Marking Bird's Mouth & Soffit Cuts

What happens here

The next step is to mark the bird's mouth cut and the soffit overhang. Using the speed square, align the '4' mark with the determined rafter length mark (122.25 inches) to mark the outside of the wall. Measure 1.5 inches down from the rafter edge to establish the depth of the seat cut for the bird's mouth. Then, flip the speed square and mark the heel cut at a 90-degree angle from the seat cut. Finally, for the soffit overhang, measure 12 inches from the heel cut using a framing square and mark the final plumb cut line for the overhang.