By Daniel Watkins, Noble Chimney Sweepers.
A homeowner calls a roofer about a ceiling stain near the fireplace. The roofer looks at the shingles, finds them in decent shape, and says the roof is fine. The stain comes back with the next storm. Nine times out of ten the water is not coming through the roof at all — it is coming in at the joint where the chimney passes through it, and that joint belongs to the chimney trade.
What flashing is and why it fails
Flashing is the metal that seals the seam between masonry and roof. Done correctly it is two separate layers. Step flashing consists of L-shaped pieces woven into the shingle courses running up each side of the chimney. Counter flashing is a second layer set into a groove cut in the mortar joints of the brick, then folded down to cover the top edge of the step flashing so water cannot get behind it.
It fails because it sits between two things that move differently. Brick and a wood-framed roof deck expand and contract at different rates, and around here they do it through a full cycle of summer heat and hard winter freeze-thaw. Every year the metal absorbs that movement. Eventually a fastener works loose, a sealed edge lifts, or the mortar joint holding the counter flashing crumbles and lets the metal pull free.
How to tell it is the flashing
Two clues separate a flashing leak from other chimney water problems. The first is timing: a flashing leak shows up during or immediately after rain, not days later. The second is location. Go into the attic with a flashlight during or right after a storm and look at the roof deck where it meets the chimney. Dark, damp sheathing right at that joint, or staining running down the framing beside the chase, points straight at the flashing.
Compare that with the water problems we have written about before. White efflorescence on the brick face and a crumbling crown are moisture moving through the masonry itself — a different mechanism with a different fix. If you are seeing a stain on the ceiling next to the chimney and nowhere else, start with the flashing.
The repair that is not a repair
The most common thing we find on a leaking chimney is a thick smear of black roofing cement troweled over the joint by someone in a hurry. It works for a season. Then it hardens, the joint moves through one Illinois winter, and it cracks — except now the crack is hidden under an opaque black mass, so the leak continues quietly into the roof deck while the homeowner believes it was handled. We routinely have to chip that material off before we can see what is underneath, and sometimes the sheathing under it has already gone soft.
Proper counter flashing is let into the mortar joint, not stuck to the surface of the brick. If yours was surface-sealed with caulk or cement, it was never going to hold.
Why the calendar matters
Late August into September is the window for this. Water that gets past failed flashing soaks the deck and framing beside the chimney; when that water freezes it expands and opens the gap wider, so every thaw cycle makes the next leak larger. Homeowners across Shorewood and the nearby Will County communities tend to notice the stain in spring, which is the expensive end of that sequence rather than the cheap end [Observed].
What we do about it
We look at the flashing as part of the exterior portion of a chimney inspection, along with the crown, the cap, and the masonry, and we tell you which one is actually letting water in rather than guessing. If the metal is sound and the counter flashing is properly set into the joint, resealing is honest work. If it is rusted through, missing, or buried in cement, we will tell you it needs replacing and explain why. That work is covered on our chimney repair page.
Noble Chimney Sweepers is certified, bonded and insured, owner-operated, and I have 14 years in this trade. You get the price up front before anything starts — see the pricing page. Call (708) 432-5747, 7 days a week, 7 AM to 7 PM.
Frequently asked questions
What exactly is chimney flashing?
It is the metal that seals the joint where the chimney passes through the roof. It comes in two layers: step flashing woven into the shingle courses along the sides, and counter flashing set into a groove cut in the mortar joints and folded down over the step flashing. Both layers have to be present for the joint to shed water.
Why is flashing the most common chimney leak point?
Because it is a joint between two materials that move differently. Masonry and a wood-framed roof deck expand, contract, and settle at different rates through Illinois freeze-thaw cycles, and the metal in between takes that movement every year until a fastener backs out or a sealed edge opens.
How do I tell a flashing leak from a crown or brick leak?
Timing and location. A flashing leak shows up as a ceiling stain right at the chimney chase or a damp spot in the attic on the roof deck beside the chimney, and it appears during or just after rain. Crown and brick water problems tend to show as white staining, damp masonry, or interior dampness that lags the weather.
Is roofing cement a real repair?
No. Troweled roofing cement over a flashing joint is a stopgap. It is rigid, it cracks as the joint moves through a winter, and it usually needs to be chipped off before a proper repair can be done. It also hides the failure long enough for the roof deck underneath to rot.
When does flashing need to be replaced instead of resealed?
Replace it when the metal is rusted through, when the counter flashing was surface-sealed rather than set into the mortar, when step flashing is missing entirely, or when the existing metal has been buried in cement. Resealing is reasonable only when the metal is sound and the counter flashing is properly let into the joint.
Can flashing be repaired without redoing the roof?
Usually yes on the counter flashing side, since that is set into the masonry. Step flashing is woven into the shingles, so replacing it means lifting the shingle courses along the chimney — which is why it is worth addressing before a roof gets old enough that the shingles will not cooperate.
Why should I deal with this before winter?
Water that gets past the flashing soaks the roof deck and the framing next to the chimney. When that water freezes it expands, opening the gap wider for the next thaw. A small leak in September is reliably a bigger one by March.
Do you check flashing during a chimney inspection?
Yes. The exterior portion of an inspection covers the flashing, the crown, the cap, and the condition of the masonry. If the flashing is the source of a leak we will show you what we found and explain whether it needs resealing or replacement.
Published August 28, 2026