Birds Mouth Depth?

[evolvingdesigns]

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?

 

[jayrod12]

No. The limiting depth of notch is to avoid the chance of a horizontal split as the tension from the bottom of the joist in bending attempt to move upward rapidly at the notch.

Also, much of the grading of lumber has to do with knotches and knots in the extreme fibres. A 2x12 ripped down to 5 1/2" deep may not grade the same as a 2x6.