Dear Tim: My wife and I have an outdoor patio with colored precast concrete paving blocks. It doesn’t take long each year for black mold and mildew to start growing on it. We also have an issue with moss and algae growing on it. I have to power wash it at least once a year and wonder if there’s a way to prevent the moss, mildew and mold from growing in the first place. Am I damaging my patio with the power washer? Why is it growing on the precast concrete pavers? This problem can’t be that hard to solve. — Loren P., Okatie, S.C.
Loren: I used to have the same problem on two massive brick paver patios in the back of the last house I lived in. It was a mind-numbing job that took hours and hours of work to restore the patio to brand-new condition each spring. I hated doing that job.
Let’s talk about why the moss, mold and mildew grow in the first place. Many years ago I couldn’t understand how it could grow on solid rock, precast concrete or brick, but now it’s crystal clear to me.
Moss, mold and mildew need food to survive, just like you and I. The food sources are assorted, just as humans’ diets are. Dust, ultra-fine sugar aerosols from trees and bushes, tree sap, minerals, organic debris, etc., are all food sources for the unsightly things growing on your patio.
You can do a test that produces dramatic results by just pouring out a small amount of carbonated soda that contains sugar or high-fructose corn syrup on your patio. You might have mildew growing on the spill as soon as 48 hours later if you do it in a shaded area of your patio.
Water is the only other missing ingredient needed to fuel the moss, mold and mildew since their spores are constantly falling down on your patio. If you could keep your patio completely dry, you’d not have any growth. But even morning dew is enough to sustain the green and black organisms. They’re tenacious and know how to make a little water go a long way.
Let’s discuss power washing. There’s a raging debate in the home improvement community about whether power washing can be destructive to concrete, brick, precast pavers and wood. The unequivocal answer is yes — it’s destructive.
The rate of destructive force is directly proportional to the pounds-per-square-inch (psi) power the machine delivers, the angle of the spray-wand tip and the distance the tip is from the surface being cleaned. You just have to look at the Grand Canyon to understand that water flowing over rock can do damage.
Water directed at a surface with 1,500 psi or more can do immense damage on softer surfaces, and it does cumulative damage to harder surfaces with each successive washing.
In your case, power washing will rapidly remove the colored cement paste that covers the small sand and gravel particles in your precast pavers. If you saved a paver in your garage that the installer left behind, one that has never been washed or exposed to the elements, you’d notice that it’s got a uniform color over the entire surface.
After one or more washings, you’ll start to notice the individual colors of the different grains of sand and bits of gravel that was used to make the pavers. The colored cement will still be there between the individual particles of sand and gravel.
The good news is you can prevent the growth of patio moss, mildew and mold. All you have to do is borrow technology developed hundreds of years ago by mariners. Clipper ships and warships that depended on speed to make money and win wars had copper plates on their hulls so barnacles and other marine life would not grow on the wood below the water line.
Copper is a natural biocide. It’s pure, it’s pretty much harmless to mammals, and it’s found in multivitamins that you might take to stay healthy. Copper in our bodies helps us to retain iron, and it aids in producing the energy we need to get through the day.
You can’t cover your patio with copper sheets, but you can spray on a liquid solution of copper that will soak into the top surface of the concrete pavers. This copper will stop the growth of the pesky green and black organisms in their tracks.
The easiest way to apply the copper is to buy copper sulfate crystals. This is available online, and the blue crystals dissolve readily in warm or hot tap water. I’d mix 1.75 pounds of copper sulfate in each gallon of water. My guess is you’ll discover that two or three gallons of water is plenty to treat the average-size patio.
I’d apply the solution when the patio is dry as a bone. You want the solution to soak into the surface. Concrete is absorbent unless it has a shiny steel-troweled finish. Most exterior concrete is rough, so the solution will soak in. Apply just enough so the pavers get nice and wet but not so much that the solution runs off into surrounding vegetation. You don’t want to poison expensive landscaping nearby.
You’re going to have to periodically reapply the copper sulfate solution, because normal rain water will leach the copper back out of the pavers. I can’t tell you how often because it’s a function of the amount of rainfall where you live. But I do know it’s far easier to apply this solution in minutes rather that bend over for hours and hours using a power washer!
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