Freeze-Thaw and the Mortar Joints on Your Joliet Chimney
By Dan Joel, Elite Sweepers Inc.
A chimney is the most exposed masonry on your house. It stands above the roofline with no shelter on any side, it takes wind-driven rain on all four faces, and unlike a wall it has weather coming at it from above as well. In this climate, that combination has a specific consequence, and it has a name.
What freeze-thaw actually does
The short answer: water gets into the masonry, freezes, expands, and forces the material apart a fraction at a time.
Water expands when it freezes. If it has soaked into a mortar joint or a porous brick and then the temperature drops below freezing, that expansion has to go somewhere, and it pushes outward against the surrounding material. Thaw, and the slightly widened space takes on a little more water than before. Freeze again, and it pushes a little harder. Northern Illinois does not have one long freeze; it has a winter of crossing back and forth over the freezing point, sometimes within a single day. Each crossing is another cycle.
How the damage progresses
The short answer: joints recede first, then faces spall, then the structure moves.
- Receding joints. Mortar is softer than brick by design, so it goes first. The joint surface pulls back from the face of the brick and starts to look sandy or crumbly.
- Spalling brick. Once water gets past the joints and into the brick itself, the face of the brick pops off in flakes or plates. You will see fresh, lighter-colored material where the face used to be, and pieces on the roof or in the gutter.
- Open joints and loose units. With enough mortar gone, individual bricks lose their grip. At this stage you can sometimes see daylight through a joint from inside an attic.
- Structural movement. Leaning, bulging or separation from the house. This is the point where repair stops being repointing and becomes a partial rebuild.
The important thing about that list is that the first two stages are cheap to fix and the last two are not. Everything about chimney masonry rewards catching it early.
Where to look from the ground
The short answer: the crown, the joints on the weather side, and the ground beneath the chimney.
The crown is the sloped concrete surface on top. Cracks there let water straight down into the body of the chimney, and it is the single most common entry point we find. From the ground with binoculars, look at the mortar joints on whichever face takes the prevailing weather, usually the west and north sides here, and compare them to the sheltered faces. A visible difference between the two tells you the process is underway.
Then look down. Brick flakes, sand-like mortar grit, or fragments of concrete in the flower bed at the base of the chimney did not appear from nowhere. They came off the structure above.
Why fall is the deadline, not the starting gun
The short answer: repointing needs temperatures that let mortar cure, and winter removes them.
Fresh mortar needs to set before it freezes, so once night temperatures drop consistently the work has to wait until spring. That means a joint you notice in October and address in October gets protected through the winter. The same joint noticed in October and left alone takes an entire season of freeze-thaw before anyone can touch it, and it will be measurably worse by then. This is the clearest example of why timing matters more with masonry than with almost any other part of the system.
Getting an assessment
A masonry evaluation is part of a full chimney inspection, and the work itself is covered on our chimney repair page, including how we handle flashing where the chimney meets the roof. If the liner is affected as well, our chimney relining page explains that side of it. For local scheduling, see our chimney service in Joliet, and our pricing page for costs.
Elite Sweepers Inc is insured and bonded, gives you the price up front, and works seven days a week, 7 AM to 7 PM. Call (708) 432-3977.
Masonry and Mortar Questions
What does spalling brick look like?
The face of the brick flakes, pops or peels away, leaving fresh lighter-colored material exposed underneath. You will usually find the shed pieces on the roof, in the gutters, or on the ground at the base of the chimney.
Is repointing the same as rebuilding?
No. Repointing removes deteriorated mortar from the joints and replaces it while leaving the brick in place. A rebuild takes the chimney apart and relays it, and it becomes necessary only when the structure itself has moved or too many units are loose.
Can chimney masonry be repaired in winter?
Generally not. Mortar needs temperatures above freezing to cure properly, so repointing and crown work are warm-season jobs. That is why an issue found in fall is best addressed in fall rather than left until spring.
What is a chimney crown and why does it crack?
The crown is the sloped concrete surface capping the top of the chimney, shedding water away from the masonry below. It cracks because it is thin, fully exposed, and subject to the same freeze-thaw cycling as everything else up there.
Will waterproofing stop freeze-thaw damage?
A breathable masonry water repellent reduces how much water soaks in, which slows the process meaningfully. It is not a fix for joints that have already opened, though, so the repair comes first and the treatment goes on afterward.
How do I know if water is getting into my chimney?
Common signs are staining on the ceiling or wall near the chimney, a damp or musty smell from the firebox, white powdery deposits on the exterior brick, and rust on the damper or firebox components.
Do all chimneys in this area have this problem eventually?
Every exposed masonry chimney in a climate that cycles across freezing will experience some degree of it. How fast depends on the condition of the crown and cap, the quality of the original work, and whether water is being kept out at the top.
How often should chimney masonry be inspected?
Annually, alongside the sweep. Masonry deterioration is gradual, and an annual look is what turns a receding joint into a small repointing job rather than a structural one discovered years later.
Published September 10, 2026
