This time of year, I am inundated with questions about how to clean algae from every conceivable outdoor surface. Let’s start with why algae thrives on our homes in the first place.

Algae needs food and water to grow, which means you can prevent algae growth by keeping the affected surfaces clean. It’s wise to wash outdoor surfaces like siding, patios and roofs 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: use a pressure washer. These machines generate such high pressure that they can peel the paint from wood and erode wood fibers.

Homes are built with the expectation that water will be coming down from the sky, not blowing up from below. Lap siding, flashings and other features are designed to shed water that is flowing down, so standing on the ground or on a ladder and trying to spray surfaces above your head is going to cause problems. Water can get behind the siding and into places where water should not be.

If you try to clean vinyl siding with a pressure washer, you could force gallons of water behind it when your stream hits a vertical seam or corner post, or gets 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 success cleaning algae from just about every outdoor surface using liquid dish soap and an exterior cleaning brush. The brush I prefer is made to clean RVs, with soft, 2-1/2-inch bristles. It won’t scratch automotive paint, so it’s safe to use on house siding, windows and most other outdoor surfaces. To remove algae from masonry or other extra-rough surfaces, switch to a scrub brush with sturdier bristles.


Do not use any product that contains chlorine bleach in your cleaning solution. Check the label for its chemical name: sodium hypochlorite. Chlorine bleach is toxic to all of the plants, trees and flowers that are growing near your home.

I’d also caution against using “miracle products” that claim that you just spray them on and walk away. I once did a survey in my newsletter about these products, and the overwhelming response was that they do not live up to the hype.

Common sense tells you these miracle products are too good to be true. The way you get any surface clean is by agitating the surface to break the bonds of the dirt and algae. So my recommendation is to stick with soap, water, the right brush and a little common sense.

Tim Carter has worked as a home-improvement professional for more than 30 years. To submit a question or to learn more, visit