Here are some more flashing facts. We’ll look at the two options when a sloping roof meets a vertical wall.
Horizontal along the wall: In this case, a piece of 10″ flashing bent in half (or valley flashing) will do the trick. Slide one end up under the siding, and nail the bottom half over the roofing material. Be sure to caulk nails with roofing cement. If the wall is covered in brick or stucco, you’ll need another bend in your flashing. Cut a groove (a power saw with a masonry blade will do the job) in the stucco, or along a line of brick mortar, insert the flashing “flap”, and re-stucco or re-mortar the flap in the groove.
Sloping down the wall: In this case, a continuous strip could cause you problems. If water managed to get underneath a shingle, it would be trapped under there. Step flashing — short lengths of folded flashing — prevents this by working similarly to a shingle. The bottom of a piece of flashing sits on top of the shingle course, and the next shingle sits on top of it, leaving a place where water can escape. Nail the flashing to the roof, and cap the vertical section with a continuous length of cap flashing, installed with a flap, as in the horizontal method, or with a number of short lengths following the mortar lines in brick.