In my news feeds last week I came across an article on how to clean fiber cement siding. The author recommended a 3000-psi (pounds per square inch) power washer, and the accompanying photo showed a man spraying up at siding from about four inches away, with a tight spray pattern.
I had an immediate reaction. Not unless you like mold and dry rot! Too powerful, too close, and the wrong direction.
A power washer in the wrong hands is a dangerous tool. It can cause damage you can’t see and won’t discover for years. I stopped using a power washer several years ago because the spray from my garden hose is sufficient for keeping my brick, wood, and fiber cement exterior clean.
For hard to clean spots, the article recommended using a wire brush. I had another reaction, based on my personal experience: What will that do to the finish?
I use a wire brush on things like rusty outdoor grills, but not anything where appearance matters. While there are pre-finished fiber cement siding products, the best products are primed at the factory and painted on site, and I know from personal experience what a wire brush will do to latex or acrylic paint. (A the tool in the wrong hands.)
The article also recommended using bleach to remove mold. I find a mild vinegar solution to be effective on my brick house, and I don’t like what happens to green growing things when they encounter bleach.
I checked with siding manufacturers Nichiha USA and James Hardie about the article’s recommendations, and since I had a planned meeting with local Nichiha rep Jeff Knauff, I asked him about it. too. We all agree. Gentle is best.
Recommendations for Cleaning Your Fiber Cement Siding
With a little care, your fiber cement siding will last a lifetime. Regular cleaning will enable you to remove chalking, dirt and mold before they become real problems.
If you really want to use a power washer, be gentle. Follow these manufacturer recommendations:
Keep the pressure under 1500 pounds per square inch.
Use a gentle fan spray.
Keep the nozzle no closer than six feet from the wall.
Use a ladder and spray downward onto the siding. Spraying up at the siding may force moisture behind the lap.
Be careful not to spray onto nooks and crannies around windows, doors, and utility outlets.
Depending on what material is on your fiber cement siding, use the most gentle of these three methods:
Dust and chalk: soft cloth or brush and a garden hose.
Grease, oil, other goo: soft cloth or brush, garden hose, and ordinary dishwashing soap.
Mold and mildew: soft cloth or sponge and a mild house wash detergent such as Mold Armor (my favorite), Krud Kutter, or Simple Green. For tough mold spores, you may have to repeat.
Andrew Tweedy, customer service representative at Nichiha USA, put it most succinctly, reading notes from his technical support team:
“Wash your siding just the way you would wash your car.”