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A birdsmouth is the notch at the end of a rafter that lets it sit on a wall plate. It has two cuts: a seat cut that rests flat on the plate and a plumb cut that meets the outside edge. A framing square makes both cuts easy to lay out, provided you first establish the roof pitch and check that the notch does not remove too much rafter depth.
Tools and materials
You’ll need a framing square, pencil or marking knife, tape measure, and a straight rafter blank. A sharp pencil is easier to see on rough lumber; a knife line is more exact but can disappear in splintered grain. A basic steel framing square is sufficient for occasional work. A rafter square can be faster for repeated layouts, but it is not necessary for marking one or two rafters.
Use a square with legible graduations and no bent or loose parts. Before relying on it, check it against a known straight edge or use the 3-4-5 method: mark 3 inches along one edge and 4 along the other; the diagonal between those marks should be 5 inches.
Understand the layout
Roof pitch is stated as rise over 12 inches of horizontal run. A 6:12 roof rises 6 inches for every 12 inches of run. On a framing square, the long blade is commonly called the body and the shorter leg the tongue. For a 6:12 layout, put the 6-inch mark on the tongue and the 12-inch mark on the body against the same rafter edge. The resulting angle gives the plumb line.
The seat cut is perpendicular to that plumb line. The birdsmouth’s inside corner is where the two cuts meet. The distance from the rafter’s top edge to the seat cut is the notch depth; it must leave enough wood below the cut to carry the load. Check local code and the project plans before cutting. A common limit is to keep the notch depth to no more than one-quarter of the rafter depth, but requirements vary, and some conditions call for less or prohibit a notch.
Mark the plumb cut
First mark the rafter’s length and the wall-bearing position from the plan. Account for the ridge detail, overhang, and any specified adjustment at the ridge; don’t assume the rafter’s overall length is the same as the building’s horizontal span. Mark the plumb cut at the appropriate reference point, usually where the rafter meets the outside face of the wall plate. Confirm that point against the layout before drawing the line.
Set the framing square for the roof pitch: for a 6:12 roof, align 6 on the tongue and 12 on the body along the rafter’s top edge. Hold both graduations firmly against the edge and draw along the tongue to mark the plumb line across the rafter’s face. Keep the square from rocking on knots or a rounded edge. If the lumber edge is rough, mark from the straightest face and transfer the line around the rafter with a combination square or by carefully extending it with the framing square.
Mark the seat cut
At the plumb line, measure the intended bearing length along the rafter’s bottom edge and mark it. The bearing length is set by the plate width and plans; for a typical 3½-inch-wide plate, a seat around 3½ inches may provide full bearing, but the actual layout must account for the rafter position and specified overhang. Don’t make the seat longer than needed: an oversized notch removes more wood without adding useful bearing.
Draw the seat line from the bearing mark to the plumb line so it meets the plumb cut at a square corner. One practical way is to use the framing square as a reference: the seat line must be perpendicular to the plumb line, not simply parallel to the rafter’s edge. You can check the corner with a small square or by comparing the layout to a template. Mark the waste clearly, then measure the notch depth from the top edge to the seat line at the deepest point.
Before cutting, lay the rafter against a piece of plate stock or a full-size layout. The seat should sit flat, and the plumb cut should land at the intended wall face. If either line misses, correct the marks now. A cut made slightly too deep can weaken the rafter; a shallow or uneven seat can leave the load bearing on an edge.
Choosing a marking method
| Method | Best for | Trade-off |
|---|---|---|
| Framing square and pencil | One-off layouts and general framing | Low cost and versatile; needs careful alignment each time |
| Rafter square | Quick repeated pitch marks | Fast to read, but less useful for some longer layout measurements |
| Story board or template | Several matching rafters | Improves consistency; an error in the first layout is repeated across the batch |
For a small job, a framing square is usually the sensible buy. If you’re marking many rafters, a rafter square can save time, but it does not remove the need to verify the first layout. A simple template cut from scrap is useful for production work, as long as you test-fit it on the plate before using it on every rafter.
Check before cutting
Make a test birdsmouth in scrap of the same size, then seat it on the actual plate stock. Check for full contact across the seat and confirm the plumb cut is in the right place. Repeat the layout on the first production rafter and fit it before marking the rest. Differences in lumber width, crown, or edge condition can make a copied mark inaccurate.
When cutting, stop at the inside corner rather than sawing past it. An overcut creates a stress concentration at the notch corner and can extend beyond the intended cut. Use a handsaw or circular saw for the straight cuts, finishing the corner carefully with a hand saw if needed. If the notch depth exceeds the allowed limit, the seat rocks, or the plans call for a different connection, don’t trim more material to force a fit; check the design or ask the responsible builder or designer.