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Should I Be Worried about Black Streaks on My Roof?

Are you feeling nervous about the black streaks you recently noticed on your roofing system? If so, don’t stress — these streaks are common (even if they aren’t pretty to look at) and don’t cause any immediate threat to your roofing system

However, if left unchecked, these black streaks can result in premature damage to your roofing system, so addressing them as soon as possible is crucial. In this post, we will explain what these black streaks are, what causes them, and how to get rid of them — for good!

What Causes Black Streaks on My Roof?

Though you may think those unsightly black streaks on your roof are caused by mold, mildew, or dirt, they are actually caused by a certain type of algae called gloeocapsa magma.

This type of algae is usually blue or green in color, but when set against your roofing system or other surfaces, it can appear black. This is because this type of algae can form a darker coating to protect itself from harmful ultraviolet rays.

By the time you have identified dark streaks on your roofing system, the algae has usually already been there for a few months. It starts with unnoticeable black spots and then grows into more prominent black streaks that will catch your eye.  

Why Do My Asphalt Shingles Collect Algae?

Wind & Animals

Algae spores are spread through the wind. They can also circulate if an animal (like a squirrel) scurries across your roof after coming in contact with algae. 

Since these spores are airborne, it isn’t uncommon for them to spread from one rooftop to another in your neighborhood. Chances are, if you have an algae problem — at least one of your neighbors does too.


Asphalt roofing shingles are the most common victims of algae growth. These shingles were once exclusively made from asphalt, but modern manufacturers have added limestone fillers to reduce the manufacturing cost.

Unfortunately, algae feasts on limestone, so when it lands on your roof, it begins to eat away at your roofing shingles and makes them brittle, weak, and vulnerable. 


Your home is more prone to develop black streaks from algae growth if you live in certain regions of the United States.

Algae growth is most common in hot, humid environments, like the Southeast, and can be severe in wet climates, like the Pacific Northwest. However, because of the limestone in asphalt roofing shingles, algae growth is even present in cold climates like the Midwest and Northeast.

Damp Surfaces

Algae thrives in damp, shady, humid environments. If your home’s roof is shaded for most of the day, or if your gutters or downspouts are constantly clogged with water, it’s more likely that your home will suffer from algae growth.

Algae retains moisture, so when it sits on your roof for extended periods, it can cause many other problems — like mold and mildew growth, rotting, and more.  

How Can I Remove Black Streaks from My Roof?

If you are ready to remove the black streaks on your roofing system, it’s best to call a professional roofing contractor. 

Climbing on your roof is a safety risk, especially if you don’t have the right equipment or experience. Plus, if you don’t know which tools and cleaning solutions to use, you may damage your roofing shingles and void your warranty. 

Your contractor will gently clean your roofing shingles by applying a mixture of water, bleach, and trisodium phosphate (TSP) to the black streaks. Power washing is not advised since the pressure can damage your shingles.

How Can I Prevent Algae from Growing on My Roof?

It is important to note that even after algae removal, these unsightly black streaks will return if you do not take any of the following preventive measures to keep your roof algae-free.

Zinc or Copper Strips

Copper and sheet metal are toxic to algae, so adding a strip of metal along each side of your roof (just below the ridge) can prevent streaking and algae growth.

You must leave at least two to four inches of the metal exposed to the elements for this method to work correctly. They can be attached to your roof with nails, screws, or washers by your roofing contractor after the algae removal.

Roof Maintenance

Keeping up with general roof maintenance can lower your chances of developing black streaks on your asphalt roofing shingles.

Algae spores thrive in moist, dark, humid environments. So, when you remove damp leaves and debris from your roofing system, keep your gutters and downspouts dry, and trim the trees around your roof, you will reduce the likelihood that algae spores make your roof their home!

Roof Replacement

If your roof is already old and needs to be replaced, or if your algae damage is severe, you may opt for an entire roof replacement instead of algae removal.

Ask your contractor about the options available to you for algae-resistant roofing materials. For example, some asphalt shingles are made with copper granules to stop the spread of algae. Or you may opt for another roofing material, like metal, to say goodbye to algae growths forever.

Inspect Your Home for Algae Damage with Remodeling Concepts

Whether you notice black streaks on your roofing system or identify other costly problems, like missing shingles, mold growth, or water damage — it’s wise to hire a professional contractor to complete an inspection and create a plan of action for your roofing system.

Remodeling Concepts offers thorough roof inspections for homeowners in Bucks County, Pennsylvania. Learn more about the roof inspection services we offer, and when you’re ready, schedule your inspection for peace of mind about the state of your roofing system.