White chalky staining on brick is efflorescence: mineral salts left behind as water passes through masonry and evaporates at the surface. The staining is harmless, but it proves water is actively moving through your chimney, which in this climate leads to freeze-thaw damage.
What it is telling you
Brick and mortar contain soluble salts. When water penetrates the masonry, it dissolves those salts, carries them to the surface, and evaporates, leaving the salt behind as a white powdery deposit. Scrub it off and it returns, because the cause has not changed.
That is the useful part. Efflorescence is a free diagnostic: it tells you, without any inspection, that water is getting into the chimney and moving through it. In Montgomery this is most common on homes near the Fox River, where ambient moisture is higher year round.
Why water in masonry is a serious problem here
Water expands roughly nine percent when it freezes. Northern Illinois crosses the freezing point dozens of times each winter, so any water sitting inside brick is subjected to that expansion over and over.
The result is spalling: the faces of the bricks flake, pit, and eventually pop off. Spalling cannot be reversed. Once a brick face is gone, the only remedy is replacing the brick, and matching brick on an older chimney is often imperfect. Stopping the water before spalling starts is dramatically cheaper than dealing with it afterward.
Where the water is getting in, and what to do
Usually one of four places: deteriorated mortar joints, a cracked or improperly built crown, a missing or damaged cap letting rain fall straight down the flue, or failed flashing at the roof.
The fix depends on which. Receded and cracked joints call for repointing, with mortar matched to the existing masonry rather than a hard modern mix that would cause the brick to spall instead. A cracked crown needs recasting with a proper overhang and drip edge. An open flue needs a stainless cap.
One thing to avoid: sealing the chimney with a film-forming waterproofer. It traps moisture already inside the brick and accelerates exactly the damage you were trying to prevent. Only vapour-permeable sealers belong on masonry.
Talk to a local chimney pro
If you have noticed white staining on your chimney in Montgomery, it is worth an assessment while the weather still allows masonry work. Mortar needs sustained temperatures above about 40 degrees to cure properly.
Written by Josh Amber, co-owner of Ember Chimney Sweepers, Oswego IL. Published September 24, 2026.