What Is a Birdsmouth Cut on a Rafter?

What the cut does

A birdsmouth cut is a triangular notch cut into the underside of a rafter where it crosses the top of the wall plate. It lets the rafter sit flush and level on the plate instead of balancing on its lower corner. The notch has two faces: a seat cut that rests horizontally on the plate, and a heel cut (or plumb cut) that sits vertically against the outer edge of the plate.

Why it's necessary

Without the notch, a sloped rafter would touch a level plate only along one edge, concentrating the load and making the rafter unstable and hard to fasten. The notch gives it a flat bearing surface matched to the roof's pitch.

It also locates every rafter at the same height. With each heel cut butted against the outside of the wall, every rafter lands in the same position, which keeps the roof plane flat and the fascia straight.

Parts of a birdsmouth

• Seat cut — the level face that bears on top of the wall plate.
• Heel (plumb) cut — the vertical face against the outside edge of the plate; its height is set by the seat length and the pitch.
• Height above plate (HAP) — the vertical distance from the top of the plate, along the heel line, to the top edge of the rafter. Keeping it identical on every rafter keeps the roof plane true.
• Tail — the part of the rafter beyond the heel cut that forms the overhang.

The geometry behind the cut

For a given seat length, heel height = seat length × (rise ÷ 12). The depth removed, measured square to the rafter's edge, is the heel height ÷ the slope factor, where slope factor = √(rise² + 12²) ÷ 12. A plumb line through the rafter is longer than its square depth by the same factor, so HAP = rafter depth × slope factor − heel height. If you don't know the pitch yet, measure it first.

Example: a 6/12 rafter made from a 2×8 (7.25 in actual depth) on a 2×4 wall with a 3.5 in seat. Heel height = 3.5 × 6 ÷ 12 = 1.75 in. With a slope factor of 1.118, the depth removed is 1.75 ÷ 1.118 = 1.57 in (about 22%), leaving 5.68 in, and HAP = 7.25 × 1.118 − 1.75 = 6.36 in. The Birdsmouth Cut Calculator works out the heel height, depth removed and remaining depth for any pitch, seat length and rafter depth.

How deep is too deep?

In the 2021 International Residential Code (IRC), Section R802.7.1 applies the sawn-lumber rules of R502.8.1 to rafters: notches at the ends of a member must not exceed one-fourth of its depth. R802.7.1.1 permits notches on the cantilevered portion of a rafter (the overhang) where at least 3½ in of rafter remains and the cantilever is no longer than 24 in. Measured square to the edge, a 2×6 at 12/12 with a 3.5 in seat keeps only 3.03 in.

The cited text doesn't name the birdsmouth, say whether it counts as an end notch, or say how to measure the depth of a sloped notch. The approved plans and your building official decide how these limits apply, and local amendments or another code edition can change them. The often-quoted one-third rule is a framers' rule of thumb, not code text, and it is looser than one-quarter.

Bearing is a separate check: R802.6 requires at least 1½ in of rafter bearing on wood or metal and 3 in on masonry or concrete, so a seat shorter than the plate can still meet it.

Worked example: sizing the seat on a 2×6 wall

A 6/12 roof (slope factor 1.118) with 2×8 rafters, 7.25 in deep, on a 2×6 wall with a 5.5 in plate, tested against one-quarter of the rafter depth: 7.25 ÷ 4 = 1.81 in. Readings A and B below are alternative geometric interpretations; the cited code text defines neither, so neither seat is code-compliant by default. Use the method your plans or building official specify.

• Full-width seat: heel height = 5.5 × 6 ÷ 12 = 2.75 in; depth removed = 2.75 ÷ 1.118 = 2.46 in (34%) — over one-quarter on either reading, and over one-third.
• Reading A, depth removed measured square to the rafter: largest seat = 7.25 ÷ 4 × 1.118 ÷ 0.5 = 4.05 in. A 4 in seat removes 1.79 in (24.7%), leaves 5.46 in and gives HAP = 7.25 × 1.118 − 2.00 = 6.11 in.
• Reading B, heel (plumb) height compared with the rafter depth: largest seat = 7.25 ÷ 4 ÷ 0.5 = 3.63 in.
• Bearing: both seats exceed the 1½ in minimum in R802.6, taking the seat length as the bearing length.

Getting the layout right

Both cuts depend on the pitch and the rafter size, so lay them out with a framing square or a calculated cut sheet rather than by eye:

• Lay out the plumb cut at the ridge first, then measure the rafter's line length along the edge to find the heel line. The Rafter Length Calculator gives the line length from run and pitch.
• Mark the heel (plumb) line using a framing or speed square set to the pitch.
• Mark the seat line square to the heel line, at the chosen seat length.
• Cut one pattern rafter, test-fit it at both ends, then use it to mark the rest so every HAP matches.
• Stop saw cuts at the intersection and finish with a handsaw rather than over-cutting past the corner, which weakens the rafter.

When there's no birdsmouth

Not every roof uses one. Manufactured trusses bear on the plate through their heel joint, so there is nothing to notch — see the Roof Truss Calculator for spacing and counts. IRC R802.7.2 prohibits cuts and notches in trusses, I-joists, glued-laminated members and structural composite lumber unless the manufacturer permits them or a registered design professional accounts for them.

A birdsmouth isn't an uplift connection either: IRC R802.11 requires rafter-to-wall connections that resist wind uplift, and allows ordinary nailing only in limited cases, such as an uplift force of 200 lb or less per rafter.

When to check the plans or a professional

The birdsmouth doesn't size the rafter — span tables, the approved plans or an engineer do. Check the plans, or ask the building official or a structural engineer, when:

• You don't know how your building official measures the notch, or no seat meets the limit while still giving the required bearing.
• The overhang is longer than 24 in, or less than 3½ in of rafter would remain at a notch on it.
• The rafters are engineered lumber, or the plans call for a raised-heel, ridge-beam or hanger detail.
• The plans specify a different notch, connector or bearing detail — the approved plans govern.

Common mistakes

• Making the seat as wide as the whole plate on a 2×6 wall, removing too much rafter depth.
• Over-cutting past the corner of the notch with a circular saw.
• Laying out each rafter separately instead of from a tested pattern, leading to uneven HAPs and a wavy roof plane.
• Cutting a birdsmouth into an engineered I-joist flange.