Birdsmouths: the third-depth rule, seat and plumb cuts

How deep the notch goes, why two rules disagree by three per cent, and the three different lengths that one notch produces.

The birdsmouth is the notch that lets a rafter sit down on the wall plate instead of balancing on one arris: a seat that lies flat on the plate, and a plumb face bearing against its outside edge.

A third of the depth

The working rule is that the notch goes no deeper than a third of the rafter depth. On a 47 by 147 rafter that is 49mm, leaving 98mm of uncut timber above it.

The reason is what the rafter has left. Cut half the depth away over a bearing and you have taken the section down where the load is concentrated. A third keeps enough behind the notch to carry the roof and resist splitting from the internal corner.

Note the wording. It is a limit, not a target: nothing says cut a third out, it says cut no more. Set the notch on d over 3 and every rafter sits exactly on the line, with nothing in hand.

Goss asks for a bit more

Goss's reckoner states the same rule the other way round, and tighter: a birdsmouth should leave at least 0.7 of the rafter depth. A third off leaves 0.667, so the rule most people quote sits a whisker the wrong side of the book.

On that 47 by 147 it is a 44mm notch against 49, leaving 103mm instead of 98, and a height above plate of 126 against 120.

CutRoof defaults to the book's figure, because it is the one that satisfies both rules: 0.7 left is comfortably inside a third off, and a third off is not inside 0.7. Convention C4 switches to the third-depth rule if you want the number most carpenters quote, and the schedule prints which one produced its figures.

One notch, three lengths

The depth is measured square to the rafter edges, and the two cuts you mark are both longer. On the same rafter at 35 degrees:

Notch depth, square to the edges: 44mm
Birdsmouth plumb cut: 44 ÷ cos 35° = 54 mm
Birdsmouth seat cut: 44 ÷ sin 35° = 77 mm

Three numbers from one notch, and they get mixed up. Mark 44 along the seat instead of 77 and the rafter sits on the arris and rocks. The app names all three, and the diagram shows which line each belongs to.

When the plate is too narrow

The seat has to fit on the wall plate. On a 100 plate at 35 degrees a 77 seat is fine; drop the pitch or deepen the rafter and it stops being fine. A 47 by 220 at 30 degrees wants a 66 notch, asking for a 132 seat on a 100 plate: it would run past the inside face and take the bearing with it.

CutRoof shortens the seat to the plate width, reduces the notch to suit, and says so in a warning: a 50 notch and a 100 seat, with the height above plate moving to 196 to match.

Marking it

Set out the plumb line first, because it is the end of the true length. Measure the notch depth square from the top edge down it, then square the seat off towards the tail.

Finish the internal corner with a handsaw. Over-cutting past it with a circular saw is the quickest way to start a split in a rafter that has already lost a third of its depth.

Every figure above is worked out by the same engine that produces CutRoof's cut schedules, and is checked against it on every build.