Birdsmouth Cut Calculator
Plan your roofing project with our free birdsmouth cut calculator. Get precise measurements, material lists, and budgets.
Reviewed by Abdullah, Technical Content Specialist
Formula
Plumb Cut = Seat Depth / cos(angle); Seat Cut = Seat Depth / sin(angle)
The roof angle is calculated from the pitch as atan(rise/12). The plumb cut length equals the seat cut depth divided by the cosine of the roof angle. The seat cut length equals the depth divided by the sine of the roof angle. The heel height is the rafter width times the cosine of the roof angle. The maximum seat cut is one-third of the rafter width per building code.
Worked Examples
Example 1: 2x6 Rafter at 6/12 Pitch
Problem:Calculate birdsmouth dimensions for a 2x6 rafter (5.5 in actual) with a 6/12 pitch and 1.5 inch seat cut depth.
Solution:Roof angle = atan(6/12) = 26.57 degrees\nPlumb cut length = 1.5 / cos(26.57) = 1.677 inches\nSeat cut length = 1.5 / sin(26.57) = 3.354 inches\nMax seat cut = 5.5 / 3 = 1.833 inches\n1.5 < 1.833, so code compliant
Result:Plumb cut = 1.677 in, Seat cut = 3.354 in, Code compliant
Example 2: 2x8 Rafter at 8/12 Pitch
Problem:Calculate birdsmouth for a 2x8 (7.25 in) with 8/12 pitch, 2.0 inch seat depth, and 12 inch overhang.
Solution:Roof angle = atan(8/12) = 33.69 degrees\nPlumb cut = 2.0 / cos(33.69) = 2.404 inches\nSeat cut = 2.0 / sin(33.69) = 3.606 inches\nTail length = 12 / cos(33.69) = 14.42 inches\nMax seat = 7.25 / 3 = 2.417, so compliant
Result:Plumb = 2.404 in, Seat = 3.606 in, Tail = 14.42 in
Frequently Asked Questions
What is a birdsmouth cut on a rafter?
A birdsmouth cut is a notch cut into the bottom of a rafter where it sits on the top plate of a wall. It consists of two cuts: a horizontal seat cut (or level cut) that rests flat on the wall plate, and a vertical plumb cut (or heel cut) that fits against the outside edge of the wall. The birdsmouth allows the rafter to sit securely on the wall while maintaining the correct roof pitch angle. It transfers the roof load directly down through the wall framing.
How deep should a birdsmouth cut be?
Building codes generally limit the birdsmouth seat cut depth to no more than one-third of the rafter depth. For a 2x6 rafter (5.5 inches actual), the maximum seat cut depth is about 1.83 inches. For a 2x8 (7.25 inches actual), the maximum is about 2.42 inches. Cutting too deep weakens the rafter at the bearing point and can cause the rafter to split under load. The remaining wood above the seat cut must have sufficient depth to carry the bending loads from the roof.
How do I mark a birdsmouth cut using a framing square?
Place the framing square on the rafter with the rise measurement on the tongue (short side) and 12 inches on the blade (long side). The tongue side gives you the plumb cut line and the blade side gives you the seat cut line. Mark the plumb line first at the bearing point location, then measure down along the plumb line the depth of your seat cut. From that point, draw the horizontal seat cut line perpendicular to the plumb cut. The intersection of the two lines forms the inside corner of the birdsmouth notch.
Can I use a birdsmouth cut on engineered lumber or I-joists?
No, you should never cut a birdsmouth into engineered lumber such as LVL beams, I-joists, or manufactured trusses unless the manufacturer specifically approves it. Cutting notches in engineered lumber compromises the structural integrity and voids the warranty. Instead, use manufacturer-approved hangers, clips, or bearing connectors designed for engineered products. For manufactured trusses, the bearing connection is already designed into the truss and no additional cutting is needed or permitted.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy