How to Cut a Birdsmouth: Notching Rafters for a Pent Roof
The rafters of our log cabin roof needed a solid connection to the log walls. The quick way is metal rafter brackets: the rafter sits on a wall plate and is nailed to the bracket. We wanted the rafters to finish flush with the log walls, though, and a bracket would have been in the way. Our pent roof also has no wall plate – the rafters sit directly on the top log, which is only 5 cm (2 in) thick. That leaves very little timber to nail brackets into securely. So we chose the more work-intensive but stronger timber joint: a birdsmouth in every rafter and a matching seat in the top log.
Quick answer:
- Notch the top log at every rafter position so the rafters cannot slide and the spacing is fixed.
- Lay one rafter on the walls and take the roof angle with a sliding bevel.
- Transfer the angle to every rafter: a plumb cut at the roof angle and a seat cut at a right angle to it.
- Saw both cuts with a fine saw and chisel out the waste. Our notches are about 5 cm (2 in) deep in 16 cm (6 in) rafters.
Step 1: Seats for the rafters in the top log
Once the log walls were up to the top course, we prepared them for the roof. Wherever a rafter would sit, we cut a shallow notch into the top log. This has three advantages:
- The rafters cannot slide sideways.
- The rafter spacing – 60 cm (about 24 in) in our case – is fixed automatically.
- The rafter bears on the full width of the wall, not just on the tongue of the top log.
We marked the notches and cut them out with a sharp saw and a chisel. With that, the walls were ready.
Step 2: Preparing the rafters
For the rafters we used ordinary construction timber, 8 × 16 cm (approx. 3 × 6 in), spaced 60 cm apart. Rafter size and spacing must be worked out by a structural engineer: snow loads in winter can be considerable, and they depend on the roof build-up. Ours is a light roof with timber boards (guide in German) and bitumen felt. A green roof is much heavier and needs bigger rafters at closer spacing.
The timber arrived dry and rough-sawn. Rough-sawn timber holds water on its surface and soaks up a lot of stain or preservative, so we planed all four sides smooth with a power planer and then cut the rafters to length with a mitre saw.
Step 3: Marking the birdsmouth
Because a pent roof slopes – anything else would make little sense – the notch cannot simply be cut at a right angle. We laid one rafter on the walls in its final position and took the angle between wall and rafter with a sliding bevel. We then transferred this angle to every single rafter.
The photo shows that the notch is deeper on one side than on the other – it follows the roof slope. So that the rafter sits flat on the wall over its full width, the seat cut (the horizontal face that rests on the wall) must be at a right angle to the plumb cut (the vertical face against the wall).
Step 4: Cutting the notch
For the base frame we could use a chainsaw, but the birdsmouth called for finer tools. We made the cuts with a Japanese pull saw. With a good dozuki or kataba, these fine, accurate cuts are quick and easy. The kataba is especially suitable because it has no spine above the blade, so the cutting depth is not limited.
Then we chopped out the waste with a chisel and mallet. The notch is about 5 cm (2 in) deep. It gives a strong connection between rafter and wall and keeps the walls at exactly the same distance apart along the whole roof. A common rule of thumb is not to cut away more than a third of the rafter depth – with 5 cm out of 16 cm we stayed well within that.
Once all the birdsmouths were cut, the rafters could be set on the log walls and fixed. Then the roof was covered with tongue and groove roof boards.
Frequently asked questions
What is a birdsmouth cut?
A birdsmouth is a notch cut into the underside of a rafter where it sits on a wall or wall plate. It consists of a plumb cut and a seat cut at right angles to each other. The seat cut lies flat on the wall, so the rafter bears on its full width and cannot slide off.
How do you mark a birdsmouth at the right angle?
Lay one rafter in its final position on the walls and take the angle between wall and rafter with a sliding bevel. Transfer this angle to every rafter. The seat cut is then drawn at a right angle to the plumb cut. On a sloping roof the notch is deeper on one side than on the other.
How deep should a birdsmouth be?
Our birdsmouths are about 5 cm (2 in) deep in rafters that are 16 cm (approx. 6 in) high. A common rule of thumb is not to cut away more than a third of the rafter depth, so enough timber remains above the notch to carry the load. Rafter size and spacing should be checked by a structural engineer.
Birdsmouth or metal rafter brackets?
Metal rafter brackets are quicker: the rafter sits on the wall plate and is nailed to the bracket. We chose a birdsmouth because we wanted the rafters to finish flush with the log walls, and because the top log of our cabin is only 5 cm (2 in) thick – too little to fix brackets securely.
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