Cleaning Stained Patio Brick
Solutions for moss, mildew, and other unsightly marks
How should I clean the green and brown stains on my brick patio? I tried power washing, but it didn't work.
— Marcia, Short Hills, NJ
Roger Cook replies: Power washing is usually enough to remove moss (probably your green stains) as well as mildew stains (usually black) from brick and other patio surfaces. You'll get better results when the moss is actively growing, but you should be able to remove the stains even in cold weather. Just be careful not to use nozzles with a tightly focused spray or too high a pressure—anything above 3,000 psi may damage the brick.
For stubborn stains, try a 10-percent solution of chlorine bleach to water, scrub the area with a stiff-bristle brush, and then rinse thoroughly with clean water. Be sure to wear rubber gloves as you scrub, and don't forget eye protection—scrubbing and spraying can splatter the bleach mixture toward your face. Try that approach on a small sample area first, however, and wait a week to see if there are any unwanted color changes in the brick.
As for the brown stains, it could be something that was spilled on the patio, or a mineral leaching out of the brick itself. Without knowing for sure what the stain is or the kind of brick you have, I can't recommend a single approach. So before you do anything, take a look at the Brick Institute of America's Web site (www.bia.org). It has a good section on how to get rid of a variety of stains. Then, no matter what solution is recommended, be sure to test it first on an inconspicuous area of your patio.