Granted a structural ridge can lessen the amount of force on a rafter but even then i would.
How to cut a birdsmouth in a roof rafter.
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.
This ensures that you make the birdsmouth cut at the right length in the rafter.
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.
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.
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.
Due to the width.
72 inches wide divided by 2 36 inches.
Calculate rafter or joist length to birdsmouth location.
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.
Multiply the figure from above by it s square and do the same for the height to the top.
With the ripped strip installation there would be no horizontal bearing surface to resist that force.
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.
This new line marks the horizontal cut for the bird s mouth.
Use the square root figure to measure along the top of the rafter.
Measure and mark plumb cut.
Make a mark along the top edge of the rafter where this measurement ends.
This point is the start of the bird s mouth.
Take a break from the measuring by cutting the top end of the rafter first.
When you cut the rafters you ll need to make 3 separate cuts.
Divide the width of building by 2 e g.
