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

Not a week goes by that I don’t get a question on the Ask Tim page on about cleaning this or that household surface. It turns out I happen to know a lot about cleaning. I attribute much of this to my formative years at Skyline Chili in Cincinnati.

I worked there after school during high school and most of college. I became an expert string grumbler. You wouldn’t think there’s an art to grumbling a floor, but it is. I discovered the best way to get grease off surfaces. I also washed tens of thousands of dishes, polished chrome and discovered the magical way to get windows crystal clear.

At this time of year, however, I am inundated with requests about removing algae from every exterior surface imaginable. It can be green or black algae. It could be simple mildew.

Let’s start with why algae thrive. My major is geology, not biology, but I can tell you that algae need food and water. It’s not much different from humans. Knowing this can help prevent algae growth by keeping affected surfaces clean.

Just as you probably shower every day to prevent body odor, it is wise to wash outdoor surfaces regularly. Once they start to look a little dingy or you see a very light haze of algae, it’s time to take action.

Here’s what you shouldn’t do in my opinion: Do not use a pressure washer. These machines generate such high pressures that they can peel the paint off wood, they can even corrode wood fibers, and they can spray water behind the exterior surfaces of your home where there shouldn’t be water.

The biggest mistake I see homeowners and professionals make with pressure washers is pointing the wand upward. In other words, they stand on the ground or a ladder, trying to clean surfaces above their heads. I have news for you.

The way we build houses is that we rely on water coming from the sky, not blowing up from below. Overlapping siding, flashing and so on are all designed to divert water that flows toward the center of the Earth and doesn’t shoot out into space, for God’s sake.

Cleaning vinyl siding with a pressure washer may force 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 direct the water flow in the wrong direction on the overlap or trim, the water can easily be blown out behind the siding. I’ve had fantastic success removing algae from just about any exterior surface with liquid dish soap and my favorite exterior cleaning brush.

This brush is made to clean motorhomes. It has many bristles about 2.5 inches long and they are soft. They do not scratch car paint, so it is safe to use on your siding, windows and any other surface. I would use a regular scrubbing brush with aggressive bristles to remove algae from any masonry surface, both vertical and horizontal.

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

I saw a neighbor slowly poison a beautiful tall maple tree years ago. Every spring she poured gallons of chlorine bleach on her patio to get rid of the algae. The tree provided shade on the terrace and all the roots were under the stones of the terrace. I warned her about this, but she treated me like an idiot. Oh, well, the tree eventually died, and she had it cut down.

I would also warn you against buying miracle products that say you just spray them on and walk away. I did a survey in my Ask the Builder newsletter about these products, and the overwhelming response was that they are worthless.

Common sense tells you that these miracle products are too good to be true. Think about it. The way you get surfaces clean is that you have to shake the surface mechanically to break the bonds of dirt and algae.

Example: Suppose you are dirty and sweaty. If you just jump in the shower and let the warm water run over you while standing dead still, you’ll come out of the shower dirty. Imagine what the towel would look like.

You get clean by using your hands with soap and rubbing your skin, then you rinse off the dirt. The same goes for cleaning clothes. Imagine taking a load of dirty clothes and putting them in a giant stainless steel tub. Add soap and water and let them soak for three days. Empty the water. Look at the clothes. They will still be dirty. Clothes get clean in your washing machine because the machine sets them in motion. The clothes rub against each other and this removes the dirt with the help of the soap.

Cleaning algae is easy. Just use common sense, soap, water and the right brush.

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