How to Layout and Cut a Bird's Mouth Notch with a Framing Square

Introduction: Anatomy of the Bird's Mouth Notch

When framing a gable, shed, or hip roof, the rafters do not simply sit balanced on top of the exterior walls. Because rafters are angled at an incline, resting a flat board on a flat wall plate creates only a knife-edge point of contact. Under heavy roof loads, this point contact would slip, roll, or split the timber.

To give the rafter a solid, level bearing area that distributes the roof weight evenly onto the wall studs, carpenters cut a triangular notch into the underside of the timber. This notch is known as the Bird's Mouth.

A bird's mouth consists of two intersecting straight cuts:

• The Seat Cut (Level Cut): The horizontal bottom cut that sits flat against the top of the wall's double top plate. Under the International Residential Code (IRC Section R802.4), the seat cut must provide at least 1-1/2 inches of direct bearing surface on wood framing (3 inches on concrete or masonry). On exterior walls framed with 2x4 lumber, carpenters usually make this cut 3-1/2 inches long to span the full width of the wall plate.
• The Heel Cut (Plumb Cut): The vertical cut that runs down flush against the outside face of the wall plate or sheathing.

Phase 1: Understanding H.A.P. (Height Above Plate)

Before you mark your wood, you need to know about the most important measurement on a rafter: H.A.P., which stands for Height Above Plate.

H.A.P. is the vertical thickness of solid wood that remains directly above the corner of your seat cut up to the top edge of the board.

Phase 2: The IRC 1/3-Depth Notch Limit (IRC R802.7.1)

Can you make the bird's mouth cut as deep as you want so the rafter sits low on the wall? No.

Under IRC Section R802.7.1, notches in roof rafters cannot exceed one-third (1/3) of the rafter's actual board depth. Cutting deeper than one-third removes too much solid wood and can cause the rafter to split horizontally along the grain under snow loads.

Depth Limits for Common Lumber Sizes

If your roof pitch is steep (such as 8/12 or 10/12) and a 3-1/2" seat cut would force your vertical plumb cut deeper than the 1/3 limit, reduce the seat cut length to 2-1/2" or 3", or upgrade your rafter lumber from a 2x6 to a 2x8.

Phase 3: Scribing the Cut with a Framing Square

Here is the job-site method for marking the bird's mouth cleanly using a steel framing square and a pair of brass stair gauges:

1. Inspect and Crown the Timber: Sight down the narrow edge of your 2x board. Find the crown (the slight arch curving upward) and face that edge upward.
2. Set Your Framing Square: Clamp one brass stair gauge on the tongue (the short 16" arm) at your roof pitch rise (for example, 6 inches for a 6/12 pitch). Clamp the second brass gauge on the body (the long 24" arm) at 12 inches.
3. Mark the Building Line (Heel Plumb Line): Lay the square on the crowned timber near the lower end. The body gives you the level direction, and the tongue gives you the vertical plumb direction. Scribe a plumb line along the tongue. This is your heel line, which will sit flush against the outside edge of your wall plate.
4. Measure Down for Your Seat Cut: Along that heel line, decide your cut depth. For a standard 2x6 with a 6/12 pitch, a 3-1/2" seat cut creates a plumb cut depth of 1-3/4 inches (3.5" × 6/12 = 1.75"). Mark a tick mark 1-3/4" down from the bottom of the rafter on that heel line.
5. Scribe the Level Seat Line: Rotate your framing square 90 degrees, lining up the body of the square with your tick mark so it runs perpendicular to the plumb line. Draw the horizontal line out toward the rafter tail.
6. Mark the Waste Block: Mark an "X" in the triangular notch so you do not cut on the wrong side of your pencil line.

Phase 4: Cutting the Notch Safely (The 2-Tool Method)

Just as with stair stringers, cutting a bird's mouth requires care at the inside corner.

Step-by-Step Cutting Procedure:

• Step 1: Cut along the horizontal seat line with your circular saw, stopping the blade about 1/2 inch before the inside corner.
• Step 2: Cut down the vertical heel plumb line, again stopping the circular saw blade 1/2 inch short of the corner.
• Step 3: Pick up a handsaw or jigsaw, place the blade into the kerf, and saw the remaining 1/2 inch cleanly by hand. The scrap wood will drop free, leaving a sharp, square corner with zero overcut.

Phase 5: Making a Master Pattern Rafter

Do not measure each rafter individually from scratch with your tape measure and framing square. Small differences in pencil lines can add up and make your rafters slightly different lengths.

Instead, cut your first rafter completely:

1. Test-fit this first rafter directly on your building framing against the ridge board and the exterior wall.
2. Verify that the seat cut sits flat against the top plate and that the top plumb cut fits flush against the ridge.
3. Once confirmed, write MASTER PATTERN — DO NOT INSTALL on the board in permanent marker.
4. Use this master board as a physical template to trace the cuts onto all your other rafter boards.