth of wood left above the wall line, measured plumb. Keep it the same on every rafter so the ridge stays level. A 2x6 rafter at 4:12 with a 3-1/2 in. seat has an HAP of about 4-5/8 in.
How do I mark a birdsmouth with a speed square?
Set the square to the pitch, draw the plumb heel line where the rafter crosses the outside of the wall, then the level seat line to its length; measure the HAP to check.
Does the birdsmouth change the rafter length?
No. Rafter length is measured along the top edge to the ridge plus the overhang; work it out with the rafter length calculator, then lay out the birdsmouth where the rafter crosses the wall.
Can I cut a birdsmouth in an LVL or I-joist rafter?
Only if the manufacturer allows it; most require a beveled bearing plate or a hanger instead. Follow their installation guide.

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