Why Chimney Flashing Fails First: A Field Note

A roof is mostly one material moving as one system. Shingles, underlayment, and decking are all wood-framed, all fastened to the same structure, and all expand and contract together through a Michigan year. A chimney breaks that agreement. It is masonry, a separate structure with its own footing in most cases, meeting a wood-framed roof at a hard right angle. Where those two systems meet is where a roof asks the most of a strip of metal, and it is why chimney flashing tends to be the first thing on an aging roof to actually let go.

01 /Two materials, one joint, different rules

Brick and mortar respond to temperature slowly and by a small margin. A wood deck and asphalt shingles respond faster and by more. Over ten or twenty years of freeze and thaw, that small gap works on whatever bridges the two materials, the flashing, thousands of times over. Nothing else on a typical roof carries that kind of repeated differential stress at one seam. It is a mechanical fact of the joint, not a defect in either material, and it is why code requires step flashing and counterflashing here rather than a single continuous piece.

02 /How the joint is supposed to work

A correct chimney flashing detail is two separate pieces working together, not one. Step flashing, small L-shaped pieces of metal woven course by course into the shingles, sheds water down the roof side of the joint. Counterflashing, set into a cut reglet joint in the mortar, laps down over the step flashing and keeps water from ever reaching the masonry seam. Done right, the two pieces move independently, the roof side flexing with the shingles and the masonry side staying anchored to the brick, and water never has a straight path into the house.

03 /The shortcut we photograph on almost every job

When counterflashing corrodes or a reglet joint opens up, the correct fix is to cut a new reglet and set new counterflashing. That is masonry work layered into a roofing repair, and it takes longer than most other flashing fixes on a house. The shortcut crews see often enough to earn its own line here is a heavy bead of roofing tar smeared over the old metal and the seam instead. It can hold back water for a season, sometimes longer. It is not a repair. It is a delay, and it usually means someone already found a leak at this chimney once and covered the symptom rather than fixing the joint underneath.

We keep this qualitative on purpose. We are not handing you a made-up percentage of Macomb County chimneys hiding a tar patch, because we have not run that count and would not trust a number we invented. What we can say plainly is that it is common enough that a tar-smeared chimney is one of the first things a trained eye photographs on every inspection, right alongside the shingles themselves.

04 /What a correct rebuild looks like

A proper rebuild removes the old metal and any tar buildup, re-cuts or cleans out the mortar reglet, and installs new step flashing woven into the shingle courses along both sides of the chimney, then new counterflashing set into the reglet and sealed at the joint itself rather than across the whole seam. On a chimney with a cricket behind it, the small peaked structure that diverts water around a wide chimney on the uphill side, that piece gets rebuilt too if it was ever missing or undersized, since an undersized cricket is its own common failure point on wider chimneys.

This is masonry-adjacent roofing work, not a five-minute patch, and it is priced as a repair rather than a full reroof in almost every case we see. Chimney flashing repairs generally fall inside the repair range of $350 to $3,200, with the top of that range reserved for jobs that also need cricket rebuilding or masonry repair alongside the metal. Our chimney flashing replacement cost guide walks through what drives a job toward either end of that range, and the flashing cost guide covers the other flashing types on a roof if the chimney is not the only spot in question.

None of this means every chimney on an older Macomb County roof is leaking today. It means this is the joint we check first, tar or no tar, on every inspection, because it is the one place on the roof where two different materials have been arguing with each other since the day it was built. If your chimney has a smear of black cement around the base, or you have simply never had it looked at, our roof repair page covers how we scope a job like this, the 60-second cost calculator can put a range on it before anyone climbs a ladder, and a free inspection turns this field note into an answer specific to your chimney, with photos either way.