If brick or mortar is falling onto the roof or a walkway, keep people clear of the area below and have it assessed instead of waiting for the next storm.
Spalling is when the face of a brick pops off and leaves a rough open crater. It happens because water gets into the brick, freezes, expands and breaks it from the inside, and a chimney goes through that cycle far more often than the walls below it, since it is exposed on all four sides and on top with no roof over it. The important thing about spalling is that it accelerates. The intact face of a brick is the part that sheds water; once it is gone, the brick absorbs much faster, so the next winter does more damage than the last. That is why the instinct to seal a spalling chimney is exactly wrong. A film-forming sealer over masonry that is already wet traps the water inside, where it goes on freezing out of sight. Repair first, then a breathable repellent.
Checks you can do safely
- Look at the stack from the ground with binoculars and photograph it
- Check the roof and gutters for brick fragments or mortar debris
- Look for white chalky staining, which indicates water movement
- Note whether the damage is worst on one face, which points at exposure
- Check whether the crown looks broken or the cap is missing
- Do not attempt to patch it from a ladder
Spalling brick: causes in the order worth checking
Not an emergency; do not leave it for a season
| Possible cause | Rank |
|---|---|
| Water entering the brick and freezing through repeated cycles | Most common |
| A failed crown above, feeding water into the masonry | 2 |
| A missing cap letting water down the flue and into the structure | 3 |
| Failed mortar joints giving water an easy path in | 4 |
| An impermeable sealer applied previously, trapping moisture inside | 5 |
| Hard modern mortar used to repoint soft old brick | 6 |
| Acidic condensate attacking the masonry from the flue side | 7 |
Services this maps to
Related problems
- My Chimney Crown Is Cracked
- Water Is Getting In Around My Chimney
- There Is an Animal or a Bird in My Chimney
- Buying a House With a Fireplace, Stove or Unused Chimney
If what you have is not spalling brick after all, the full list of chimney problems is grouped by urgency, and the service triage tool takes a symptom and returns the stop condition and the service it maps to.
Common questions about spalling brick
What exactly is spalling?+
The face of a brick popping off, leaving an open crater. Water inside the brick freezes, expands and breaks it apart from within. The standard failure mode for masonry that is repeatedly wetted and frozen, which describes every chimney in this climate.
Why does it get worse each year?+
Because the intact face is what sheds water. Once it has gone, the brick absorbs far more, so more water is in it for the next freeze. Spalling is a self-accelerating failure, so it is worth addressing early, not watching.
Can I just seal it?+
The most common wrong move. A film-forming sealer over masonry that is already taking water traps it inside, where it still freezes and still breaks the brick: now invisibly. Repair first, then a vapour-permeable repellent if appropriate.
Is spalled brick replaced or just repointed?+
Replaced. Repointing addresses mortar joints; a spalled brick is a failed unit and pointing around it does not restore the face that was doing the work. Where enough units have failed, the honest answer moves from replacement to rebuilding.
How much spalling means a rebuild?+
It turns on whether the loss is in the brick or only in the joints. A handful of failed units are swapped out. Once many courses are affected, or the stack is off plumb, pointing would only be binding failed material together, and rebuilding becomes the correct scope.
Can hard mortar cause spalling?+
Yes, and it is a real cause on older buildings. Modern hard mortar used to repoint soft old brick makes the joint stronger than the brick, so movement and moisture stress get taken by the brick face instead of the joint. The mortar has to suit the brick.
Is falling brick dangerous?+
It can be, depending on what is below. Keep people clear of the area under the stack, and treat continuing loss as a reason to get it assessed instead of a slow cosmetic decline. Masonry that has started coming down commonly continues.
Why is only one side of my chimney spalling?+
Exposure. The face taking wind-driven rain, or the one that stays wet longest and gets the least sun, will fail first. A useful clue and it commonly confirms that water, not anything structural is driving the damage.
Should I stop using the fireplace?+
Exterior spalling on its own is not commonly a reason to stop, but it is a reason to have the flue inspected too. Exterior and interior damage frequently share a cause, and finding a cracked liner after the staging comes down is an expensive sequence.
Can spalling come from inside the chimney?+
Yes. Acidic condensate from an appliance venting into an oversized or unlined flue attacks masonry from the flue side. When damage looks like it is working outward instead of inward, that is worth investigating, not assuming weather.
When can the repair be done?+
In the warmer months, because mortar needs to cure above freezing. Winter is for stabilisation and protection. Planning it for spring or summer instead of reacting in November is how this work gets done well.
How do I stop it recurring?+
Fix what is letting water in, commonly the crown and the cap, repoint with a mortar suited to the brick, and then use a breathable repellent, not a sealer. Spalling is a water problem, so it recurs whenever the water source is left in place.