Hi, unsure about the depth of the birdsmouth in my roof rafters.?

Question

Hi, im currently getting an extension built and the depth of the birdsmouth is about 3/4 through the rafter. This doesnt seem correct to me. The roof is approx 18 degrees. Any advice would be very helpful.

Thanks

Mike

Answer 1

An 18 degree roof should be 4/12. Cutting a 3 and half inch birdmouth to fit the top wall plate should only take about 1and a quarter inches off the bottom. (Or about 1.75 inches on 2X6 walls.) Are you sure you are marking the right side of the rafter? The plumb line for the birdmouth is the same as the ridge cut on the rafter, and is in line with the outer wall. For the level cut, (the base of the birdmouth), go 90 degrees from the outside plumb line, 3 and a half inches to where it hits the inside of wall plumb line/bottom of rafter junction.

Here's a picture I found, http://daveosborne.com/newsletters/image...

The steeper the roof the more you would have to cut out, but (like for a very steep roof), you don't have to span the whole 3 and a half inches, just get enough to nail, or just use rafter ties.