By Johnathan Mason, Ace Chimney Service.
A wet firebox, a stain on the ceiling beside the chimney breast, or a damp smell after every rain all point at the same thing: water is getting into a structure designed to move hot gas out. Chimney leaks are common, and the frustrating part is that the entry point is almost never where the stain shows up. Here is how we find it on a Fox River Grove call.
Where does chimney water actually get in?
There are five usual suspects, and a leak is often two of them at once. Rain falls straight down an uncapped or badly capped flue. The crown — the concrete slab across the top — cracks and lets water into the brickwork below. The flashing where the chimney meets the roof lifts, rusts, or was never sealed properly. The masonry itself absorbs water through porous brick and failed mortar joints. And a rusted-out chase cover on a factory-built chimney holds standing water until it finds a seam.
Why the stain is never where the hole is
Water entering at the crown travels down inside the masonry, often several feet, before it reaches something that stops it and pushes it sideways into the room. That is why chasing the stain rarely works, and why sealing the obvious crack sometimes changes nothing. The right approach is to inspect the whole system from the top down and eliminate each entry point in order.
The signs worth acting on
- Water, drips, or standing damp in the firebox after rain.
- White chalky staining, called efflorescence, on the exterior brick.
- Rust on the damper, the firebox, or the metal cap.
- Ceiling or wall stains next to the chimney breast.
- A musty odor from the fireplace that gets worse in humid weather.
- Loose or missing mortar joints and spalling brick faces.
Why our climate accelerates it
Northern Illinois cycles above and below freezing many times each winter. Water that soaks into brick expands about nine percent when it freezes, and it does that over and over. The result is spalled faces, opened joints, and a crown that cracks a little wider each year. A leak that is cosmetic in October can be structural by April, which is why we would rather look at it now than after another freeze-thaw season.
What the fix usually involves
It depends entirely on the entry point: a correctly sized cap, a repaired or recast crown, new flashing sealed properly to the roof, repointed mortar, or a breathable water repellent that lets the masonry dry outward while shedding rain. What it should never involve is a coat of paint or a hardware-store sealant that traps moisture inside the brick and makes the damage worse. See chimney repair and masonry and chimney caps and crowns for what each job covers.
Booking in Fox River Grove
Ace Chimney Service is insured and bonded, based in Crystal Lake, and has been solving leaks in McHenry County chimneys for 29 years. Seven days a week, 7 AM to 7 PM, price up front. Call (815) 320-0883.
Frequently asked questions
Can I just seal the top of the chimney?
No. The flue has to stay open to vent. Sealing a working flue creates a far more dangerous problem than the leak.
Will waterproofing stop a leak?
Only if the leak is through porous masonry. Waterproofing does nothing for a cracked crown, failed flashing, or a missing cap, and applying it over those hides the real fault.
Is a cracked crown urgent?
A hairline crack can often be sealed. A crown broken through, missing sections, or with no overhang is letting water in every rainfall and should be dealt with before winter.
Who repairs the flashing, a roofer or a chimney sweep?
Either can, and the important thing is that whoever does it understands both surfaces. We handle flashing as part of leak work and will tell you if a roof problem is the real cause.
What is efflorescence?
The white powdery deposit left when water moves through masonry and carries salts to the surface. It is harmless in itself but it is proof that water is passing through the brick.
Can a leak cause a chimney to be unsafe to use?
Yes. Water corrodes dampers, breaks down mortar between flue tiles, and rusts metal liners, all of which affect whether the chimney vents safely.
How do you find the entry point?
A top-down inspection of cap, crown, flashing, and masonry, plus a look at the flue interior. Where it is not obvious, controlled water testing of one area at a time isolates it.
Do you serve Fox River Grove and nearby towns?
Yes. We are based in Crystal Lake and serve Fox River Grove, Cary, Algonquin, Barrington-area communities, and the wider McHenry County region seven days a week.
Published August 14, 2026