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A birdsmouth is the notch at the lower end of a common rafter. It lets the rafter sit flat on the wall plate while the plumb cut at the heel lines up with the outside face of the wall. A framing square can mark both cuts accurately, but only if you establish the roof pitch and wall bearing point first.
What you need
You will need a framing square, a sharp pencil or marking knife, a tape measure, a straightedge, and the rafter stock. A pair of framing squares with stair gauges is convenient, but one square works perfectly well if you mark carefully.
Use a square with clear, legible graduations. The long blade is normally 24 inches and the short tongue is 16 inches. Check that the inside corner is clean and that the edges are straight. A bent square or one with a damaged corner will transfer errors to every rafter.
You also need the roof pitch, such as 6-in-12, and the required overhang. “6-in-12” means the roof rises 6 inches for every 12 inches of horizontal run. Confirm the pitch from the plans rather than guessing from an existing roof.
Understand the two cuts
The birdsmouth has two intersecting lines:
- Plumb cut: the angled cut across the rafter’s width. It runs vertically when the rafter is installed and usually forms the heel at the wall’s outside edge.
- Seat cut: the horizontal cut that rests on the wall plate.
The seat cut should provide a full, stable bearing surface. Do not make the notch deeper than one-third of the rafter’s actual depth unless the design has been specifically engineered for it. On a nominal 2-by-6 rafter, the actual depth is about 5 1/2 inches, so one-third is about 1 13/16 inches. Local codes, span tables, and the building designer may require less.
Measure the seat-cut length from the plumb-cut line toward the tail. It must be long enough to bear on the wall plate, commonly 3 1/2 inches for a nominal 2-by-4 plate, but verify the actual plate width. The cut should not extend past the inside edge of the plate unless the plans call for a different detail.
Mark the plumb cut with the framing square
Start at the lower edge of the rafter. Mark the point where the outside face of the wall will meet the rafter. This location depends on how you calculated the rafter length and overhang. If you are laying out from the building, use the outside wall line, not the inside edge of the wall.
Place the framing square on the face of the rafter with the tongue pointing toward the rafter’s tail. Use the blade’s rise numbers and the tongue’s 12-inch run scale. For a 6-in-12 roof, align the 6 on the blade with the top edge of the rafter and the 12 on the tongue with that same edge. Hold the square firmly and draw along the blade. This is the plumb-cut line.
For a 4-in-12 roof, use 4 on the rise scale and 12 on the run scale. For an 8-in-12 roof, use 8 and 12. Keep the square’s reference edge against the same rafter edge every time. A small change in orientation can alter the cut noticeably over a long rafter.
Mark the seat cut
From the intersection of the plumb-cut line and the lower edge of the rafter, measure along the bottom edge toward the tail by the required bearing length. Mark that point. Use a carpenter pencil and marking knife if you want a durable layout line; the pencil is faster, while a knife line is easier to follow with a handsaw.
Now place the framing square so one leg runs along the rafter’s bottom edge and the other crosses the face at your bearing mark. Draw a line across the rafter to connect the bearing mark with the plumb-cut line. This is the seat-cut line. The two lines should meet cleanly at a sharp inside corner.
Do not extend the seat cut farther than needed. An overlong notch weakens the rafter and can leave too little wood at the heel. If the seat depth exceeds one-third of the rafter depth, stop and recheck the roof geometry rather than simply cutting it.
Use stair gauges or mark each rafter
Stair gauges clamp to the framing square at the selected rise and run marks. They make repeated layout faster because the square registers in the same position on every rafter. A set of framing-square stair gauges is useful when cutting more than a few rafters, but it is not essential for a small repair or shed.
| Layout method | Best for | Trade-off |
|---|---|---|
| Square without gauges | Small jobs and one-off cuts | Cheap and flexible, but requires careful repositioning |
| Square with stair gauges | Repeated common-rafter layout | Faster and more consistent, with a small extra cost |
| Template rafter | Large batches of identical rafters | Saves time, but an error is repeated on every piece |
After marking one rafter, cut it and test-fit it on the wall plate or a mock-up made from matching lumber. Check that the seat lies flat, the plumb cut lands at the wall line, and the overhang is correct. Use that rafter as a template only after it fits. Mark the rest from the template, then verify several pieces with the square rather than trusting the template blindly.
Cutting and common errors
Cut the plumb line first, then cut the seat line. Stop at the intersection and avoid cutting beyond the layout lines. A handsaw gives good control, while a circular saw is faster but can overcut the inside corner. If you use a circular saw, finish the corner with a handsaw or chisel instead of allowing the blade to chew past the lines.
The most common failures are using the wrong pitch scale, measuring from the wrong wall face, confusing nominal and actual lumber dimensions, and making the seat cut too deep. Another mistake is reversing the square, which produces the opposite roof angle. Before cutting a batch, label the rafter’s crown, tail, wall line, and waste side. That simple step prevents many mirrored or backward birdsmouths.
For a structural roof, compare the finished layout with the plans and local code requirements. A framing square makes accurate marks, but it cannot correct an incorrect rafter length, insufficient bearing, or a design that needs an engineer’s review.