Water and moisture
What causes white stains on chimney brick?
A white, powdery film on brick is common. It is a clue about water, not the problem itself.
Short answer. The white film is usually efflorescence: salts dissolved by water that travel to the surface and are left behind as the water evaporates. It signals that moisture is moving through the masonry. Cleaning it off treats the symptom, so the useful step is finding where the water enters.
What efflorescence needs
According to the Brick Industry Association, efflorescence needs three things at once: soluble salts in or near the masonry, water to dissolve them, and a path for the solution to reach the surface. Take away any one and it does not form.
On a chimney the water is the part you can change. Brick that keeps getting wet from above or behind will keep showing it.
Efflorescence and spalling are different
Efflorescence on the surface is mostly cosmetic. Salts crystallizing inside the brick are another matter, because that can push off the brick face. If the surface of your brick is flaking, read Why are chimney bricks flaking apart?.
Tracing the water
| Where the white shows | Likely source |
|---|---|
| Just below the crown | Crown or cap leak |
| Along mortar joints | Open or failed mortar |
| Below roof line, one side | Flashing or the roof |
| Everywhere, evenly | General absorption; check all of the above |
What to do
- Dry-brush light deposits if you wish. Avoid acid washes unless a mason advises them, since they can harm brick and mortar.
- Have the crown, cap, flashing and joints checked.
- Repair first. A breathable water repellent can be considered afterward, but it does not fix defects.
Sources
Quick triage
| Observation | Possible cause | Next step |
|---|---|---|
| Powdery white on brick, dry | Efflorescence | Brush dry, find the moisture source |
| White crust near the crown or flashing | Water entry from the top | Inspect crown and flashing |
| Returns after cleaning | Moisture still present | Address the source first |
| White and flaking brick | Freeze-thaw damage | See spalling guide |
Common misconceptions
"Efflorescence is the damage." It is a sign of moisture moving through masonry.
"Washing it off fixes it." It comes back if the water source remains.
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
Crown repair
Repair of the cement or masonry top of the chimney.
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Replacing deteriorated mortar joints.
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Sealing where the chimney meets the roof.
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A condition check of the chimney system.
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Spalled, cracked or loose brick.
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Trace water to its source.
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A breathable repellent on sound masonry.
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