Here is a little jig I made for cutting asphalt shingles for cut roof valleys. It consists of two strips of 1/2-in. plywood, one wider than the other. I drilled a series of holes in the ends of the plywood and made a hinge from some plastic cable ties. That’s it.
To use the guide, I slip an upside-down shingle between the two pieces of plywood, align my layout marks along the edge of the upper layer, and use a utility knife held against the guide’s edge to make my cut.
I use the wire anchor line at the top of the guide to tie it to a scaffold plank to keep the guide from sliding off the roof.
Don Mathis, Macomb, IL