Q&A / 

How to Remove Algae

DEAR TIM: My house must be a magnet for algae. I have green algae on the shaded parts of my deck, it's on my patio and on my vinyl siding. Furthermore, I believe there's black mildew on my siding. Then there's the roof. I have black streaks of algae up there. How can I safely remove the algae from all these surfaces without harming me, my pets and my plants? I've heard I should use bleach, but something tells me that this is not the right approach. What would you do? Caroline B., Jackson, MS

DEAR CAROLINE: I have some algae issues at my own home. I have two decks, one on top of another. The lower deck has lots of shade which is a perfect place for algae to grow. I also have algae coating stone steps on my property. I work at keeping them clean because when wet, the algae-covered steps are like walking on wet ice. They are so slippery! You, I, and millions of others suffer from algae around our homes.

Oxygen bleach was used to remove the algae from the decking boards on the right. Photo Credit: Tim Carter

The good news is that it's not too hard to clean algae. The bad news is that there's no silver bullet to inhibit its growth on all surfaces.

The first thing you might find interesting is there's more than one type of bleach. For many years growing up, I thought bleach was bleach. My mom used to buy those white plastic bottles of smelly chlorine bleach for years to use in our laundry. She would save the bottles, rinse them out and then fill them with water in case the water main broke outside our house. It was a big joke around our home, but my mom always was prepared!

Much to my surprise I discovered, about seventeen years ago, there's at least one other bleach - oxygen bleach. It does many of the same things chlorine bleach does, but it's not toxic. Oxygen bleach is the preferred bleach to use on the exterior, and interior, of your home because it will not harm you, your pets or your plants.

Chlorine bleach, on the other hand, is highly toxic to plants, humans and animals. It's so powerful that it will remove the color from wood decking and it can discolor painted exterior surfaces much like it takes out the color from fabrics.

Oxygen bleach will not take the color out of your wood decking, your vinyl siding, your painted surfaces, or your roof. Typically, oxygen bleach is a powder you mix with water. You can easily find oxygen bleach online or at many stores. Once mixed with water, all you get is more water, oxygen ions that do the cleaning and some harmless organic soda ash. Not all oxygen bleaches are the same. Some are completely organic, while others contain fragrances, dyes, color crystals, and excessive fillers.

This past weekend, I mixed up some oxygen bleach to clean algae from my lower deck and from the deck railing. I simply dissolved the powder in warm water, poured it into a simple garden hand-pump sprayer and squirted it on the algae-coated surfaces. I allowed it to soak for about ten minutes, then used a scrub brush to remove the green algae. After rinsing with a garden hose, the surfaces looked brand new!

You can clean roofs with oxygen bleach, but it can be challenging. The biggest problem is that it's dangerous working up on a roof. The algae-covered roof, once wet with the oxygen bleach solution, can be very slippery. You need to make sure you do not fall from the roof.

It's imperative the roof surface, or any surface you're cleaning, does not dry out before you get to scrub it. You must keep the surface wet with the solution at all times. This is hard to do on a roof on a sunny day. It's best to clean roofs on overcast days when air and roof temperatures are cool.

You need to scrub the surface after the oxygen bleach solution soaks. Don't ever think that you can just spray on any magical cleaner and get perfectly clean surfaces. Marketers that make that claim are yanking on the heartstrings of your inner self that wants the easy way out. Don't believe the spray-and-rinse claims for a second.

Once you have the surfaces clean, you can inhibit the growth of algae if you coat the surfaces with a biocide. This is not easy, it can be expensive and it's a constant maintenance headache.

Copper, however, is a natural biocide as is zinc to a degree. If you have an exposed copper or zinc strip about 3 inches wide along the top of your roof, each time it rains tiny metal atoms are deposited on the roof. Algae will not grow in the presence of copper or zinc.

It's hard to get copper to coat siding, decking or patios. You can see why you'll just have to suck it up and do periodic cleaning to keep your home looking great.

You can watch two informative video showing how to use oxygen bleach to clean a roof. Just type "removing algae stains video" into the www.AsktheBuilder.com search engine.

Column 935


9 Responses to How to Remove Algae

  1. My fountain was full of algae, I emptied the water. Now i have algae stains. If i do not remove the stains, will that algae come back to life?

  2. Hi Tim
    I have 2 questions:
    1. Can Stain a Solver be used on painted stucco house siding?
    2. On the inside of a wall around a deck - there is a railing on top - would it make sense to put a strip of copper to stop the algae from growing down the inside of the wall?

Leave a Reply

Your email address will not be published. Required fields are marked *