Calculate the precise seat and heel cuts for roof rafters to ensure a perfect fit on the top plate of your wall.
Last updated: March 2026
e.g., 6 for a 6/12 pitch
2x6 = 5.5", 2x8 = 7.25"
2x4 = 3.5", 2x6 = 5.5"
A bird's mouth cut is a notch cut into the bottom of a rafter that allows it to sit securely and flat on the top plate of a wall. This cut is essential in roof framing because it provides two critical functions: a horizontal bearing surface (seat cut) for the rafter to rest on, and a vertical surface (heel cut) that prevents the rafter from sliding outward under roof load.
The bird's mouth consists of two perpendicular cuts: the seat cut (horizontal) matches the width of the top plate, and the heel cut (vertical) is calculated based on the roof pitch. The HAP (Height Above Plate) is the remaining vertical thickness of the rafter after the cuts are made—this must be adequate to maintain the rafter's structural integrity.
A critical rule in carpentry is never to cut more than one-third of the rafter's depth for the bird's mouth. Cutting too deeply weakens the rafter and can lead to structural failure under snow load or high winds.
Follow these steps to determine accurate bird's mouth cuts:
Roof framed with 2×6 rafters (5.5" actual width) on 2×4 top plate (3.5" actual width) with 6/12 pitch:
Disclaimer: This calculator provides geometric guidance for birdsmouth cuts based on standard roof framing relationships. Results are for reference and planning purposes only. It does not constitute a full structural assessment or account for all design considerations, regional code variations, or installation-specific factors. Cutting rafters incorrectly can weaken structural integrity and cause roof failure. Always follow local building codes and consult a qualified builder, structural engineer, or licensed contractor before cutting. Never cut more than 1/3 of the rafter depth.
The heel cut should never exceed 1/3 of the rafter's width. Cutting too deep significantly weakens the rafter and violates building codes.
HAP (Height Above Plate) is the remaining vertical thickness after cutting. It must be adequate to maintain structural integrity—generally at least 1/3 of original rafter width.
Yes, the seat cut should match the plate width for maximum bearing surface and code compliance. Use actual dimensions (3.5" for 2×4, 5.5" for 2×6).
Use a framing square set to the roof pitch. Mark the plumb line first, then measure the seat cut horizontally from that line using the calculated dimensions.
Not recommended. Engineered rafters (LVL, truss components) may have specific fastening requirements. Consult manufacturer instructions before cutting.
If HAP is insufficient, use a higher-grade lumber (larger rafter), reduce roof pitch, or use raised heel trusses to maintain structural integrity.
These calculations are geometric and suitable for field layout. Always check fit with a test rafter and adjust for saw kerf and actual lumber dimensions before cutting all pieces.
Engineered trusses are factory-designed and often should not be altered in the field. Consult the truss manufacturer or a structural engineer before modifying.
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