I'm fitting some 3m rafters on a shallow-sloped, lean-to extension, and I'm trying to figure out an easy/reliable way of getting the geometry right...
What's the best way to measure, mark up and cut the rafter/wall board angle and rafter/wall plate bird's mouth so that the rafter meets the wall board at the correct angle, rests on the outer skin of the cavity wall and conforms to the wall plate accurately?
I've been told that if you rest the uncut rafter on the wall plate at the same angle as the slope of the roof, then mark a line perpendicular to the rafter's lower edge from the point where it touches the wall plate, then mark a right angle to form the bird's mouth, this will give the correct cut out --- but I can't see how you'd get the correct depth (or, practically, how you go about holding a 3m rafter in position while you mark it up).
Is there an easy way to do this with a right angle, tape measure and a bit of dead reckoning? ;-)
Any help appreciated.