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.
How to cut a birdsmouth in a roof rafter.
Due to the width.
This new line marks the horizontal cut for the bird s mouth.
Use a square placing one side against the plumb cut line.
Calculate rafter or joist length to birdsmouth location.
Subtract half the width of the ridge board beam in this case 0 5 inches.
Make a mark along the top edge of the rafter where this measurement ends.
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.
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.
Multiply the figure from above by it s square and do the same for the height to the top.
This ensures that you make the birdsmouth cut at the right length in the rafter.
This point is the start of the bird s mouth.
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.
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.
Use the square root figure to measure along the top of the rafter.
Take a break from the measuring by cutting the top end of the rafter first.
With this information determined and the rafter ends cut to the proper pitch lay out of the bird s mouth is fairly easy.
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.
Possibly the most satisfiying part of the roof yet.
Measure and mark plumb cut.
