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Birds mouth cut question

pizza's picture

Hi. When cutting the birds mouth, I see that most people don't account for the sheathing applied on the exterior wall. Is that normal or does it just depend on how you are going to finish the eaves?

I've seen some make the seat cut of the birds mouth a little wider, say 1/2" if using 1/2" sheathing. But most just cut the bird's mouth so that if butts right to the top plate with no gap for the sheathing.

Any suggestions? What's typically done?

I will be building a 10:12 shed roof (using 2" x 6" rafters and a 2" x 8" ridge beam) with an 18" overhang and plan to leave the rafters exposed, not closing it all in with a soffit.

I do not want to notch the sheathing around all of those rafters.

Thanks to all in advance.

Sheathing need not notch (post #215753, reply #1 of 8)

Sheathing need not notch around rafters.  Simply butt sheathing (and siding) to the 2x6 bird blocking.

birds mouth (post #215753, reply #2 of 8)

Hi . Thanks for the reply but I can't picture what you are saying. What is bird blocking?

bird blocking (post #215753, reply #4 of 8)

That is simply blocking between the rafters at a point located at the outer edge the wall that closes the space between the rafters, wall and roof deck. If the blocking is located such that the inter edge of the blocking and the outer edge of the top plate is aligned, the sheathing and the siding can be butted up to it. It eliminates the need for notching, and as the name implies, keep the birds out of the attic. :) The blocking can be plumb with with the wall or perpendicular with the rafters. Sorry, I can't do pictures. I think that there are pictures in the International Residential Code book.

I usually account for sheathing (post #215753, reply #3 of 8)

I notch for sheathing so that I have the full 3" at the top of the wall for nailing. Remember, among other things sheathing serves to negate sheer. For example, for 1/2" sheathing I make a 5/8" allowance.

That said, in very steep pitch situation I take into consideration how much the tail end of the rafter may be weakend by taking off an additional amount for sheathing. 

Mel Fros

slow down.. (post #215753, reply #5 of 8)

Put the wall sheathing on before the rafters.


Shoot (post #215753, reply #6 of 8)

oldhand wrote:

Put the wall sheathing on before the rafters.


Square and sheet the wall before standing it up......

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


But why shoot the poor bird? (post #215753, reply #7 of 8)

But why shoot the poor bird?

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

I dunno (post #215753, reply #8 of 8)

Can't see it from my house.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.