How do I design a bird's mouth for my rafters?

Question

I've built 2x4 walls for my shop, and now I need to cut the bird's mouth into the rafters for the roof. I understand the concept, but I'm a bit shy on the exact details.

I will have 2x6 rafters in a vaulted ceiling (the drywall will be directly attached to the inside of the rafters until it hits the collar ties) sitting atop the 2x4 top plate. What I've designed so far has 5-1/2" of rafter left vertically at the inside edge of the wall, leaving only 3-1/4" of wood at the outside edge of the wall. Here's a drawing:

Note: Disregard the sheathing there, it will not be inside the bird's mouth as drawn.

1. Is this the proper way of doing it?
2. Do I need to change the angle of the rafter so there is 5-1/2" of wood above the outside edge of the wall?
3. Does the bottom of the rafter need to meet up with the inside edge of the top plate, leaving however much wood above that leaves?
4. Is there something totally different I need to do?
5. Am I over thinking this too much?

As I've been writing this up, I believe that option 3 is the correct answer. Since the ceiling is vaulted, the way I've drawn it will leave me with a short section of horizontal ceiling (about 1-1/2") at the wall because of the flats on all the other rafters not at the gable end. (Remember, this is a vaulted ceiling, drywall will be attached directly to the bottom of these rafters.) If I change the pitch of the roof to bring the bottom of the rafter to the inside edge of the wall, that will give me a nice, clean joint between the wall & ceiling finish material.

Answer 1

When I am cutting rafters and the rafter tails do not need to meet a specific design detail, like going low enough to meet a top of a window with certain size facsias, etc. I will make the birds mouth seat cut the width of the plate it sets on, in your case 3 1/2". This will allow a large enough width to nail on either side. It could be smaller, I think code allows down to 1 1/2" with collar ties/ceiling joists (needs verification) The consideration would be what is the narrowest width of the rafter will be at the birds mouth and how wide the overhang will be, and will that narrow dimension support the overhang you are considering. This decision is also influenced by whether you will be adding "lookouts", blocking that is added under the overhang to create a level soffit.

Answer 1) There are a number of way to do it, what you have drawn is one of them.

Answer 2) What you have drawn will, beyond the gable wall have the corner, made by the cut of the birds mouth, to be unsupported. Anything that is unsupported, essentially reduces the size of the material you are using, since it can split, not being supported. The 5 1/2" dimension you have drawn is not illustrated properly in the way the strength of the material is. The strength run the length, not at an angle, in respect of how the rafter lays, if I stated it properly. My apologies if it leads to some confusion.

Answer 3) I believe some wood beyond the inside edge of the plate is allowable, if it is too much a metal strap may be required or desired to keep the rafter from splitting along the unsupported edge. A ceiling rafter would help, but since you are doing cathedral, that cannot be an option, ceiling rafters that is. Setting the cut for the edge of the birds mouth at the edge of the plate line creates an easy line for finishes to follow without adding nailers.

Answer 4) Nope I don't think so.

Answer 5) Maybe, I do the same thing.

Answer 2

The birdsmouth looks very large, but I can't say if it is bad.

The thinnest part of the rafter above the top plate must be sufficient to support the single ended eaves overhang load and fattest part must be enough for the main roof load.

Other than that, whatever suits your design.

Do I need to change the angle of the rafter ...

perhaps you could raise both ends instead of changing the angle.