By Johnathan Mason, Ace Chimney Service.
Where the chimney passes through the roof, two rigid structures meet that move at different rates, expand at different rates, and are made of entirely different materials. The joint between them is bridged by flashing, and when flashing fails the water does not stay near the roof — it runs down the framing and shows up as a stain on a ceiling somewhere else. This is one of the most misdiagnosed leaks we see in Island Lake.
What is chimney flashing?
Flashing is the layered metal that seals the gap between the chimney and the roof deck. A proper installation has two parts. Step flashing is woven into the shingle courses up each side. Counter flashing is set into a groove cut in the mortar joints and folded down over the step flashing, so water running down the brick is directed out onto the roof surface rather than behind it. Both parts are needed. One without the other is a leak waiting on the weather.
How flashing fails
- Sealant used in place of proper counter flashing, which dries and cracks within a few years.
- Counter flashing pulling out of the mortar joint as the joint erodes.
- Galvanized flashing rusting through at the fold lines.
- Nails backing out as the roof deck moves through freeze and thaw.
- New shingles laid over old flashing during a roof replacement.
- Debris and ice damming behind the chimney on the uphill side.
The uphill side is where the trouble is
Water running down a roof hits the back of the chimney and has to go somewhere. A correctly built chimney has a cricket, a small peaked structure behind it that splits the flow and sends it around each side. Without one, water and debris pool against the masonry, and in an Illinois winter that pool becomes ice that lifts flashing and forces water uphill under the shingles. Any wide chimney on a sloped roof deserves a look at whether it has a cricket at all.
How to tell flashing is your problem
Flashing leaks tend to follow rain rather than temperature, appear on the ceiling near the chimney rather than in the firebox, and show up first as a stain that dries between storms. A leak that puts water in the firebox is more likely to be the cap or crown. A leak that stains the ceiling but leaves the firebox dry points hard at the roof joint. From the ground with binoculars you can often see rust streaks, lifted metal, or a black smear of old sealant doing a job it was never meant to do.
Getting it looked at in Island Lake
We inspect flashing as part of any leak call and as part of a full chimney inspection. If the masonry around it has opened up, that gets addressed at the same time under chimney repair and masonry — new flashing set into crumbling mortar will not hold. Ace Chimney Service is insured and bonded, 29 years in, serving Island Lake and McHenry County seven days a week, 7 AM to 7 PM, price up front. Call (815) 320-0883.
Frequently asked questions
Can flashing just be sealed with caulk?
Sealant is a short-term patch at best. It has a service life of a few years and it hides the condition of the metal underneath. Proper counter flashing set into the mortar is the durable fix.
Should flashing be replaced when the roof is replaced?
In almost every case, yes. Reusing old flashing under new shingles is a common shortcut and a common source of leaks two winters later.
What is a chimney cricket?
A small peaked saddle built on the uphill side of the chimney that splits water and debris around it. Wide chimneys on sloped roofs generally need one.
How long should flashing last?
Well-installed copper or heavy-gauge metal can last decades. Thin galvanized steel and sealant-only installations fail much sooner.
Is a flashing leak urgent?
It should not wait a season. Water tracking down framing rots sheathing and joists, and that damage costs far more than the flashing.
Can I inspect it from the ground?
You can spot obvious problems with binoculars — rust, lifted edges, visible sealant smears. Confirming it takes a look from the roof, which is not a homeowner job.
My ceiling stain dried up. Is the leak gone?
No. Stains dry between rains. A dry stain means the water stopped for now, not that the entry point closed.
Do you serve Island Lake and nearby towns?
Yes. We are based in Crystal Lake and serve Island Lake, Wonder Lake, McHenry, Cary, and the wider McHenry County area seven days a week.
Published August 16, 2026