Without a birdsmouth the downward force will be transferred across a point to point contact. The birdsmouth also makes it easier to position the rafter, hold it in place, and nail the upper end to the ridge board, especially when working alone. Yes blocking material is installed at the wall, in between the rafters.
Wherefore, how do you calculate Rafter cuts?
By the way, how do you cut rafters with a speed square?
How big should Birdsmouth cut be?
When rafters are gang cut, the saw's cutting capacity helps determine birdsmouth dimensions; a 10 1/4-inch Big Foot saw is capable of making a 3-inch seat cut, which is suitable for a wide range of roof pitches and rafter sizes.
In light frame construction, a birdsmouth joint or bird's beak cut is a woodworking joint that is generally used to connect a roof rafter to the top plate of a supporting wall. ... The joint is generally fastened with nails by toenailing the rafter from the side into the top plate below.
Without an enclosed soffit, blocking is typically provided between rafters along the exterior wall line regardless of the rafter size to serve as a barrier between the outside and the attic space. ... The most direct load path is for the roof sheathing to be edge nailed to blocking between each rafter.
Drive a toe nail on each side of the rafter just above the birdsmouth into the top plate. Adding more nails than that may split the lumber and weaken the birdsmouth. Add hurricane ties to the connections at the ridge board and top plate. Hurricane ties are metal clips used to reinforce construction joints.
The length of the rafter is given from the long point of the ridge cut at the top down along the top edge the board to the same plumb cut on the rafter's notch. The next result is the run. This is the horizontal measurement from the outside of the wall to the inside of the ridge.
RAFTER LENGTHS PER FOOT RUN I.E., assume the roof has a 22" rise per foot run. With a 22" rise, the length per foot run of a common rafter is 25.06 inches. Assume build- ing is 50' wide. The run of the common would be 25' ( 1/z of width).
Notches in solid lumber joists, rafters and beams shall not exceed one-sixth of the depth of the member, shall not be longer than one-third of the depth of the member and shall not be located in the middle one-third of the span. Notches at the ends of the member shall not exceed one-fourth the depth of the member.