Birdsmouth notch depth — the IRC D/4 limit and 1½-inch minimum seat bearing

The birdsmouth notch is limited to D/4 of the rafter depth (IRC R802.7.1) with at least 1½″ of seat bearing (R802.6.1). The “one-third” rule some sites cite is not the IRC figure — cutting deeper weakens the tail.

A large detail of a rafter seated on a double top plate at a 6:12 slope. The birdsmouth is the triangular notch removed so the rafter sits flat: a horizontal seat cut resting on the plate and a vertical plumb (heel) cut. The International Residential Code section R802.7.1 limits the notch depth to one-quarter of the rafter depth — D over 4 — not the one-third figure many websites cite, and section R802.6.1 requires at least one and a half inches of seat bearing on the plate. Cutting the notch deeper than D over 4 eats into the tension edge and can shear the rafter tail. — Source: IRC R802.7.1 (D/4 notch) + R802.6.1 (1½″ bearing)

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A large detail of a rafter seated on a double top plate at a 6:12 slope. The birdsmouth is the triangular notch removed so the rafter sits flat: a horizontal seat cut resting on the plate and a vertical plumb (heel) cut. The International Residential Code section R802.7.1 limits the notch depth to one-quarter of the rafter depth — D over 4 — not the one-third figure many websites cite, and section R802.6.1 requires at least one and a half inches of seat bearing on the plate. Cutting the notch deeper than D over 4 eats into the tension edge and can shear the rafter tail.

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<figure style="margin:0;max-width:800px">
  <a href="https://remodelcalculators.com/construction-diagrams/rafter-birdsmouth" target="_blank" rel="noopener">
    <img src="https://remodelcalculators.com/api/figure-embed/rafter-birdsmouth" alt="A large detail of a rafter seated on a double top plate at a 6:12 slope. The birdsmouth is the triangular notch removed so the rafter sits flat: a horizontal seat cut resting on the plate and a vertical plumb (heel) cut. The International Residential Code section R802.7.1 limits the notch depth to one-quarter of the rafter depth — D over 4 — not the one-third figure many websites cite, and section R802.6.1 requires at least one and a half inches of seat bearing on the plate. Cutting the notch deeper than D over 4 eats into the tension edge and can shear the rafter tail." width="800" height="470" style="width:100%;height:auto;border:0" loading="lazy">
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    Diagram: <a href="https://remodelcalculators.com/construction-diagrams/rafter-birdsmouth" target="_blank" rel="noopener">Birdsmouth notch depth — the IRC D/4 limit and 1½-inch minimum seat bearing</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
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Free rafter length calculator: common, hip, valley & jack rafter lengths, plumb/seat cut angles, birdsmouth depth, and board count. Per IRC R802. No signup.

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