By Johnathan Mason, Ace Chimney Service.
If you have noticed brick faces popping off your chimney, mortar you can scrape out with a key, or a scatter of orange grit on the roof below, you are watching freeze-thaw damage in progress. It is the most common structural problem on chimneys in northern Illinois, and it is one of the few where doing something now costs a fraction of doing it later. Here is what is happening on your Marengo chimney.
What is spalling?
Spalling is when the face of a brick flakes, pops, or crumbles away. The mechanism is simple. Brick and mortar are porous and absorb water. Water expands roughly nine percent when it freezes. That expansion happens inside the brick, pushing the face outward from within. One cycle does nothing you would notice. Northern Illinois runs through dozens of freeze-thaw cycles in a single winter, and the damage compounds season after season.
Why chimneys get it worse than walls
A chimney is exposed on all four sides with no wall cavity behind it, it takes rain directly on its top surface, and it is the coldest masonry on the house because it stands above the roofline in the wind. It also cycles between hot and cold every time you light a fire. A house wall gets none of that treatment.
What to look for from the ground
- Brick faces flaked, pitted, or missing pieces.
- Mortar joints recessed, soft, or crumbling to the touch.
- Brick fragments or orange dust on the roof or in the gutters.
- White chalky staining on the exterior masonry.
- A visible lean, bulge, or step-crack in the stack.
- Cracks or missing sections in the concrete crown at the top.
How it is fixed
Early spalling is handled with tuckpointing: cutting out failed mortar joints to depth and repointing with a mortar matched to the original in strength and appearance. Matching matters more than people expect — modern high-strength mortar on soft old brick transfers stress into the brick and accelerates the damage it was meant to stop. Where individual bricks have deteriorated too far, they are cut out and replaced. Where the crown has failed, it is sealed or recast, because a bad crown is usually the reason the brick below it got wet in the first place.
Stopping it coming back
Once the masonry is sound, a breathable water repellent keeps rain out while still letting vapor escape outward. What you must not use is a film-forming sealer or masonry paint. Those trap moisture inside the brick, and the next freeze does its damage behind a coating you cannot see through. Keeping the cap and crown in good order matters just as much, since most of the water arrives from the top.
Getting it assessed in Marengo
Late summer is the right time for masonry work: mortar cures properly in warm, dry weather, and the repair is done before the next freeze cycle starts. See chimney repair and masonry for what the work involves and the pricing page for starting figures. Ace Chimney Service, 29 years, insured and bonded, price up front, Marengo and McHenry County seven days a week, 7 AM to 7 PM. Call (815) 320-0883.
Frequently asked questions
Is spalling brick dangerous?
Localized surface spalling is not an emergency, but it is water getting in. Advanced spalling, a lean, or step-cracking is a structural concern and should be looked at promptly.
Can spalled brick be repaired or must it be replaced?
The mortar can be repointed; the brick face itself cannot be put back. Badly deteriorated bricks are cut out and replaced individually.
What is tuckpointing?
Cutting failed mortar out of the joints to a proper depth and refilling with fresh matched mortar. It restores the weather seal and the structural bond.
Should I paint or seal my chimney?
Never with paint or a film-forming sealer. A breathable water repellent is the right product because it sheds rain while letting trapped moisture escape.
How long does tuckpointing last?
Properly done with matched mortar on sound brick, it is measured in decades. Poorly matched mortar can fail in a few years and damage the brick as it goes.
What time of year is best for masonry work?
Warm, dry weather. Mortar needs above-freezing temperatures to cure correctly, which makes late summer and early fall ideal.
Does spalling mean my whole chimney needs rebuilding?
Usually not. Most cases are repointing and selective brick replacement. Rebuilds are reserved for stacks that have lost structural integrity.
Do you serve Marengo and nearby towns?
Yes. We are based in Crystal Lake and serve Marengo, Woodstock, Huntley, Union, and the wider McHenry County area seven days a week.
Published August 21, 2026