Birdsmouth Cut Calculator — Rafter Notch for Roof Framing
The birdsmouth is the L-shaped notch cut into a roof rafter at the wall plate so the rafter sits flat and secure. Enter your roof pitch, rafter size, and seat cut depth to get the exact horizontal bearing length, pitch angle, perpendicular notch depth, and a code-compliance check.
in per 12 in run
inches
inches
Horizontal length of the birdsmouth notch sitting on the wall plate
- 1
Run-to-rise ratio
12 ÷ 6 = 2Ratio of the standard 12-inch run to the pitch rise — the seat-length multiplier. - 2
Seat cut bearing length
1.5 in × 2 = 3
How does this calculator work?
Seat cut length = seat depth × (12 ÷ rise). For a 6:12 pitch with a 1.5-inch plumb seat depth, bearing length = 1.5 × 2 = 3 inches. Pitch angle = arctan(rise/12) ≈ 26.6° for 6:12. The notch depth (perpendicular to rafter) must not exceed ⅓ of rafter width per IRC R802.7.
Formula
How this is calculated
A birdsmouth consists of two perpendicular cuts: the plumb cut (vertical when the rafter is installed in the roof) and the seat cut (horizontal when installed). Together they form an L-shaped notch that lets the rafter rest squarely on the wall plate without rocking or tilting. The geometry flows entirely from the roof pitch, expressed as rise-per-12 in the North American convention.
The pitch angle θ = arctan(rise/12) determines the rafter slope. The seat cut runs horizontally for a length equal to the plumb seat depth divided by tan(θ), which simplifies to depth × (12/rise) using the pitch ratio directly. The deeper the seat, the longer the horizontal bearing on the plate — good for load transfer — but cutting too deep weakens the rafter. IRC R802.7 (US building code) limits the notch to one-third of the rafter depth measured perpendicular to the rafter face; this calculator converts your plumb depth to the perpendicular depth and flags when the limit is approached.
All dimensions are in inches using the standard North American rise-over-12-run pitch convention. For metric users: 1 inch = 25.4 mm. These calculations cover a standard stick-frame birdsmouth; if you are framing with structural ridge beams or engineered lumber, confirm requirements with a structural engineer.
Frequently asked questions
IRC R802.7 states that the depth of the notch (measured perpendicular to the rafter face) must not exceed one-third of the rafter depth. For a 2×6 (5.5 inch actual), the perpendicular notch depth must not exceed 1.83 inches. This calculator checks this automatically and flags over-depth seats.
The plumb cut is the vertical cut at the birdsmouth location — it faces toward the building exterior and is made at the same pitch angle as the ridge cut at the top of the rafter. The seat cut is the horizontal cut that creates the flat bearing surface on the wall plate. They are perpendicular to each other, forming the L-shape of the notch.
Structural ridge beams change the load path and may not require a traditional birdsmouth, or may require a different depth. This calculator applies to conventional stick-frame construction with a non-structural ridge. Consult a structural engineer for beamed or engineered-lumber roof systems, and always check your local building code requirements.
Also known as
TG we-Calculate Editorial Team. (2026). Birdsmouth Cut Calculator — Rafter Notch for Roof Framing [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/birdsmouth-cut-calculator
TG we-Calculate Editorial Team. "Birdsmouth Cut Calculator — Rafter Notch for Roof Framing." TG we-Calculate. 2026. https://we-calculate.com/calculator/birdsmouth-cut-calculator.
TG we-Calculate Editorial Team, "Birdsmouth Cut Calculator — Rafter Notch for Roof Framing," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/birdsmouth-cut-calculator
@misc{wecalculate_birdsmouth_cut_calculator, title = {Birdsmouth Cut Calculator — Rafter Notch for Roof Framing}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/birdsmouth-cut-calculator}}, year = {2026}, note = {TG we-Calculate} }
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