Spalling is the word for brick that is flaking, chipping, or popping its face off, and on the older rowhouses I manage around Petworth in DC, it is the damage I take most seriously. Not because a little surface flaking is a crisis, but because spalling is almost always a symptom of a moisture problem that will keep working until you find it. Treat the flaking and ignore the cause and you will be treating it again every couple of years.
What spalling actually is
Spalling is the brick’s hard outer face breaking away, exposing the softer, more porous body underneath. Once that skin is gone, the brick soaks up even more water and deteriorates faster, so spalling accelerates itself if left alone. It shows up as flaking faces, pitting, and little piles of brick crumbs at the base of a wall. On a historic wall it is heartbreaking, because you cannot glue the face back on, spalled brick has to be repaired or replaced.
The usual causes
Behind almost every spalling wall is water plus one aggravating factor:
- Freeze-thaw, water in the brick freezes, expands, and pops the face, the dominant cause in DC’s cold snaps
- Hard mortar on soft brick, which forces stress and moisture into the brick face instead of the joint (the mismatch I cover in matching mortar on 100-year-old brick)
- Trapped moisture, often from a film-forming sealer or paint that would not let the wall breathe
- Chronic wetting, from a leaking gutter, failed flashing, or sprinklers hitting the wall
- Rising damp, ground moisture wicking up into brick near grade
What to do about it
The order matters: find and stop the water first, then repair the brick. Repairing spalled brick without fixing the moisture source just resets the clock. Once the cause is addressed:
- Lightly spalled brick may be stabilized, but heavily spalled units usually need replacement with matching brick
- Failed joints should be repointed with the correct soft mortar
- Any film-forming sealer or trapping paint should be removed so the wall can breathe again
Prevention is cheaper than repair
Keeping brick from spalling comes down to moisture control: sound mortar joints, working gutters and flashing, water draining away from the wall, and, where a finish is truly needed, only a breathable penetrating repellent, never a film former. The Brick Industry Association’s technical notes address spalling causes and prevention through the Brick Industry Association. If your brick is starting to flake, do not reach for sealer or paint first, find the water, because the flaking is the wall telling you where it is getting in.
Painted brick and the trap it sets
A particular version of the spalling problem shows up on the painted rowhouses all over DC, and it catches owners off guard. Ordinary exterior paint forms a film on the brick, and like a film-forming sealer it blocks the wall from breathing. Moisture that gets behind the paint, and it always does, through a crack, a failed sill, rising damp, cannot escape as vapor, so it drives the paint off in sheets and takes the brick face with it. That is why so many painted brick facades spall worse than bare ones once the paint starts to fail. The trap is that the fix is not simply repainting, which just re-seals the moisture in, nor is it always safe to strip the paint, since aggressive stripping can damage soft old brick and older paint may contain lead. On a painted historic wall, the right move is usually to address the water source first, repair any spalled brick, and then use a breathable masonry coating designed to let the wall dry rather than a standard film paint. If you own painted brick and it is starting to flake, treat it as a moisture-and-breathability problem, not a paint-color problem.
Frequently asked questions
What causes brick to spall or flake? Moisture plus an aggravating factor, most often freeze-thaw (water freezing inside the brick and popping the face), hard mortar on soft brick, trapped moisture from film-forming sealers or paint, or chronic wetting from leaks and poor drainage. The face breaks away and the exposed brick then deteriorates faster.
Can spalled brick be repaired? Lightly spalled brick can sometimes be stabilized, but heavily spalled units generally must be replaced with matching brick, you cannot reattach the lost face. Crucially, the moisture cause must be fixed first, or the replacements and repairs will spall too.
Will sealing brick stop spalling? Not if you use the wrong sealer, a film-forming coating traps moisture and makes spalling worse. Spalling is a moisture problem, so the fix is finding and stopping the water (leaks, freeze-thaw exposure, hard mortar). Only a breathable penetrating repellent is appropriate, and only after the cause is addressed.