Spalling
Brick faces popping off a chimney is spalling, and it means water has been getting into the brick and freezing inside it.
Short answer
Water absorbed into brick expands when it freezes, and the fired outer skin pops off. Once that skin is gone the brick absorbs faster and the process accelerates. The usual drivers are a failed crown, no cap, failed joints, and frequently a hard mortar or a sealer that trapped moisture in the brick.
What causes it
Water absorbed into the brick, then freezing and expanding inside it. The whole mechanism in one sentence.
A failed or missing crown at the top, which lets water straight into the masonry from above.
No cap, so rain and snow go down the flue and into the structure.
Failed mortar joints, which are the easiest route in.
A mortar harder than the brick, which forces the movement and the salt damage into the brick face instead of the joint.
And a non-breathable sealer applied to the masonry, which traps the moisture inside and accelerates exactly what it was meant to prevent.
The sealer trap
A film-forming sealer on a chimney stops water getting out as well as in. The masonry still absorbs from the crown, the joints and the back face, and now it cannot dry. The result is accelerated spalling, frequently behind the sealed face. If a chimney is to be treated, the product has to be vapour permeable.
What it means for the chimney
Spalled brick cannot be repaired — the face is gone and the brick is now absorbent. It is replaced.
A few spalled bricks means cutting them out and replacing them with matching units.
Widespread spalling, particularly above the roof line, means rebuilding that section.
Spalling plus a leaning or bulging stack means a structural assessment rather than a masonry repair.
Debris falling into the flue from the inside face, which affects the draft and can block it.
And a liner question, because a deteriorating masonry chimney serving an appliance needs looking at from the inside as well.
Stopping it
Fix the crown, which is the single most important measure and is frequently the original cause.
Fit a cap, which keeps rain, snow and animals out of the flue.
Repoint with a compatible mortar, softer than the brick.
Replace spalled brick with matching units.
If sealing, use a vapour-permeable breathable water repellent rather than a film-forming sealer.
And deal with the flashing at the roof line, because water held against the base of the stack does the same thing lower down.
Common questions
Can spalled brick be repaired?
No. The fired face is gone. The brick is replaced.
Should I seal the chimney?
Only with a breathable vapour-permeable product. A film-forming sealer traps moisture and accelerates spalling.
What is the most important fix?
The crown, then a cap. Those are usually where the water is getting in.