· by Michael Blair, Nashville's Masonry
A white haze or crust on brick, block or stone, sometimes in streaks, sometimes in a band along the bottom of a wall, is efflorescence. Water inside the masonry dissolves salts from the brick, the mortar or the ground, carries them to the surface, and leaves them behind when it evaporates. On a new wall it is common and usually goes away on its own. On an old wall, appearing in one place year after year, it is a map of where water is getting in, and that is the part worth reading.
Where the salts come from
New masonry has plenty of soluble salts in the mortar and often in the brick, and plenty of water from construction. As it dries out over the first year, the salts come to the surface. This is called new-building bloom and it fades and stops as the wall dries. It is cosmetic.
On older masonry the salts come from the ground, from deicing salt, from a leaking gutter, or from within the wall where a failed cap or flashing lets rain in. The water is the thing. Efflorescence needs a path for water into the masonry and a surface for it to evaporate from, and where you see it, both exist.
Reading the pattern
A band along the bottom of a wall, a foot or two above grade, is rising damp: ground water wicking up through the brick from soil that stays wet, often because a downspout dumps there or the grade slopes toward the wall. A streak below a windowsill is water getting in at the sill joint. A patch at the top of a chimney or a parapet is a failed cap or crown. A patch on the inside of a basement block wall means water is coming through from outside.
Efflorescence on the face of a wall that has been sealed with a film-forming sealer is the worst version: the salts are being deposited behind the sealer, where they cannot be brushed off and where the pressure of their crystals is spalling the brick face. That wall needs the sealer stripped, not more sealer.
- Bottom of wall: ground water and drainage at the base
- Under sills and at joints: failed sealant or flashing
- Top of chimney or wall: cap, crown or coping
- Under a sealer: trapped salts damaging the face
Cleaning it off
Start dry. A stiff brush, not a wire brush on soft brick, removes most fresh efflorescence. If it returns, wait for a dry spell and brush again; washing it adds water, which brings more salts. Stubborn deposits come off with a proprietary efflorescence cleaner or a very dilute acid wash, applied to a pre-wetted wall, worked briefly and rinsed thoroughly. Test in an inconspicuous spot, since acid can change the color of some brick and etches limestone and cast stone badly.
Do not pressure wash it. High pressure drives water into the wall, opens the mortar joints, and produces a bigger bloom in a few weeks.
Stopping it
Cleaning is temporary until the water source is closed. Fix the downspout, regrade away from the wall, replace the failed sealant at the sill, rebuild the chimney crown, repoint the open joints. Once water stops moving through the masonry, the salts stop arriving, and a final cleaning holds. On a basement wall the fix is on the outside: drainage and grading, or a membrane and drain tile if the water table is the cause.
Once the wall is dry and clean, a breathable penetrating water repellent, not a film-forming sealer, can reduce the water the face takes in from rain while still letting the wall dry outward. Applied to a wall that is still wet from inside, any treatment makes things worse, which is the mistake most often made with efflorescence.