Chimneys present a vulnerable possible leak stage on any roof, metal, built-up or otherwise. And flashing — impervious material put on the joint where the chimney meets the roof — provides the defense. On metal roofs, flashing goes over an ice-guard membrane. In principle, you put in flashing, counter-flashing and crickets as on another roof, with all the exception that you will be riveting or screwing metal panels rather than nailing shingles.

The Order of Battle

You are going to be breaking metal and cutting brick on this undertaking. A slot cut with a diamond blade at the masonry, at least 4 inches above the roof plane, accepts a bent slot at the top of the counter-flashing. Rivet or screw flashing at the front corners and the head — top of the chimney. Meanwhile, the counter-flashings go in second, pounded in their masonry slots, and sealed with sealant. Work from the base of the chimney to the surface. Add a cricket — also called a saddle — to divert water from a larger chimney.