Birds mouth cut over 1/4 depth of rafter

Started by evolvingdesigns, February 23, 2021, 02:36:05 PM

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We have 2X12 roof rafters spanning 10'4 from ridge to wall which is less than 1/2 the distance code allows it to span. Code states a birds mouth cut can only cut into a rafter 1/4 of the depth...since a 2x6 can technically span the 10'4" is it possible to cut the birdsmouth 1/2 the depth of the 2 X 12 rafter?


OK, there's one of the stupid rules  :D
No, you are limited to a 1/4 depth notch without engineering.


I believe the D/4 provision only applies within the load span of the rafter, which ends 1 & 1/2" from the inside edge of the wall plate....beyond that, you cannot cut a notch closer than 3 & 1/2" of top edge of rafter, cantilever may not extend more than 24" beyond outside of wall. I don't believe any inspector will fail a properly cut birdsmouth in a sound 2x12 rafter....
"The lyf so short, the craft so long to lerne."


No, they did change it several code cycles ago. There is a clear drawing in the current codes showing it applies to the birdsmouth. Kind of nuts but here is the logic. In the old codes you could make the notch as large as you wanted as long as it did not extend beyond the inside face of the wall /and as long as there was at least a 2x4 of wood extending over the wall with no more than 2' of overhang. In grading rules a #2 is allows up to a 1/3D knot. So it is possible that a numbnut could leave pretty much nothing but knot supporting the overhang. I've never seen that happen so consider this logic pretty insane. If you are trying to make a compliant hip it gets really fun.

For a thinking exercise consider a bobtail rafter, no overhang just a plumb cut at the outer wall line with a level seat cut. Now there is no prohibition on the width of the seat cut.

Want an overhang on that? Apply a scabbed on cantilever tail, say a 2x4 with 2/3 inboard, 1/3 outboard and well secured. If the inspector is a real @$$ that is an engineered solution. If there is anything going on in his noggin he will allow leaving the 2x12 tail intact and applying a 2x4 canti alongside the tail to satisfy grade. Now 99% of the time it is double strong and with the odd large knot 2x12 it is single strong. You've wasted a few 2x4's.

I know where this code started, it was a young know it all carpenter that hopefully by now realizes he didn't know all that much. Couldn't talk him out of pushing it and the powers that be were all to willing to accommodate his notions. That is how we get dumb laws.


I guess they do have to write the codes for the dumbest and crappiest carpenters out there, which is unfortunate.....but let's be honest, pretty necessary... ;D
"The lyf so short, the craft so long to lerne."