I am framing my gable roof using 2 by 10 rafters, 12/12 pitch. Using blocklayer's online rafter calculator I got a warning saying "birds mouth too deep!". I calculated for 6 in. (5 1/2 top plate plus 1/2 in. sheathing) so the bottom of the rafter would line up with the inside of top plate. this still leaves 7 in. of lumber vertically from the outside of top plate to the top of the rafter. Seems like plenty and that is how I've seen it done on this site.
Is that right or is it a flaw in the program?
Any reason not to leave the rafter tails @ 10 in.? I've seen most cut them to about 6 in. I am planning an open soffit.
The calc is right. If you click on the little "calc" link right there beside where you insert the birdsmouth foot cut dimension, a pop up box will suggest the maximum foot cut to use for that pitch and rafter depth. The foot cut does not have to be the same length as the wall thickness.
If you do use a 6" foot cut all you are weakening is the tail. If the roof had a lot (3-4 ft) of overhang that might be a problem. But with a short overhang and a 12:12 that won't allow much snow build up you first idea will likely be alright.
Got it.
I wanted the full cut so I could line up drywall @ walls and ceiling (cathedral ceiling). Overhang is 18 in. so I think I will be fine.