Why chimney brick spalls, and how bad it is

What spalling looks like up close

A spalling chimney brick has lost part of its outer skin, and flaking chimney brick is the everyday name for it. Sometimes the loss is a scatter of shallow flakes. Sometimes a sheet of the face, parallel to the surface, has let go in one piece and left a rough, sandy crater behind. Either way, the fragments tend to collect on the roof below the chimney, in the gutter or on the ground.

Three other things get mistaken for it. A white, powdery bloom is efflorescence, salt left on the surface, and it wipes or brushes off without taking any brick with it. Joints that have worn back from the face are a mortar problem. A crack that runs through several bricks in a line is movement in the chimney, which is a different and sometimes more serious diagnosis.

Water is where it starts

Ask what causes spalling brick, and the first answer is water. Brick is porous. Every rain puts some water into it, and every dry spell draws that water back out, and that cycle by itself does no harm to sound brick in a wall that dries well. Spalling starts when a chimney gets more water than it can shed, or when something stops it from drying.

The extra water arrives from predictable places. A cracked crown or one with no overhang sends rain down the faces and into the top courses. An uncapped flue lets water soak the chimney wall from the inside. Lifted flashing feeds the lower courses where the chimney meets the roof. Open joints let water straight into the wall, and a lawn sprinkler that hits the base of an outside chimney keeps the bottom courses wet.

Salts that grow under the face

Water moving through brick carries dissolved salts from the mortar, the brick itself, the soil and anything that has been washed onto the wall. When the water evaporates, the salts stay behind. The National Park Service's repointing guidance, Preservation Brief 2, describes the two outcomes: salts deposited on the surface show as efflorescence, which is mostly harmless, while salts that crystallize just below the surface build up pressure that can push the outer face off.

That second process needs no frost at all. It only needs water moving through the brick and drying from its face, over and over. Guides written for cold climates blame freezing, but the salt mechanism works in any climate where brick gets wet and then dries.

Mortar and coatings that stop the wall drying

The joints are meant to be the easy path for water and movement. Preservation Brief 2 explains that mortar stronger than the brick will not give, so the stresses of expansion, contraction and settling are relieved through the brick instead, with cracking and spalling as the result. It also notes that when mortar is less permeable than the brick, moisture and salts leave through the brick, with the same damage following.

Coatings can do the same thing from the outside. Paint and film-forming sealers slow the rate at which a wall dries, so water that gets in behind them, through a crack, a joint or the top of the chimney, stays in the brick longer and deposits its salts just behind the coating. Breathable water repellents, the silane and siloxane types, are made to shed liquid water while letting vapor out, which is a different behavior from a film.

Heat from the inside

Some spalling starts in the flue, not the weather. A chimney fire, or years of fires in a flue whose liner has cracked, can heat the chimney wall unevenly and crack or spall the brick from within. On the outside that can look like weather damage, so a chimney with spalling on several faces gets its flue checked as well.

The firebox is a special case. Where a firebox is lined, section R1001.5 of the residential code wants that lining in firebrick to ASTM C27 or C1261, set in refractory mortar. Ordinary building brick used where the flames touch it is not made for that heat and can spall at the face.

Why the chimney spalls before the house walls

A spalling chimney on a house whose brick walls look fine is a familiar sight. No other masonry on the building is as exposed. Above the roof it takes weather on all four faces, its top is open to rain unless it is well capped and crowned, and on the inside it is heated and cooled by every fire. A house wall has a roof overhang, one exposed face and a steady interior.

So the chimney is the first place water and salts do their work, and it tells you early when something is feeding water into the masonry. Treat spalling there as a report on the chimney's top and flashing, not as a sign that the whole house is next.

Judging how bad it is

A few shallow flakes on a sound chimney can be watched. A face that has lost its skin down to the soft body of the brick cannot, because that body now soaks up water faster and wears faster. Binoculars and a look at the roof below will tell you a lot without climbing.

• Scattered, shallow flaking on a few bricks: note it, find the water source, and look again after the next rainy stretch.
• Faces lost down to a rough, sandy core: those units are candidates for replacement.
• Spalling concentrated below a crown crack, a downspout or a flashing line: the source is right above it.
• Loose or crumbling bricks near the top of the chimney: get it checked soon, since pieces can fall onto the roof or a walkway.
• Spalling on several faces plus cracked flue tiles or a past chimney fire: leave the fireplace cold until the flue is checked.

What to fix first

The order matters more than any single product. Stop the water first, whether that means a crown repair, a cap, flashing work or repointing open joints. Then replace the bricks that have lost their faces, which is the job of our spalling brick repair page. After the mortar has cured, a breathable repellent can be considered on a chimney that stays wet, following the product maker's instructions.

Two reactions make spalling worse. Painting or film-sealing a spalling chimney traps the moisture and salts that are causing the problem. High-pressure washing can strip the fired surface off soft brick and drive water deep into the wall, so any cleaning is done gently and only after the repairs.