Understanding the remaining-depth check
The check compares how much solid rafter material is left above the notch against a commonly used two-thirds rule of thumb, not a universal code minimum.
| Check text shown | Meaning |
|---|---|
| ok | At least two-thirds of the rafter's original depth remains above the birdsmouth cut, per the common rule-of-thumb convention used by this calculator. |
| tooDeep | Less than two-thirds of the rafter's original depth remains — the notch may be cutting too deeply into the rafter's bearing section, weakening it at its most loaded point. |
- The two-thirds remaining-depth convention is a widely used framing rule of thumb, not a universal fixed code minimum — some jurisdictions specify their own notch-depth limits for rafters, which should always be checked directly.
- A wider seat (plate) width produces a deeper heel cut for the same pitch, since heel depth scales directly with seat width — a wider top plate is not automatically 'safer' for the rafter notch.
- This calculator checks remaining depth only; it does not evaluate whether the resulting bearing area on the plate is adequate for the rafter's actual load, which is a separate structural consideration.
What is a birdsmouth cut?
A birdsmouth cut is a notch cut into the underside of a rafter where it crosses and bears on the top wall plate, made up of two cuts: the 'seat cut', a roughly horizontal cut that rests flat on the plate, and the 'heel cut' (also called the plumb cut), a vertical cut that defines how deep the notch goes into the rafter. Together they let the rafter sit securely and evenly on the wall plate at the correct roof angle, transferring the roof's load into the wall below.
Because the notch removes material from the rafter's original cross-section, cutting it too deep weakens the rafter right at the point where it's most heavily loaded (the bearing point). A common framing convention is to leave at least two-thirds of the rafter's original depth intact above the birdsmouth cut, though the specific limit is set by the applicable local building code.
How to use this birdsmouth cut calculator
- Enter the roof pitch as X-in-12 (for example, 6 for a 6:12 pitch) — this sets the roof angle the heel cut must match.
- Enter the plate (seat) width — the width of the top wall plate the rafter will bear on, typically the width of the wall's top plate lumber.
- Enter the rafter's full depth (its dimension measured perpendicular to its length, e.g., the depth of a 2×8 or 2×10).
- Read the seat cut length, the heel cut depth, the roof angle, and whether enough rafter depth remains intact above the cut per the common two-thirds convention.
The formula behind birdsmouth cut geometry
The heel (plumb) cut depth equals the seat width multiplied by the tangent of the roof angle, since the seat cut, the heel cut and the rafter's top edge form a right triangle matching the roof's slope. The remaining rafter depth above the cut equals the full rafter depth minus the heel cut depth; the common rule of thumb requires this remaining depth to be at least two-thirds of the rafter's full depth.
Worked example (calculator defaults): a 6:12 pitch (angle ≈ 26.57°) with an 8.9 cm seat width and an 18.5 cm rafter depth. Heel cut = 8.9 × tan(26.57°) = 8.9 × 0.5 ≈ 4.45 cm. Remaining depth = 18.5 − 4.45 = 14.05 cm. The two-thirds threshold is (2 ÷ 3) × 18.5 ≈ 12.33 cm, and since 14.05 cm exceeds that, this cut passes the common remaining-depth check.
Common mistakes
- Cutting the heel too deep on a steep-pitch roof with a wide top plate, since heel depth grows with both pitch angle and seat width — steeper roofs and wider plates both increase the cut depth for the same rafter.
- Assuming a birdsmouth cut that passes the two-thirds remaining-depth rule of thumb automatically satisfies the local building code, which may specify a different or more precise limit.
- Cutting the seat and heel cuts at slightly different angles than the actual roof pitch, which prevents the rafter from seating flat and evenly on the plate.
- Forgetting that a birdsmouth notch reduces the rafter's effective depth right at its bearing point — the point of highest shear — which matters more than reducing depth elsewhere along the rafter's length.
Questions fréquentes
How deep should a birdsmouth cut be?
The heel (plumb) cut depth equals the plate's seat width multiplied by the tangent of the roof angle. For an 8.9 cm seat width at a 6:12 pitch (26.57°), the heel cut is about 4.45 cm deep — but the resulting remaining rafter depth should still meet the applicable notching limit.
What is the two-thirds rule for birdsmouth cuts?
It's a common framing rule of thumb requiring at least two-thirds of a rafter's original depth to remain intact above the birdsmouth notch, so the cut doesn't excessively weaken the rafter at its heavily loaded bearing point. Specific building codes may set a different or more precise limit.
Why does a wider top plate mean a deeper birdsmouth cut?
The heel cut depth is directly proportional to the seat width (heel depth = seat width × tan(roof angle)), so for the same roof pitch, a wider top plate requires a proportionally deeper heel cut to seat the rafter flat.
What happens if a birdsmouth cut is too deep?
Cutting too deep removes too much material at the rafter's bearing point — the location of highest shear stress — which can weaken the rafter beyond what the remaining wood can safely carry. This is why a minimum remaining-depth rule of thumb (commonly two-thirds of full depth) is applied.
Références
- American Wood Council (AWC) — Wood Frame Construction Manual, rafter notching and bearing conventions.
- International Code Council (ICC) — International Residential Code (IRC), rafter cut and notch limitations.
- Standard trigonometric relationships (rise/run right-triangle geometry) underlying rafter seat and heel cut calculations.