How to Clean a Concrete Retaining Wall

Why walls go dark

A retaining wall holds back soil, so one side of it is always against damp ground. Water moves through the concrete, splash from rain and sprinklers hits the face, and plants on top drop leaves and dirt down the front. Over a few years a light gray wall turns blotchy and dark, often with a white crust near the bottom.

Stepped concrete walls from one of our jobs, before and after.

What the makers say

Allan Block, a retaining wall block maker, lays out the steps in its care guide: wet the wall with a hose or pressure washer to loosen the grime, apply a cleaner, scrub with a stiff brush, rinse so you “don’t want to leave lasting streaks,” and let it dry, which “may take a few hours up to a day” in shade (Allan Block).

Prosoco, which makes masonry cleaners, adds three warnings on its block cleaner sheet (Prosoco):

• “Do not clean block with muriatic acid.”
• Do not spray the cleaner itself above 50 psi, because it “drives the cleaner into the surface, making rinse difficult,” and can stain.
• “ALWAYS TEST before overall application,” and rinse from the bottom up.

It also says to protect plants, painted surfaces and metal nearby from splash and runoff, which matters on a wall with a flower bed on top of it.

The white crust

The white powder near the bottom of a wall is efflorescence, salt carried out by water moving through the concrete. Allan Block says to dry the surface completely after washing, because “if moisture remains the efflorescence is likely to return” (Allan Block). More on it here: white powder on brick and pavers.

One visit for the whole yard

Walls rarely get dirty alone. The patio below them and the walk beside them usually need it too. See driveway and concrete cleaning and patio cleaning.