Counter-flashing
Also called: chimney flashing, step flashing, cap flashing, reglet flashing, chimney cricket, flashing repair, roof to chimney flashing
The upper layer of metal at a chimney's base that's let into a cut in the mortar joints and bent down over the step flashing woven into the shingles, so water running down the brick is shed onto the roof rather than behind the flashing. The two-part system is what keeps the joint dry.
A chimney is a masonry column poked through a roof, and the two move differently, so the joint between them can’t be sealed rigidly — it has to be lapped, like shingles, so that every path water might take leads back out onto the roof. Two layers of metal do that. Step flashing is a series of L-shaped pieces, each woven under a shingle course and bent up the side of the chimney, overlapping the one below. Counter-flashing (or cap flashing) is a second layer that starts inside the brick — its top edge bent into a reglet, a 1-inch groove cut into a mortar joint, and sealed there — and hangs down over the step flashing without being attached to it, so the roof can move against the chimney and the metal stays lapped. On the uphill side of a chimney wider than 30 inches, a cricket — a small peaked saddle — diverts water around the stack instead of letting it pool behind it.
That’s the system, and most leaking chimneys are missing part of it. The common shortcuts: flashing surface-mounted to the face of the brick with roofing cement, which lasts until the cement cracks; a single layer of flashing with no counter-flashing; old counter-flashing whose reglet has rusted or worked loose; and no cricket behind a wide chimney. A leak that shows in the rain — not the steady seep of a cracked crown, but water within hours of a storm — is usually flashing.
Replacing it is a roofer’s job: shingles lifted, step flashing woven in, the reglet cut with a grinder, the counter-flashing let in and sealed, a cricket built and shingled. Aluminum is $130 in material; copper is three times that and outlasts the roof. The cost page prices the job at $500–$1,800 and puts it on the hire side, because a flashing job that fails puts water into the framing.