Copper solution can prevent patio mildew, mold, moss
Q: My wife and I have an outdoor patio built with colored, precastconcrete 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. I have to power wash at least once a year. Is there a way to prevent the growth in the first place?
— Loren P., Okatie, South Carolina
A: I used to have the same problem on two massive brick paver patios in the back of the last house I lived in. I hated the hours and hours of work it took to clean the patio each spring.
Let's discuss power washing. There's a raging debate in the home-improvement community about whether or not power washing can be destructive to concrete, brick, precast pavers and wood. The unequivocal answer is yes — it's destructive. You just have to look at the Grand Canyon to understand that water flowing over rock will do damage.
The rate of destructive force is directly proportional to the pounds-per-squareinch (psi) power the machine delivers, the angle of the spray-wand tip and the distance the tip is from the surface being cleaned. Water directed at a surface with 1,500 psi or more can do immense damage to 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.
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.
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.
You can't cover your patio with copper sheets, but you can spray on a liquid solution of copper that will soak into the surface of the 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 2 or 3 gallons of water is plenty to treat an average-sized patio.
I'd apply the solution when the patio is dry as a bone. 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 rainwater will leach the copper back out of the pavers. I can't tell you how often because it's a function of how much rainfall your area gets. But I do know it's far easier to apply this solution than it is to bend over for hours and hours using a power washer.