The ridge cut also called the plumb cut at the top of the rafter the birdsmouth cut which itself comprises 2 different cuts where the rafter meets the building s wall and the tail cut at the base of the rafter.
How to cut a birdsmouth in a roof rafter.
Granted a structural ridge can lessen the amount of force on a rafter but even then i would.
Subtract half the width of the ridge board beam in this case 0 5 inches.
This new line marks the horizontal cut for the bird s mouth.
This point is the start of the bird s mouth.
Measure and mark plumb cut.
With the ripped strip installation there would be no horizontal bearing surface to resist that force.
Use the square root figure to measure along the top of the rafter.
Multiply the figure from above by it s square and do the same for the height to the top.
A lot of force is transferred down the length of the rafter and the seat cut on the birdsmouth transfers that force directly to the top plate.
Make a mark along the top edge of the rafter where this measurement ends.
Start by measuring perpendicularly from the tail end of the rafter to the point that the overhang length meets the edge of the inside part of the rafter forming a triangle.
With this information determined and the rafter ends cut to the proper pitch lay out of the bird s mouth is fairly easy.
In my photo above the green rafter line length is from the plumb cut face or ridge board face to the plumb cut face of the bird s mouth notch.
Divide the width of building by 2 e g.
Make one rafter that has the bird s mouth cut in place and extends out past that for your roof overhang test fit it in place and mark it pat for pattern rafter to use in cutting its sisters.
Possibly the most satisfiying part of the roof yet.
When you cut the rafters you ll need to make 3 separate cuts.
Use a square placing one side against the plumb cut line.
Draw a line at right angles to the bird s mouth plumb cut starting 3 inches up from the bottom of the rafter but no more than half the thickness of the rafter.
72 inches wide divided by 2 36 inches.
This ensures that you make the birdsmouth cut at the right length in the rafter.
