What Are The Dark Streaks On My Roof? All About Algae

Residential | October 4, 2021  | By: John Esh

stained roof with red brick chimney

Dark streaks on your roof are unattractive and cause your shingles to look aged and stained. These dark areas are usually not caused by dirt and grime – they are actually algae growth. 

This algae is called “gloeocapsa magma,” also known as blue-green algae. It appears black because it covers itself in a UV ray protective layer. While it often begins as spots on your roof, over the following months of its initial growth is when streaks start to form.

How does algae grow on my roof?

Algae is an airborne plant, so it spreads relatively easily. It thrives in humid conditions because it needs heat, moisture, and nutrients to grow. Most times, you’ll see this algae forming on asphalt shingle roofs. This is because the spores of the blue-green algae feed off of the minerals found in the components of asphalt shingles.

Is algae harmful for my roof?

The biggest harm dark streaks cause to your roof is resale value. The streaks make the roof look unappealing and could worry potential buyers that they’ll have to replace it soon.

Other than being aesthetically displeasing, the algae doesn’t cause any immediate harm. Over time, the algae could prematurely age your shingles by breaking down the shingles as it grows and attaches itself to the roofing shingles.


gray roof stained with algae

Do I need to clean the algae off my roof?

While it’s not urgent to remove the algae, if you don’t clean it off then it will continue to grow and potentially produce darker streaks. 

Algae stains come off pretty easily. If you want to clean the algae off, do not power wash your roof! The high pressure can damage shingles.



The Asphalt Roofing Manufacturers Association recommends applying a 50/50 mix of chlorine bleach and water to a stained roof, and then rinsing it off after twenty minutes.

As always, use proper safety equipment if you’re working on your own roof, or contact a professional.This method won’t prevent algae from coming back, but will usually keep your roof clean for about a year.

We recommend against the use of pressure washers. Pressurized water may damage shingles, causing more harm than good.


Can I prevent algae growth?

Newer shingle manufacturers have created shingles with upgraded technology to deter the algae from developing. The shingles have metals incorporated into the granules on the surface which provides a hostile environment for algae.

However, these granules degrade over time, so GAF takes it a step further by concentrating the copper in a layer of the shingles with StainGuard Time Release Technology.These shingles come with a 25 year warranty against blue-green algae growth, which is the best warranty currently available. 

brick house with tress and bushes in the front yardYou’ll notice that areas surrounding roof penetrations such as pipes and chimneys, don’t have the algae growth. That is because rainwater runs off and around these objects. The water contains a small amount of metal making the area inhabitable for algae. 

To prevent potential algae growth on the remainder of your roof, you can consider copper or zinc strips under the shingles at the ridge of the roof. The elements of copper and zinc fight against the growth of not only blue-green algae, but also against moss and other types of fungus on the roof. As rain hits these strips, the copper or zinc slowly oxidizes and washes down over the roof surface, cleaning the shingles as it goes. However, these products really only clean the first 5-10 feet. So to keep a roof relatively clean, you’d need to install multiple lines down through the roof. Furthermore, zinc and copper strips lose efficacy as they age.


Talk to Joyland Roofing about your roof algae

We want your home or business’s roof looking its best! It can be challenging to find a contractor that is focused on only the customers needs. At Joyland Roofing, we want to make sure your process is straightforward and educational.

To learn more about how to treat or prevent algae on your roof, or for assistance with any other roof-related question, please contact us today!

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Editor’s Note: This post was originally published in February 2018 and has been revamped and updated for accuracy and comprehensiveness in October 2021.

    Pennsylvania Home Improvement Contractor License (HIC) # PA124258