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Old 07-03-2015, 02:20 AM   #1
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Should birdmouth cuts be snug against top plate?


I've been building roof rafters for my shed, and generally there has been a little play in between the bird mouth cuts and the roof rafters (maybe a quarter of inch). I've been knocking myself out trying to make these birdmouth cuts snug against the top plates. Is this necessary? I know I can glue some shims in the gaps, but I don't know if this is good practice.

Thanks
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Old 07-03-2015, 04:17 AM   #2
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Quote:
Originally Posted by darsunt View Post
I've been building roof rafters for my shed, and generally there has been a little play in between the bird mouth cuts and the roof rafters (maybe a quarter of inch). I've been knocking myself out trying to make these birdmouth cuts snug against the top plates. Is this necessary? I know I can glue some shims in the gaps, but I don't know if this is good practice.
No, it is not necessary to cut the birdsmouth to be snug against the top plate. Think about it. Rafter forces want to push downward or outward, not inward. If the rafters were not fastened to the top plate (discounting structural ridge, ceiling joists, collar ties, etc.) the heel cut of the birdsmouth would want to go outward, not inward. Shimming gaps in the heel cut will not do anything to strengthen the structure.

But it is a good thing to have the seat cut be the width of the top plate, and in conjunction, it is also a good thing to have a stud directly below each rafter. Roof loads will push down on the seat cut, through the top plate and down through the wall stud. If rafters are not directly above a wall stud, then a double top plate is necessary to distribute the roof load between the wall studs. If rafters are directly above wall studs, then a single top plate will suffice.

The depth of the heel cut and the size of the rafter determines the amount of overhang allowed.

HRG

Last edited by Homerepairguy; 07-03-2015 at 04:26 AM.
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Old 07-03-2015, 12:45 PM   #3
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If they are all sitting a quarter of an inch off then you are doing something wrong, don't have your clips set correctly on the framing square, are working from a pattern that is off, etc. MUST they sit flush? No....but SHOULD they? Yup if the building is framed correctly e.g. Walls straightened, ridge straight, etc. but keeping mind that you are building a shed with dimensional lumber and not a piano! Ron
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