Chimney Flashing: Why Most 'Chimney Leaks' Are Roof Details
A large share of water coming in near a chimney never passed through the chimney. Counter-flashing, step flashing and the cricket — and why sealant is not a repair.
When water appears in a ceiling near a chimney, the chimney gets the blame. Frequently the masonry is sound and the water is entering at the junction between the stack and the roof, which is a roofing detail rather than a chimney one.
01The assembly that is supposed to be there
Flashing at a chimney is a two-part system. Base or step flashing is integrated with the roofing material and turns up against the masonry. Counter-flashing is set into a cut joint in the masonry and laps down over the base flashing.
The lap is what makes it work. Water running down the masonry is intercepted by the counter-flashing and directed onto the base flashing, which carries it out onto the roof surface. Neither piece works alone.
02Why sealant fails and keeps failing
A bead of sealant where the flashing meets the brick is the most common attempted repair and the most reliably temporary. It is a material with a short service life in full sun and thermal movement, applied to a joint that moves.
Worse, it hides the defect. Once the visible gap is filled, the failure moves out of sight and the water keeps entering while everyone believes it was fixed.
03The cricket, on the upslope side
A chimney of any width on a sloped roof needs a cricket — a small peaked structure on the uphill side that divides water and directs it around the stack. Without one, the upslope face becomes a dam collecting water and debris against the masonry.
Its absence on wider chimneys is a defect, not an omission of preference, and it is a common finding on older roofs.
04Establishing where the water is actually entering
The candidates are the crown, the flashing assembly, the masonry face itself and, less often, the flue. They are distinguished by inspection and, where necessary, by controlled water testing of one area at a time.
Diagnosing correctly matters because the four have entirely different repairs, and treating the wrong one produces a leak that comes back with the next storm.
Ask which of the four sources was established and how. If the proposed repair is a bead of sealant at the flashing line, it is a deferral. Proper counter-flashing set into the masonry, and a cricket where the width calls for one, is what actually ends the problem.
Every recommendation here starts from one place: a graded, photographed chimney inspection done to one standard, with a paper trail you keep.
The Chimney Standard Team
This article was written and reviewed by the CSIA-certified crew at Chimney Standard. Every recommendation reflects NFPA 211 and CSIA trade standards and the same documented, photographed approach we hold every job to.
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