Freeze-thaw damage happens when water soaks into brick, mortar or concrete, freezes, and then thaws, over and over. Each cycle can flake a little more of the surface off. Chicago gets about 121 days a year with a low at or below freezing, so a wet wall or slab faces plenty of chances. The damage shows up in spring as spalling brick, crumbling mortar and scaled concrete. The fix is to keep water out and let it drain away.
Freeze-thaw damage is water doing the work, not the cold itself. A porous material like brick, mortar or concrete soaks up water. When that water freezes, the material takes the stress. When it thaws, more water gets in. Chicago gives a wet wall a lot of chances to repeat that.
How many freeze-thaw days does Chicago get?
The National Weather Service's 1991 to 2020 normals for Chicago show about 121 days a year with a low at or below 32 degrees Fahrenheit, and about 43 days when the high stayed at or below 32 (NWS Chicago). Days where it dips below freezing at night and warms above it later are where the cycling happens. NWS does not publish a freeze-thaw cycle count, and a day count is not an engineering cycle count. By our own arithmetic, 121 minus 43 is roughly 78 days that start below freezing and end above it, which is a rough proxy, not a published number. Annual snowfall averages 38.4 inches (NWS Chicago), and the melt is the water.
What does freeze-thaw do to brick, mortar and concrete?
Brick. Saturated brick that freezes can spall, meaning the face flakes or pops off. Salts crystallizing inside the brick are a second cause that does the same thing (BIA and NPS guidance). If the mortar is harder and less permeable than the brick, moisture and salts move out through the brick instead, which is how faces get pushed off (National Park Service). For the long version, see how freeze-thaw damages Chicago brick.
Concrete. The surface flakes off in a pattern called scaling, and deicing salts make it worse by keeping the surface wetter and adding cycles (NRMCA). Another sign is small cone-shaped pits, called popouts, where a stone near the surface absorbed water and fractured when it froze (NRMCA). Concrete that will be wet in freezing weather should be air-entrained (NRMCA). Salt is its own subject: read road salt and your building.
Mortar. The joint is usually softer than the brick, so it is the first thing to open. Open joints let in more water, which is why a small mortar repair can prevent a bigger brick one.
What does it look like, and what should you do?
White crust on a wall means water is moving through it. It is usually harmless by itself, but if it shows up on a building more than a year old it usually points to excess water getting in (BIA Technical Note 23A). Other signs: flaking brick faces, sandy or missing mortar, rough scaled concrete, pits in a slab.
Walk the building after the thaw using our post-winter checklist. If you find brick faces coming off or steps crumbling, a mason can tell you whether it is a repair or a rebuild. Our masonry crew does that walk-through, and the Chicago maintenance calendar shows where it fits in the year.
Common questions
Water freezes at 32 degrees Fahrenheit, but damage depends on how wet the material is when it freezes, not on a single temperature. A saturated brick or slab that freezes and thaws repeatedly is at risk. We do not have a verified temperature threshold to give you.
That depends on your policy and the insurer, and we cannot answer it for you. Read the policy, ask your agent, and photograph the damage before repairs. For brick specifically, see whether insurance covers spalling brick.
Sources
- National Weather Service Chicago: 1991 to 2020 Climate Normals for Chicago
- National Park Service: Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings (PDF)
- National Ready Mixed Concrete Association: Concrete in Practice 2: Scaling Concrete Surfaces (PDF)
- National Ready Mixed Concrete Association: Concrete in Practice 40: Aggregate Popouts (PDF)
- National Ready Mixed Concrete Association: Concrete in Practice 27: Cold Weather Concreting (PDF)
- Brick Industry Association: Technical Note 23A: Efflorescence, Causes and Mechanisms (PDF)
Facts last checked: October 7, 2026.
This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.