When "bird mouthing" a rafter, how deep can the cut be?

Question

we are building a foof over our patio, 12x24. we are birdmouthing the rafters, but can not find out how deep a cut is too deep (10 inch boards)

Answer 1

Usualy the cut is 1 1/2 inch deep (perpendicular)so that the low edge is even with the top plate(bottom).What you want is the whole top plate  to be covered so you have plenty of room for fasteners(you won't regret useing metal plates /tiedowns,if you do) of course the debth is at an angle(ie square)

Answer 2

The "birdsmouth" is actually called a seat cut and it will be determined by the pitch of the roof your trying to achieve . the seat cut will not weaken the roof only the overhang so if your more than 6 inches into the board you will need to "sister" a 2x4 at the cut to strengthen the overhang. remember your seat cut need be only as deep as the wall it is sitting on "i.e. if its a 2x4 wall it needs only to be 4 inches hope this helps you

Answer 3

the birdsmouth cut is a pre-determined cut to get the rafter to lay on the top plate at the pitch angle of your roof. you lay a framing square on the rafter at the wall line, mark the rise and run points, and connect the dots(trace the outline of the square.