How Freeze-Thaw Damage Wrecks Chicago Brick (and What Stops It)

Freeze thaw damage in brick is a water problem that shows up in winter. The brick itself handles cold fine. What it cannot handle is being soaked and then frozen over and over. Chicago has a lot of freezing mornings, and a lot of old soft brick, so the combination shows up on porches, parapets and chimneys across the city. Here is how it works, why some walls suffer and others do not, and what actually helps.

How does freeze-thaw damage brick?

Brick is porous. Water gets into the pores, through joints, through the face, or up from the ground. If that water freezes inside the brick, it pushes outward from inside, and the surface layer can break off. The Brick Industry Association lists freeze-thaw of saturated brick as one cause of a face spalling or delaminating (BIA).

The key word is saturated. Brick that is not saturated is at far less risk. A wall that gets wet and dries out between cold snaps does fine. A wall that stays wet all winter does not. So the real question is not "how cold does it get," but "how wet does my wall stay."

Salt makes it worse and complicates things. Salt crystallizing inside brick creates its own pressure that can make the face spall (BIA). Frost and salt often work together on the same wall. See are spalling bricks dangerous for what it looks like and when it matters.

Why is Chicago hard on brick?

Chicago winters give brick a lot of chances to freeze while wet.

• The average year has about 121 days with a low at or below 32 degrees Fahrenheit, and about 43 days when the high stays at or below 32 (NWS Chicago, 1991 to 2020 normals).
• That leaves roughly 78 days a year that start below freezing and rise above it, if you subtract one count from the other. That is our own arithmetic, not a published figure, and it is only a rough stand-in for freeze-thaw days, not an engineering count of cycles.
• Chicago averages 38.4 inches of snow a year (NWS Chicago). Snow melts onto walls, chimneys and parapets and refreezes.
• Lake Michigan moderates temperature and adds snowfall (Illinois State Climatologist), which keeps the swings coming.

We are not promising a cycle count. The point is that many mornings start frozen and the afternoon thaws, and each thaw lets the wall take on water that can freeze again.

Which walls are most at risk?

Not all brick takes a beating the same way. These spots tend to go first:

• The top of the wall. Parapets and chimneys are exposed on every side, with no roof above them. Caps and copings should channel water away from the wall, and a brick cap with many mortar joints can be a poor choice at the most exposed location (BIA).
• Below leaks. Under a failed gutter, a leaking downspout or a window sill with no drip.
• The base of the wall. Splash from the ground, snow piled against the wall and soil sloping toward the building keep the base wet. Sloping soil away from the foundation and extending downspouts help (University of Minnesota Extension, general building science).
• Walls with failed flashing. Water that gets behind the brick has nowhere to go without it (BIA).
• Soft, old brick. Soft, porous brick takes in more water than hard brick.
• Walls repointed with hard mortar. This one is on the owner and the contractor, not the weather.

How does the wrong mortar turn freeze-thaw into spalling?

Mortar is supposed to be the weak link. Repointing mortar must be softer and more vapor-permeable than the brick, and no harder than the original mortar (NPS). When mortar is less permeable than soft brick, moisture and dissolved salts leave through the brick face instead of the joint, and the freezing and salt damage lands on the brick (NPS). The brick sacrifices itself to protect the mortar, which is backward.

That is why a wall repointed with a hard modern mortar can look perfect for two winters and then start shedding brick faces. A strong mortar is not automatically wrong on a modern brick wall, but on soft historic brick it can be. Our mortar and brick failure article covers it in detail.

What actually stops freeze-thaw damage?

You cannot change the weather. You can change how wet the wall gets.

1. Stop the water at the source. Gutters cleaned and pitched, downspouts discharging away from the wall, sills and copings with drips and slopes, flashing intact. This is the biggest lever and the cheapest.
2. Keep the joints closed. Open joints let water into the wall. Tuckpointing, done on time and to the right depth, closes them (BIA).
3. Use mortar that is softer than the brick. So the joint, not the brick, takes the stress (NPS).
4. Let the wall dry. A wall has to be able to dry out between wet spells. Walls should breathe.
5. Fix the salt source. White powder on the brick points to water moving through the wall carrying salts. The BIA says to find and fix the moisture and salt cause before removal (BIA). Efflorescence appearing after the first year usually points to excess water penetration or poor drainage, and is often worst in winter (BIA). See efflorescence on brick.

Will a sealer protect brick from freeze-thaw?

Usually not, and it can backfire. The BIA does not recommend coatings alone for moisture problems (BIA), and the National Park Service says water-repellent coatings are appropriate only after a building is made watertight and dried (NPS). A coating can trap water in the wall, where it freezes. We explain it in should you seal brick after tuckpointing.

What if the brick is already damaged?

Surface spalling that has just started can sometimes be managed once the water is fixed (can you fix spalling brick without replacing it). Brick that has lost its face and keeps shedding usually has to be cut out and replaced. Either way, fix the cause first or the repair repeats. Masonry repair in Chicago is permit work through a licensed mason, so confirm details with the Department of Buildings.

If your Chicago brick is flaking and you are not sure whether it is frost, salt, mortar or all three, our masonry team will look at the wall and tell you which. For the full picture, see our brick and masonry repair guide.

Common questions

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.