Remove Algae Stains Off Roof

By Robert Robillard on Roofing

Cleaning Algae Stains Off Roofs:Remove Algae Stains Off Roof

I hate seeing black or green stains on a roof.  There are many reasons that cause mold, lichen and algae to grow on a roof but that doesn’t mean you have to tolerate it.   There are ways to eliminate these ugly stains and this article will explain how you can remove algae stains off roof.

People often ask me if these stains are a sign of roof wear.  The answer to that is “no” the black staining is typically caused by algae and it will not damage the shingles, and it won’t make your roof leak.   These stains usually show up on areas on the North side of a the roof  or in areas that are blocked by trees or do not get much sunlight.

The black mold-like stains and streaks that appear on roofs is usually a blue-green algae (Gloeocapsa magma)  These stains mostly appear on light colored roofs but do grow on all roofs.Remove Algae Stains Off Roof

Roof Cleaner

There are all types of consumer products that you can buy to remove algae stains off roof  but the best solution I’ve found is to use a mixture of two parts water to one part bleach and apply it with a garden sprayer.

Applying Cleaner To Remove Algae Stains Off Roof

This is the tricky part.  You need to be able to access a ladder or the your roof and do it safely.  I suggest that you wear a respirator and protect your eyes from bleach over-spray.  Covering plantings below or at least rinsing them before and after with water will help prevent the bleach from damaging the vegetation.Remove Algae Stains Off Roof

Using the garden sprayer apply you’re mixture to the affected areas of your roof with your two-part solution, let the solution soak for about ten minutes, and then spray the cleaning solution off your roof with a garden hose.

When spraying the roof, spray in the direction that water drains. [down] Do not spray up and under the shingles as this can cause a roof leak or damage the shingles.  Ideally a pressure washer with a low pressure fan tip to do the rinsing, produces better results than just a garden hose, but is also much more likely to damage the shingles.

Avoid the Sun

For best results do not attempt to clean your roof when the sun is beating down on the roof, the roof temperature can get too hot and evaporate your solution.  Wait for an over cast day or early or later in the day to do this.

Stubborn AreasRemove Algae Stains Off Roof

On stubborn areas you may need to scrub the shingles with a long-handled push broom. It may take more than one application to completely remove algae stains off roof.  when satisfied  rinse the roof with clear water from a garden hose. In some cases, you may need to apply a second spraying of oxygen bleach to clean all of the roof algae away.

Preventing Stains From Re-Appearing:

Re-roofing with darker shingles will make a difference and so will using shingles laced with copper granules. Copper, lead and zinc kill algae.

I’ve had GREAT success installing strips of lead, zinc or copper under the row of shingling closest to the roof peak, leaving an inch or two of the metal’s  lower edge exposed to the weather.  Whenever it rains, some of the metal molecules wash down the roof and kill any algae.  It’s like an “algae -roof -wash” that automatically works to remove algae stains off roof every time it rains.

You can see this metal “algae killing” effect  working on roofs in your neighborhood. Look for chimneys with copper flashing; the areas directly below the flashing will be free of any algae stains.


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About the author

Robert Robillard

Carpenter / Remodeler / Editor

Rob Robillard is “The Concord Carpenter” Rob is a builder, general contractor, carpenter, woodworker, and editor of Concord Carpenter and ToolBoxBuzz As a General Contractor and carpenter, Rob owns and operates Concord Carpenter LLC. A full-service remodeling and construction company. Rob is a recognized leader in home building best practices and a source for how-to information for building professionals. On this website, Rob covers all aspects of home construction, building science, home improvement, woodworking, remodeling, and some of the best product and tool reviews. Rob is in charge of our Tool and Product Review series - Concord Carpenter Videos where we post all of our tool reviews and video tutorials. Rob approaches remodeling and building construction with a pragmatic and problem-solving approach. He enjoys using his knowledge and experience to help and educate building professionals as well as DIYers on best practices in the construction and remodeling industry. He's a strong advocate for "raising the bar" in the construction trades and promoting the trades to youth. #BeAMentor #Green2Great Craftsmanship, quality, and pride guide his journey on this channel The Concord Carpenter's motto: "Well done is better than well said!" : Read more about Rob

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