By James Rodriguez, Central Chimney Service.
When water shows up on a ceiling near a chimney, the chimney usually gets the blame and the flashing is usually the culprit. Flashing is the metal transition between two materials that expand and contract at different rates, exposed to every storm the roof takes, and it fails long before the masonry does. Knowing whether yours needs a patch or a proper reflash saves both money and a repeat leak.
What flashing actually is
Flashing is a two-part sheet metal assembly that keeps water out of the joint where a chimney passes through a roof. Neither the brick nor the shingles can seal that joint on their own, so metal bridges it.
The lower half, step flashing, is a series of overlapping L-shaped pieces woven into the shingle courses, one per course, each lapping the one below. The upper half, counter flashing, is embedded into a mortar joint on the chimney and folds down over the step flashing. Water hitting the chimney runs down the face, onto the counter flashing, onto the step flashing, and out onto the roof without ever reaching the deck.
Why it fails before the masonry does
The joint moves. Masonry and a wood-framed roof expand, contract, and settle at different rates and in different directions, and the flashing absorbs that movement season after season.
The failure points are predictable. The mortar holding the counter flashing erodes and the metal works loose. Sealant at the top edge dries, shrinks, and cracks — it has a service life measured in a handful of years, not decades. Older galvanized steel corrodes at the fasteners. And on plenty of homes across Montgomery chimney sweep calls and nearby, the original installation used roofing tar rather than proper step flashing, which was never going to last.
The signs, roughly in the order they appear
Flashing leaks announce themselves indoors before they look dramatic outdoors. Staining on the ceiling or wall near the chimney, usually a few feet from it rather than directly against it, is the classic first sign, and it often appears only after wind-driven rain from a particular direction.
Watch also for a damp or musty smell in the attic near the chimney chase, dark streaks on attic sheathing around the penetration, peeling paint or bubbling drywall on the wall the chimney runs through, and deteriorated framing visible where the chimney passes the roof deck. From the ground with binoculars you may see lifted or rippled metal at the base, visible tar patches, or a gap between the counter flashing and the brick.
Patch versus reflash
A patch is appropriate when the assembly is fundamentally correct and one specific thing has failed — a section of sealant at the counter flashing reglet, a few loosened fasteners, one displaced step piece. That is a targeted repair on sound components.
A reflash is the answer when the system itself is wrong or spent: no true step flashing, tar used as the primary waterproofing, corroded metal, counter flashing that was surface-mounted rather than let into a mortar joint, or repeated patching that has not held. Replacing it means removing shingles along the chimney, installing new step flashing woven course by course, cutting a proper reglet into the mortar for the counter flashing, and sealing that reglet correctly.
The honest rule of thumb: if the same area has been sealed more than once and leaked again, sealing it a third time is not a repair. It is a delay, and the framing under it is paying for the delay.
Why timing matters here
Flashing leaks are cumulative and they damage things that cost far more than flashing. Water tracking into the chase soaks framing, rots roof decking, saturates insulation, and reaches the masonry from behind where it drives freeze-thaw damage from the inside out.
Late summer and early fall are the working window in northern Illinois — dry enough to do the job properly and early enough to be finished before the freeze cycle starts. Our chimney repair page covers flashing alongside the crown and masonry work that often accompanies it, and a chimney inspection is what establishes which of those you are actually dealing with, since crown cracks and flashing failures produce very similar stains.
You get the price before the work, from our pricing page, not after. Insured and bonded, 29 years working on chimneys in this area, reachable 7 days a week from 7 AM to 7 PM at (815) 305-5510.
Frequently asked questions about chimney flashing
How do I know the leak is the flashing and not the chimney crown?
Location and timing are the clues. Flashing leaks typically stain near the roofline and often only after wind-driven rain, while crown and masonry leaks tend to show lower down, near the firebox or on the chimney breast, and appear in any sustained rain. An inspection that looks at both from above settles it.
Can flashing be repaired without removing shingles?
A targeted sealant repair at the counter flashing can be done without touching shingles. Replacing step flashing cannot, because step flashing is woven into the shingle courses — anything installed over the top of shingles instead is a surface patch, not step flashing.
How long should chimney flashing last?
Properly installed metal flashing generally lasts as long as the roof covering, while the sealant at the counter flashing reglet is a consumable that needs periodic renewal. Tar-based installations fail much sooner, because tar hardens and cracks with normal thermal movement.
Is roofing cement a legitimate fix?
Only as a short-term measure to stop active water until a proper repair. As a permanent solution it fails predictably: it becomes brittle, cracks along the same joint that moves, and it makes the eventual correct repair harder because it must be cleaned off first.
Who handles this, a roofer or a chimney company?
Either can, and what matters is that whoever does it understands both halves — the shingle weave and the masonry reglet. Because a leak at a chimney frequently involves the crown, cap, or mortar joints as well, having the whole chimney assessed at once avoids fixing one path and leaving another open.
Will the ceiling stain go away once the leak is fixed?
No. The stain is permanent discoloration and needs sealing and repainting after the area has dried thoroughly. What matters first is confirming the leak has actually stopped, which typically means watching it through a few heavy rains.
Can flashing leak even though the chimney looks fine?
Yes, and that is common. The masonry can be in good condition while the metal at the roofline has corroded, loosened, or lost its sealant. Flashing is inspected separately from the brick for exactly that reason.
Does a chimney chase cover need flashing too?
Yes. A framed chase on a prefabricated fireplace penetrates the roof the same way a masonry chimney does and needs the same step and counter flashing treatment, in addition to a sound metal chase cover on top. Both are common leak sources on that style of chimney.
Published August 31, 2026 · Last updated August 27, 2026