What the stain is telling you
Water enters the brick, dissolves salts inside it, travels to the surface and evaporates, leaving the mineral behind. No water, no efflorescence. So the stain is a reliable indicator of active moisture intrusion.
Where the water is coming in
The usual sources, in the order we find them:
- A cracked or missing crown at the top
- No cap over the flue opening
- Deteriorated mortar joints
- Unsealed, highly porous brick
The right repair order
Fix the entry point, let the masonry dry, clean the residue with a masonry-safe method, then apply a breathable waterproofing sealer. Pressure washing and sealing a wet chimney traps the problem inside.