Expert: Bruce E. Johnson - 1/29/2005

Question
How do I figure the angles for the birds mouth cut in the roof Rafters for differant pitched roofs?  Thank You.

Answer
Hi Cameron, the birds mouth angle is related to the rafter angle. Are you familiar with a framing square and know how to make your peak angle cut using the framing square?  Say you have a 4/12 pitch: the outside edge of the framing square has numbers on it, you set the four on one side and the twelve on the other at the bottom edge of the rafter. By marking on the rafter along the "four" side of the framing square you get your "plumb cut" or peak angle.  This will be the same angle as the verticle cut, or outside cut, on your birds mouth.  The inside flat or horizontal cut of the birds mouth that sits on top of the wall is merely a cut that is squared off of the verticle birds mouth cut.  Now the tricky part is determing the location of the birds mouth in relation to the peak cut. Looking at your rafter as it lays in front of you with the peak side to your right and the wall side to your left. Start at the peak of your rafter and mark your peak cut (just mark it don't cut it yet).  Now the shortest side of the peak cut will be the point you will do your measuring from. Using your framing square set the twelve at this point and the four to your left at the bottom of the rafter and make a mark on the four side.  This distance will equal one foot of rafter run.  So if your rafter "run" (this is the flat horizontal distance from the center of the room to the outside of the bearing wall the rafter sits on) from the peak to the outside of your wall is ten feet then you keep stepping down your rafter ten times, one per foot.  The last mark on your left after ten steps will be the birds mouth verticle cut.  I usually measure up 1 1/2 inches on this verticle cut and then square over for the top of the birds mouth.  If your wall is thicker than four inches and you want more bearing on top of the wall with your birds mouth, then increase the height of your verticle birds mouth cut and then square over.  The higher your verticle the longer your flat cut.    I hope this information helps, feel free to write again regarding this or other matters, sincerely bruce e johnson