How to Layout a Bird's Mouth Cut with a Framing Square
Learn the exact placement of brass gauges for the seat cut and heel cut, ensuring at least 1.5 inches of solid bearing surface on the wall plate.
DIY Rafter Length & Bird's Mouth Cut Calculator
Calculate theoretical rafter lengths, bird's mouth notches, ridge deductions, and tail overhangs with easy tape-measure fractions.
Step 1: Your Roof & Building Dimensions
Code Compliant
Half-Span Run
7' 11 1/4"
True horizontal run (after deducting 3/4" for ridge).
Total Peak Rise
3' 11 5/8"
Vertical height from top plate to the top of the ridge.
Total Rafter + Tail
10' 7 7/8"
Tip of ridge cut to the tip of the tail fascia.
Pitch: 6 / 12 (26.6°)
Seat Cut: 3-1/2" (Passed ≥ 1.5")
Recommended lumber to purchase:
1x 12-Foot 2x6 (per rafter)
Framing square gauge settings:
Tongue: 6" Rise | Body: 12" Run
Ridge plumb cut deduction:
Deduct 3/4" (half of 1-1/2" ridge)
Bird's mouth horizontal seat cut:
3-1/2" flat resting surface
Bird's mouth vertical plumb heel cut:
1-3/4" vertical (Within 1/3 max rule)
Remaining wood above notch (H.A.P.):
3-3/4" solid lumber (Unbroken strength)
Tail overhang length (sloped):
13 7/16" long (for 12" eave run)
Roof Framing Field Guides & Carpentry Manuals
Learn how to mark plumb cuts, calculate bird's mouth notches without weakening rafters, and attach roof framing securely to exterior walls.
Roof Pitch Explained: Rise, Run, Degrees & Framing Ratios
Demystifying 4/12, 6/12, and 8/12 roof slopes. How to measure pitch on existing roofs and pick the right angle for drainage and shingles.
The Ridge Board Deduction: Why Rafters End Up Too Long
Why forgetting to deduct half the ridge beam pushes walls outward. The simple geometry of true rafter runs and apex plumb cuts.
Rafter Tails, Overhangs & Hurricane Ties (IRC R802)
Plumb fascia cuts, sub-fascia blocking, and how to fasten rafters to top plates with Simpson H2.5A hurricane ties against wind uplift.
Frequently Asked Questions
A bird's mouth is an angled triangular notch cut into the underside of a rafter where it rests on the exterior wall's top plate. It consists of two cuts: a horizontal seat cut that sits flat on the 2x4 or 2x6 wall plate, and a vertical heel cut (plumb cut) that rests flush against the outside edge of the wall plate.
Under the International Residential Code (IRC Section R802.7.1), the vertical plumb notch cannot exceed one-third (1/3) of the rafter's total width. For example, on a 2x6 rafter (5.5" actual width), the vertical cut should never exceed 1-13/16". Cutting deeper weakens the lumber and can cause the rafter to split horizontally along the grain under snow loads.
Rafters do not meet tip-to-tip; they sandwich a center ridge board between them. If your ridge board is a standard 2x (which is 1-1/2 inches thick), you must subtract half of that thickness (3/4 inch) horizontally from the top of each rafter. If you forget this deduction, your rafters will push your exterior walls outward by 1-1/2 inches, making the building out of square.
H.A.P. stands for "Height Above Plate." It is the vertical thickness of solid wood that remains above the horizontal seat cut of your bird's mouth. Maintaining a consistent H.A.P. across all your rafters is critical because it ensures that your roof plane is completely flat and that your ceiling drywall or soffit panels will line up straight.