From this site

One project, every trade

Each calculator adds its lines to a single estimate — consolidated BOM, schedule and cash-flow included.

Open My Project →

Rafter Bird's Mouth Cut Calculator

Calculate the seat cut and heel cut dimensions for a rafter's bird's mouth notch at the wall plate.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Heel cut height

1.75 in

High confidence
Seat cut length (= plate width)
3.5 in

At the values currently entered, the heel cut height works out to 1.75 in. The largest intermediate quantity is seat cut length (= plate width), at 3.50 in — check that step first if the total looks off. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Add the equipment this sizes

This result is a specification — 1.75 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Standard carpentry bird's mouth layout: seat cut length equals the wall plate width; heel cut height = wall plate width × tan(roof pitch angle)

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

What are the seat cut and heel cut on a bird's mouth?
The seat cut is the horizontal cut that rests flat on top of the wall plate, and its length equals the plate width. The heel cut is the vertical cut at the outer edge of the notch, and its height grows with a steeper roof pitch since the rafter's underside angles away from the plate more sharply.
Why does a steeper pitch produce a taller heel cut?
The heel cut height is the plate width times the tangent of the roof's slope angle — as the pitch (and therefore the angle) increases, tan(angle) grows faster than linearly, so the heel cut height rises quickly at steep pitches.
Does this account for the rafter's overall depth or the remaining structural depth after the notch?
No — this calculates the notch geometry only. Checking that enough rafter depth remains above the bird's mouth to carry the design loads (see NDS notch provisions) is a separate structural check.