White stains on brick: what efflorescence means and how masonry should be cleaned

Updated November 11, 2026 · KeystoneProGuide editorial team

What is the white powder on brick?

Usually efflorescence. The National Park Service (NPS) explains that water moving through masonry carries dissolved salts, and when the water evaporates at the surface the salts are left behind as a white bloom. It describes the deposit as a whitish, fuzzy layer that can be brushed off.

Efflorescence tells you that water is moving through the wall and drying at that spot. The useful question is where the water is coming from.

Is efflorescence harmful?

The NPS treats the white bloom and the damage as two outcomes of the same salts. On the surface they form the bloom. When they crystallize inside the brick instead, the growing crystals break it down from within and peel off slices of its face, a process called spalling. Left unchecked, the NPS says, salts and water can make brick and stone crack and crumble.

So the bloom is worth taking seriously where it keeps coming back or where the brick under it is flaking. Both point to a steady supply of water that needs to be found.

What is a tide line on a basement wall?

A sign of rising damp. The NPS describes rising damp as moisture wicking up masonry from the ground by capillary action, the drier parts of the wall drawing it up like a sponge. In basements it often leaves a "high tide" line, often 3 to 4 feet above the floor, where the damp lower wall meets the dry wall above, with efflorescence deposited along it.

The NPS's warning is that moisture at this level has started breaking down the bonds that hold the masonry together, which can lead to delamination, surface chipping or disintegration. Coating the wall to hide it can make it worse: in the NPS's Iowa example, an impervious coating at the base of a sandstone wall drove ground moisture up into the stone just above it.

Where does the water usually come from?

The NPS lists the sources: rain and snow on the wall, sprinklers, the ground, roof leaks and even air conditioning. Some are easy to check from the yard.

Downspouts that empty against the wall. Penn State Extension says all downspouts should be directed away from foundations.
Soil or paving sloping toward the wall.
Lawn sprinklers that hit the masonry.
Failed mortar joints or cracks above the stained area.

How should brick be cleaned?

Gently, and only after testing. The General Services Administration (GSA) specification for cleaning historic masonry rests on one rule: use the mildest, least abrasive method that works. It says the gentlest cleaning method for masonry is water and soft bristle brushes, and that this should be tried first.

The GSA's other limits for historic buildings:

Test patches first. Small patches, no larger than 6 by 6 inches, in out-of-sight places, with several methods tried side by side to find the gentlest that works.
Low pressure. Water washing at no more than 300 psi; the GSA notes that most commercial pressure washers run far higher, and rinsing after chemical cleaning stays at or below 400 psi with the nozzle at least 18 inches from the wall.
No raw acids or alkalis. Proprietary cleaners only after test panels, and every surface neutralized and pH-tested afterwards.
No metal brushes. Natural-bristle brushes and wood scrapers only.
The right weather. Air and surface at 40 degrees F and rising, and no rain, snow or freezing expected within 24 hours.
Watertight first. Cracks and openings sealed so cleaning water does not get into the building, and runoff collected rather than sent into plants or drains.

What about sandblasting?

The NPS says abrasive cleaning such as sandblasting removes the hard outer surface of brick and exposes its softer core. Water can then soak into the brick, and where winters freeze, freeze-thaw damage follows. It traces many badly deteriorated brick walls to an earlier abrasive cleaning. The Occupational Safety and Health Administration also lists abrasive blasting with sand among the activities that expose workers to crystalline silica dust.

Should brick be cleaned before or after repointing?

Before. The GSA's repointing specification has the masonry cleaned first and repointed afterwards, and it requires any mortar smeared on the brick during repointing to be removed at once. Its final cleanup uses wood scrapers, natural-fiber brushes and clean water, with no acidic or alkaline cleaners.

Do you need approval in Philadelphia?

On a property listed on the Philadelphia Register of Historic Places, yes. The Historical Commission's review covers "masonry cleaning", as well as pointing, painting and coating. The review is free and is part of the building permit process.

For brick that is flaking or crumbling rather than stained, see warning signs. If someone proposes a sealer to stop efflorescence, read whether to seal or waterproof brick first. Masons are listed on the masonry contractors page.

Sources

Mortar, Unsung Hero of History, U.S. National Park Service
This Masonry is for the Bees!, U.S. National Park Service
Common Problems with Brick Masonry, U.S. National Park Service
Cleaning historic masonry, U.S. General Services Administration
Repointing historic masonry, U.S. General Services Administration
Get approval for work to a historic property, City of Philadelphia
How Can I Control Stormwater on My Property?, Penn State Extension
Silica, Crystalline - Overview, U.S. Occupational Safety and Health Administration