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

Tim Carter, Tribune Content Agency on

Not a week goes by that I don’t get a question at the Ask Tim page on AsktheBuilder.com about cleaning this or that household surface. As it turns out, I happen to know lots about cleaning things.

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.

The biggest mistake I see homeowners and pros make with power washers is aiming the wand up. In other words, they stand on the ground or a ladder and try to clean surfaces above their head.


I’ve got news for you. The way we build homes is we count on water coming down from the sky, not blowing up from below! Lap siding, flashings and so forth are all designed to shed water that is flowing toward the center of the earth, not shooting to outer space, for goodness sake.

If you clean vinyl siding with a pressure washer, you might be forcing gallons of water behind it when you get next to a vertical seam in the siding, at a corner post, or next to a window or door. If you aim the water stream the wrong way toward the overlap or the trim pieces, the water can easily be blown behind the siding.

I’ve had fantastic success cleaning algae from just about every outdoor surface using liquid dish soap and my favorite exterior cleaning brush. This brush is one that’s made to clean RVs. It’s got lots of bristles that are about 2.5 inches long and they’re soft. They won’t scratch automotive paint, so it’s safe to use it on your house siding, windows and every other surface. I would use a normal scrub brush with aggressive bristles to clean algae from any masonry surface either vertical or horizontal.

Whatever you do, do NOT use chlorine bleach in your cleaning solution. Do not use any product that contains chlorine bleach. You can identify this by looking for its chemical name on the label: sodium hypochlorite. Chlorine bleach is toxic to all of the plants, trees and flowers around your home.


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