I thought I would test an assumption. If the span and loads mean you could have a smaller timber (say 3x2) and you choose a larger timber for aesthetic reasons, can you do a larger birds mouth than the usual rules would allow (often D/3)?

I have a simple porch roof that is spanning a gap of 800mm at a pitch of near 45 degrees. Span tables don't go down as low as 3x2 (70mm x 45mm), but a 89mm x 38mm could do a span of 1.45m even at the most onerous loading [link], so I'd expect 3x2 would be fine at 800mm but look weedy so I'm using 5x2 (120mm x 45mm). My geometry means that the birds mouth cut I'd make would leave about 58mm which would be a sensible birds mouth for 3x2, but not 5x2. I've seen people talk about a crack propagating back up the rafter, but in my situation I expect the main issue would be the eave overhang (320mm horizontally).

Is there any guidance when you are using oversized rafters? Is it logical to assume that you should leave as much timber at the birds mouth as the smaller timber would need and

so you can ignore the D/3 rule of thumb?

The D/3 rule is there for a reason that doesn't go away just because the rafter is oversized. The notch leaves a sharp internal corner right at the support, and that's where timber likes to split along the grain, especially with an overhang levering down on the other side of the plate. With 320mm of overhang, the biggest bending moment on the rafter sits right over the wall plate, which is exactly where you'd be taking out the most wood.

So I'd treat 58mm left as too little. It's less than a whole 3x2, which is the section you've worked out the span needs, and a notched piece is weaker than a plain one of the same depth. As a rough guide I'd want at least the full depth of the smaller timber left above the seat, and ideally stick to the third so you keep about 80mm on a 120mm rafter.

A couple of ways round it without losing the look:

• Bevel the wall plate, or fix a splayed packer on top of it, so the seat cut can be much shallower.

• Fix each rafter to the plate with a rafter tie or truss clip as well as skew nails, so you're not relying on the notch alone.

On an 800mm span porch the loads are small, but if it's going through building control, a quick sketch to them showing the overhang usually gets you a straight answer.

1 hour ago, JpManning said:

The D/3 rule is there for a reason that doesn't go away just because the rafter is oversized. The notch leaves a sharp internal corner right at the support, and that's where timber likes to split along the grain, especially with an overhang levering down on the other side of the plate. With 320mm of overhang, the biggest bending moment on the rafter sits right over the wall plate, which is exactly where you'd be taking out the most wood.

So I'd treat 58mm left as too little. It's less than a whole 3x2, which is the section you've worked out the span needs, and a notched piece is weaker than a plain one of the same depth. As a rough guide I'd want at least the full depth of the smaller timber left above the seat, and ideally stick to the third so you keep about 80mm on a 120mm rafter.

A couple of ways round it without losing the look:

• Bevel the wall plate, or fix a splayed packer on top of it, so the seat cut can be much shallower.

• Fix each rafter to the plate with a rafter tie or truss clip as well as skew nails, so you're not relying on the notch alone.

On an 800mm span porch the loads are small, but if it's going through building control, a quick sketch to them showing the overhang usually gets you a straight answer.

Thanks. I do intend to use a truss clip (as well as the 5x100 screw that pulls the joint tight).

58mm left of a 3x2 rafter would feel fine (83%) but I am inclined to agree with your thought that it's too little for the geometry I have. I suspect it's structurally fine, but the BC might think otherwise.

The beam that the rafters come down on is very oversized, so notching the top at each rafter would be fine if somewhat of a nuisance. It would require a notch depth of 32mm to meet the D/3 spec for the 5x2 timber. I guess the notch could be just knocking the corner of the beam off, leaving a rather complex rafter cut with separated seat and heel (trapezium notch, second picture). Easily done with a jigsaw which is how I cut my birds mouth anyway. On first inspection, it make the rafter beam contact somewhat over constrained in that there are 3 mating surfaces rather than a more sensible 2. In reality, I am cutting the heel slightly away from the beam to allow space for the truss clip to slide in. I'm tempted to go this more complex route in the spirit of making my life more complicated :-).

1 hour ago, MortarThePoint said:

Thanks. I do intend to use a truss clip (as well as the 5x100 screw that pulls the joint tight).

58mm left of a 3x2 rafter would feel fine (83%) but I am inclined to agree with your thought that it's too little for the geometry I have. I suspect it's structurally fine, but the BC might think otherwise.

The beam that the rafters come down on is very oversized, so notching the top at each rafter would be fine if somewhat of a nuisance. It would require a notch depth of 32mm to meet the D/3 spec for the 5x2 timber. I guess the notch could be just knocking the corner of the beam off, leaving a rather complex rafter cut with separated seat and heel (trapezium notch, second picture). Easily done with a jigsaw which is how I cut my birds mouth anyway. On first inspection, it make the rafter beam contact somewhat over constrained in that there are 3 mating surfaces rather than a more sensible 2. In reality, I am cutting the heel slightly away from the beam to allow space for the truss clip to slide in. I'm tempted to go this more complex route in the spirit of making my life more complicated :-).

Fair enough, if you keep a decent seat bearing that's the main thing. A shallow notch into the top of the beam to suit is a reasonable way round it given how oversized it is, just keep it well within D/3 of the beam depth and clear of any bolt or fixing positions.

The trapezium cut works too, but every extra cut is another chance for a gap, so make sure the rafter bears fully on the seat rather than hanging off the heel, and that the truss clip still gets its full nail pattern into both members. Either way I'd send BC a quick sketch with the dimensions before you cut, as they're usually happy once they can see the bearing and the fixings spelt out.