Ask the Builder: How to clean algae off siding, roofs, patios

I attribute much of this to my formative years working at Skyline Chili in Cincinnati. I worked there after school during high school and most of college. I became an expert string mopper. You wouldn’t think there’s an art to moping a floor, but trust me, there is. I found out the best way to get grease off surfaces. I also washed tens of thousands of dishes, polished chrome, and discovered the magic way to get windows crystal clear.

This time of year, though, I get inundated with requests about how to clean algae from every conceivable outdoor surface. It might be green or black algae. It might be simple mildew.

Let’s start with why algae thrives. My college major is geology, not biology, but I can tell you that algae needs food and water. It’s not much different than you and me. Knowing this, you can prevent algae growth by keeping the affected surfaces clean. Just as you probably shower each day to prevent body odor, it’s wise to wash outdoor surfaces on a regular basis. Once they start to look a little dingy or you see a very light haze of algae starting to take hold, it’s time to take action.

Here’s what not to do, in my opinion: Do NOT use a pressure washer. These machines generate such high pressure that they can peel the paint off wood, they can actually erode wood fibers, and they can squirt water behind the exterior surfaces of your home where water should not be.