Freeze-Thaw Damage and St. Louis Brick: A Spring Checklist

Brick does not fail because it gets cold. It fails because it gets wet and then gets cold, over and over. A wall in Duluth that freezes in November and stays frozen until March takes less punishment than a St. Louis wall that freezes every night in February and thaws every afternoon.

That is the part worth understanding before you spend money on your building. The damage mechanism is the cycle rather than the temperature.

What actually happens inside the brick?

Water expands by roughly nine percent when it turns to ice. In an open bucket that does not matter. Inside the pore structure of a soft common brick it matters a great deal.

Brick is porous. It takes up water through the face, through open joints, and through anything above it that is failing: a cracked chimney crown, a flat parapet coping, a worn stone sill, a downspout dumping against the wall. Once the pores near the surface are close to saturated, the water has nowhere to expand into. It freezes, it grows, and it pushes the outer layer of the brick away from the body of the brick.

One cycle does nothing visible. Forty cycles a winter, on brick that was already holding water, and the outer layer separates. You get spalling: the face shales off in flakes or pops off as a thin plate, exposing the softer interior, which then absorbs water even faster. The failure accelerates on its own once it starts.

The same thing happens in the mortar joint, which is why joints on south and west walls go first. Those elevations take the sun, so they are the ones that thaw and refreeze most often in a single week.

How many freeze-thaw cycles does St. Louis get?

NOAA’s 1991 to 2020 climate normals for St. Louis show about 85 days a year with a minimum temperature at or below 32 degrees, and only about 19 days a year where the temperature never climbs above freezing. Subtract one from the other and roughly 65 days a year the wall freezes and then thaws again.

That is the number that matters for masonry. Our winters are not severe by national standards. They are relentlessly repetitive, and repetition is what breaks brick. Add our rainfall, and the wall is often wet going into each freeze.

Efflorescence or spalling: what is each one telling you?

These get confused constantly, and they mean different things.

What you see	What it is	What it means
White powdery film on the brick face	Efflorescence, salts left behind as water evaporates	Water is moving through the masonry in volume. Early warning
Faces flaking, shaling or popping off in plates	Spalling	Water got in and froze. Permanent damage, replacement needed
Mortar crumbling to sand under a key	Failed joint	The sacrificial part of the wall has done its job. Repoint
Dark damp staining that persists after dry weather	Continuing water path	Something above is feeding water in

Efflorescence on its own rarely harms the brick. The deposit brushes or washes off. The reason to take it seriously is that it is proof of a water route through the wall, and if you leave the route open you are lining up the freeze damage that comes next.

There is a nastier relative worth naming. When the salts crystallize inside the brick just below the surface instead of on the face, the crystal growth pushes the face off in the same way ice does. You get the spalling without the tidy white warning first.

What should you walk your building for in March?

Do this on a dry day after the last hard freeze, with the sun low so shadows show the wall texture.

Stand back and scan each elevation. Look for lighter patches where faces have come off, and for any bulge or dish in the plane of the wall.
Check south and west first. They take our weather and fail earliest.
Key-test the joints at every height you can safely reach. Sound mortar scratches. Failed mortar comes out as sand.
Look under every sill. A worn stone or brick sill without a drip loses its ability to throw water clear, so the wall under it stays wet.
Look at the chimney. The crown, the top three courses and the flashing.
Follow the downspouts. A spout discharging against brick or onto a porch deck soaks that area every storm.
Check the foundation and porch piers. On the clay soils around Kirkwood and the inner suburbs, stone piers and foundations move seasonally and open joints at the mortar.
Go inside and look at the plaster on outside walls, especially in closets and behind furniture.
Count the spalled bricks and photograph them with something for scale. Next March you will know whether it is getting worse.

Does sealing the brick prevent this?

Sometimes, and sometimes it makes things worse. This is the most common expensive mistake on St. Louis brick.

A film-forming sealer, an elastomeric coating or a masonry paint blocks water from entering the face. It also blocks water from leaving. Old brick walls have no cavity and no drainage plane. They handle water by absorbing it and then drying out through the face. Seal that face and the water that gets in another way, through a failed joint, a bad crown, a leaking sill, now cannot get out. It migrates until it finds an edge and freezes there. The Park Service guidance on water-repellent treatments is blunt about the risk and treats these coatings as a last resort rather than routine maintenance.

A breathable penetrating water repellent is a different product. It reduces liquid absorption while still letting vapor out. It is a reasonable step, with two conditions. Fix the joints, the crown, the sills and the drainage first. And apply it only after the pointing has fully cured, otherwise you seal moisture into fresh mortar.

The order matters more than the product. Water management first, repellent second, never repellent instead.

What does the repair work look like?

Spalled brick gets cut out and replaced with salvaged or matched units of similar hardness. Failed joints get raked to depth and repointed with a mortar softer than the brick. Crowns get recast with an overhang. Sills get repaired so they throw water clear again. Downspouts get extended away from the wall.

None of it is exotic. It is a sequence, and skipping to the end is what wastes money.

Planning the season

March and April are the right time to scope masonry work, because mortar needs to cure above about 40 degrees and the calendar fills quickly once the weather turns. If your spring walk turned up more than a few spalled faces, start with a look at masonry waterproofing to see how we sequence water management against sealing. We will tell you plainly if a repellent is the wrong spend on your wall.