Water and moisture
Why are chimney bricks flaking apart?
Flaking brick is the visible end of a process that starts with water. Replacing the brick without stopping the water starts it again.
Short answer. Spalling is physical damage to the face of a brick. Water absorbed by the brick expands when it freezes, and repeated freezing and thawing breaks the surface. Salts crystallizing inside the brick can do the same. Stopping the water entering is the first step, before replacing any brick.
How it happens
Brick and mortar are porous. In winter, water soaked into the surface freezes and expands. Many cycles of freezing and thawing weaken the surface until a layer flakes off. Salt crystallizing just beneath the surface can create similar pressure. A chimney is more exposed than most walls: it is wet from several sides and sits above the roof, where wind and cold hit it.
How serious is it?
| What you see | What it suggests | Next step |
|---|---|---|
| A few bricks with flaked faces | Local wear, perhaps near an open joint | Replace those bricks |
| Many bricks at the top | Water has entered for a long time | Fix the source; assess the stack |
| Flaking next to hard, fresh mortar | Mortar harder than the brick, forcing the brick to take the stress | See the repointing guide |
| White powder with flaking | Moisture and salts moving through | Find the water path |
| Bricks loose or the chimney leaning | Structural concern | Prompt assessment, do not use the fireplace |
Where the water comes from
- A cracked or failed crown.
- No cap, so rain wets the flue and masonry from the inside.
- Eroded mortar joints that admit water.
- Failed flashing that sends water behind the chimney.
- Poor drainage or a gutter overflowing at the chimney.
What helps
- Fix the source first: crown, cap, flashing and open joints.
- Replace badly damaged bricks and repoint with mortar suited to the brick. See repointing or rebuilding a chimney?.
- Avoid sealing wet brick shut. Trapped water makes damage worse.
- Consider a breathable water repellent only after the repairs, and only for sound brick.
Common misconceptions
"Painting or sealing will stop it." A film over damp brick can trap water. Fix the entry first.
"New brick will solve it." New brick in the same wet conditions flakes too.
"It is only the top few courses." The top is where it shows first. Water often travels further.
Related
White deposits are covered in what causes white stains on chimney brick?. Water entry points are covered in why water is coming into your chimney.
About this guide. Published by Dr. Sweeper as general homeowner education, last updated October 1, 2026. Sources are linked where they are cited. What applies to your home can depend on your local building code, your appliance and its manufacturer's instructions, and the actual condition of your chimney, so this guide is not a substitute for an inspection.
If you need help
Related services
Repointing
Replacing deteriorated mortar joints.
Learn moreCrown repair
Repair of the cement or masonry top of the chimney.
Learn moreChimney caps
Rain and animal protection at the flue top.
Learn moreChimney inspection
A condition check of the chimney system.
Learn moreBrick & Masonry Repair
Spalled, cracked or loose brick.
Learn moreChimney Crack Repair
Which cracks matter.
Learn moreQuestions
Frequently asked questions
Can flaking brick be repaired in place?
Individual bricks can be replaced. Widespread flaking may call for a partial rebuild.
Is spalling dangerous?
Loose pieces can fall, and the underlying water path damages the masonry. It is a reason to act.
Does it only happen in winter?
The damage happens in freezing weather. It shows afterwards.
Will a repellent help?
Sometimes, on sound brick, after the entry points are fixed.
Ready to have your chimney looked at?
Call us, or send a scheduling request. We will contact you to confirm a time. Your appointment is not confirmed until we do.