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White stains on the brick: reading efflorescence

The white, chalky staining on chimney brick is efflorescence: mineral salts carried out of the masonry by water and left behind when it evaporates at the surface. The stain itself is harmless and brushes off. What it says is not harmless: measurable amounts of water are moving through your masonry, and the stain is the exit wound.

Reading the location

Where the stain sits points at the source. A white band just under the crown is the classic failed-crown signature, water entering the top and migrating down. Staining concentrated below the flashing line implicates the flashing. Broad staining across a weather face suggests saturated, unsealed brick. Streaks below the cap on one side often track a cap or crown detail failing on that side.

The repair is upstream

Scrubbing the stain treats the symptom and it returns with the next wet season. Stopping it means closing the water path: crown repair, cap or flashing work, repointing, or a breathable repellent on saturated faces, per the diagnosis. Once the path closes, remaining staining cleans off and stays off, which makes efflorescence a free monitoring tool: if white comes back, water found a new way in.

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