The White Staining
The white powdery staining on a brick chimney is not a cosmetic problem. It is a sign that water is moving through the masonry.
Short answer
Efflorescence is salt left on the surface as water that moved through the masonry evaporates. It means water is getting in — from the crown, the joints, a missing cap or the flashing — and travelling through the brick. Washing it off removes the symptom; the staining returns until the water source is fixed.
What it is telling you
Water has entered the masonry, dissolved salts within it, travelled to the surface and evaporated, leaving the salt behind.
So the staining is a map of where water is moving and where it is leaving.
On a chimney the usual entry is the top: a cracked crown or a missing cap.
It can also be the joints, the back face against the roof, or the flashing at the roof line.
Heavy efflorescence on the interior chimney breast means water is coming through the structure into the house.
And the same water movement that leaves the salt is doing freeze-thaw damage inside the brick, which is the part that matters.
Where it appears matters
High on the stack points at the crown and the cap. Along one face points at driving rain or a flashing problem. At the base near the roof line points at the flashing or at water sitting against the stack. Inside the house on the chimney breast means the water is all the way through, which is the urgent version.
The related damage
Spalling, where the water freezing inside the brick pops the face off.
Mortar joint erosion, which is both a cause and a consequence.
Salt crystallization inside the brick, which does similar damage to freezing and happens in any season.
Deterioration of the flue liner and the smoke chamber from water inside the flue.
A rusted damper and a water-stained firebox.
And interior damage: staining, peeling paint and mould on the chimney breast.
The right order
Find and fix the water entry: crown, cap, pointing, flashing, in that rough order of likelihood.
Allow the masonry to dry out, which takes months rather than days on a wet stack.
Then clean the staining off, mechanically with a stiff brush where possible, or with a proprietary masonry cleaner if needed.
Do not use strong acid cleaners on older brick without advice, because they damage soft brick and mortar.
Expect some recurrence while the masonry dries out, which is not a sign the repair failed.
And consider a breathable repellent afterwards, once it is dry and the sources are fixed — not before.
Common questions
Is efflorescence a problem?
It is a sign of one. Water is moving through the masonry.
Can I just wash it off?
It comes back until the water source is fixed.
What does it mean inside the house?
Water is all the way through the structure, which is the urgent version.