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
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.
You need an adjustable angle finder...it's like a set square but is
adjustable at the corner, so that you can place the timber onto the
wallplate, put one edge of the finder sitting flat on the plate and adjust
the other so that it is touching the edge of the joist which is angled.
Bring everything down, mark the angle onto the end of the timber and use a
pullsaw, set at the correct angle to cut all joist ends the same.
The way you mentioned above is also correct, although you don't try and do
it alone, you need someone at the other end to take the weight and hold it
where it will be eventually fixed...you then put a spirit level against the
outside brickwork so that it is upright and draw a line on the joist
end...if you then take it down and draw a line at 90deg from that, it will
sit squarely on the wallplate...joists don't sit on exterior brickwork
I would do it in stages:
First lay the rafter on the wall plate and offer the end up to the wall
board. You can sight along the top edge of the rafter to get the
position right with respect to the top of the wall board. You will want
to aim a couple of inches high (or whatever depth you plan to make your
birds mouth joint) since you have not cut the other end yet, so that
will be sitting a bit high. Now set the angle with respect to the wall
on an adjustable bevel.
Take your rafter down and mark the bevel angle on it and cut to shape.
Offer the rafter back to the wall plate - it should now be a good
parallel fit. Position it a little high again to allow for the birds
mouth. If working alone, then tack nail a block of scrap to the wall
board to rest the rafter on while you go to the other end.
At the wall plate end you can now mark the length to the outside of the
birdsmouth joint. Take the rafter down again, and use the bevel (still
set to the right angle from last time!) to mark the "up" cut of the
birds mouth joint from your marked length. Use a square to mark the
cross cut perpendicular to the up cut.
Finally cut out the birdsmouth joint with a hand saw or an electric
jigsaw. Test fit, and if it looks ok use that rafter as a pattern to
mark all the others.
There is with some basic trig, but marking it out will be quicker unless
you have a decent protractor handy for marking out the angles.
 You can also cut the eves end of the rafter to the same angle to get
your facias vertical.
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