By Daniel Watkins, Noble Chimney Sweepers.
A homeowner sent me a photo last week of white, chalky patches spreading across the brick of her chimney and asked whether someone had splashed paint on it. Nobody had. That is efflorescence, and while the stain itself is harmless, what causes it is not. Here is how to read those white marks on a Lemont chimney and decide whether you are looking at a cosmetic nuisance or the start of real masonry damage.
What efflorescence is
Efflorescence is mineral salt left behind when water moves through brick and mortar and then evaporates at the surface. The water dissolves salts inside the masonry, carries them outward, and leaves them as a white powder or crust once it dries. The stain is the residue. The water is the actual story.
Why the white stain matters
Brick is supposed to shed water, not soak it up and push it back out. When you see efflorescence, water is getting into the chimney structure and traveling through it. In a climate that swings from humid August afternoons to hard January freezes, that trapped water expands when it turns to ice and pries the masonry apart from the inside. That is how you get spalling — brick faces popping off in flakes — and crumbling mortar joints.
Where the water is usually getting in
In the chimneys I look at, the entry point is almost always one of four places. A cracked or undersized crown at the top lets rain sit on the masonry instead of shedding it clear. A missing or damaged cap lets rain fall straight down the flue. Failed flashing at the roofline wicks water into the joint between chimney and roof. And porous, aging mortar joints simply absorb water like a sponge once the original weather skin has worn off.
Why the towns along the canal see it more
Older masonry chimneys built with soft, lime-heavy mortar are common in the historic parts of Lemont and the neighboring bluff communities. That mortar was designed to breathe and flex, which is fine, but it is also more absorbent than modern mixes. Combine a century-old chimney, a bluff-top exposure to wind-driven rain, and Midwest freeze-thaw cycling, and efflorescence shows up early and often. [Observed — from the chimneys I service locally, not a published survey.]
What not to do about it
Do not seal the brick with a general-purpose waterproofing product from a hardware shelf. Most of those form a film that traps moisture inside the masonry, and trapped water in a freeze-thaw climate does far more damage than water that can escape. If a chimney gets treated at all, it needs a vapor-permeable masonry repellent, and only after the actual leak is repaired. Sealing over an active leak just hides the evidence.
The right order of operations
Find the water path first. That means an inspection of the crown, cap, flashing and joints — from the roof, not from the ground. Then repair what is letting water in: rebuild or seal the crown, fit a proper cap, correct the flashing, or repoint the joints. Only then is it worth cleaning the stain itself, which usually comes off with a stiff brush and water once the source is dry. Our chimney repair and caps and crowns pages walk through what each of those fixes involves.
Do it before the first freeze
Masonry repairs need above-freezing temperatures to cure properly, which makes late summer and early fall the practical window around here. A crown repair done in August has months to cure before the first hard freeze tests it. The same repair attempted in December often has to wait until spring, and the chimney spends the whole winter taking on water.
Get an answer, not a guess
Noble Chimney Sweepers is owner-operated, certified, bonded and insured, with 14 years behind the work. We are available 7 days a week, 7 AM to 7 PM, and you get the price up front before anything starts — our rates are published on the pricing page. Call (708) 432-5747 and we will find out where the water is coming from.
Frequently asked questions
Will efflorescence wash off in the rain on its own?
Some of it does, and then it comes back after the next wet spell. Recurring stains mean the water path is still open. A stain that returns each season is a leak, not weathering.
Is the white stain harmful to touch or breathe?
The salt deposit itself is not a health hazard. It is a symptom, and the concern is what the moisture is doing to the structure behind it.
Can I just paint over the stained brick?
Painting a chimney that is actively taking on water is one of the worst things you can do. The paint traps the moisture, and it will peel off in sheets along with the brick face beneath it.
How do I tell efflorescence from mold?
Efflorescence is crystalline and white to off-white, feels gritty, and often brushes off dry. Mold is usually darker, sits in shaded damp areas, and smears rather than powders when disturbed.
Does efflorescence mean my chimney is leaking into the house?
Not necessarily. It means water is inside the masonry. Whether it reaches the ceiling or wall depends on where the path runs. Interior stains near the chimney chase are a separate warning sign worth checking.
Can a chimney cap alone stop it?
Sometimes, if rain falling straight down the flue was the only entry point. More often the cap is one piece of the fix alongside crown or flashing work.
Does efflorescence affect the flue liner inside?
It can. Persistent moisture in the structure degrades mortar joints between clay tiles and accelerates corrosion in metal liners, so a camera check of the liner is worth doing while we are there.
How long after repairs should the stain stop coming back?
Once the leak is closed and the masonry dries out, remaining salts may bloom once or twice more as the brick releases what it already absorbed. After that it should stop. Stains that keep appearing a year later mean something was missed.
Published August 19, 2026