Black Streaks on Roof: Removal and Prevention

Black Streaks on Roof: Removal and Prevention

black streaks on roof

Have you ever noticed black streaks on the roof of your home? If you take a quick walk around most neighborhoods, you’ll see this is not an isolated problem. Black streaks can make the youngest of roofs look run down and neglected. So, what are these black streaks on the roof? Dirt, moss, mold? Keep reading to learn the origin of these streaks and what can be done to remove and prevent them from staying on your beautiful roof.

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What are the Black Streaks on Roofs?

Roof Algae

While these sinister streaks may seem black, they are actually composed of blue-green algae known as Gloeocapsa Magma. The algae use a layer of black to protect themselves from the harmful UV rays of the sun.

These blue-green algae thrive with strong light exposure and humidity. New England is a moderate growth area, allowing these black streaks to live a comfortable life on your roof.


While it’s hard to pinpoint how the algae arrive on your roof, it could be due to your neighbors. If you have black streaks on your roof then your neighbors probably have black streaks on their roof as well. The algae particles travel through the air and settle on roofs. Every refreshing breeze brings the algae from house to house.

Are Black Streaks Harmful?

black streaks on roof caused by algae

Yes and no.

Black streaks do nothing to damage the structural integrity of your roof and home as shown by the Asphalt Roofing Manufacturers Association. But, that doesn’t mean they come without a price. Black streaks can damage the curb appeal of your home with their unsightly stain upon your roof.

Impact on Curb Appeal

A roof that is still in its infancy can look twenty years old with the arrival of black streaks. Think about it: If you saw a home with black streaks on the roof, would you buy it? These streaks make a home seem neglected and run down which can indicate deeper, unseen problems. Homebuyers don’t want a house which could be a lifelong hassle.

Black Streak Removal

It is possible to remove black streaks, but be wary of the consequences. These removal techniques can do more harm than good.

Roof Spray

Pressure Washing

DO NOT PRESSURE WASH YOUR ROOF! There are hundreds of companies who offer roof cleaning. These companies will come to your home and pressure wash the streaks off your roof with reckless abandon.

 While pressure-washing does remove the algae, it does so at the expense of your shingles. When a pressure-washer hits shingles, it removes granules designed to protect your roof. Once these granules leave, your roof will need to be replaced shortly. Try a gentler method of removal.

Roof Spray

A common mixture comprised of one-part bleach and one-part water typically helps control streaks. Spray the solution onto the area, leave it alone for twenty minutes, then gently rinse the area with water.

This solution will most likely lighten the streaks instead of completely removing them, but once their distracting look disappears you can think of them more as harmless housemates who just want some sun.


The Cleaning Cycle

As stated before, these streaks travel through the air across roofs. So, unless you and your neighbors sync up your cleaning routines, these streaks will return with the breeze, so prepare to regularly clean your roof to keep these streaks in check.

Roof Cleaning Safety

While using the solution is the best option for cleaning your roof, it’s not exactly accident free. In order to properly follow the instructions, you’ll have to deal with ladders, heights, and bleach. If these factors are not handled with care, harm can come to both you and your home.

Make sure you use protective eyewear and exercise extreme caution during the entire process. In fact, it’s better to prevent black streaks from appearing on your roof in the first place. The danger you place yourself in during the removal process may not be worth the hassle.

Black Streak Prevention

You can easily prevent black streaks when you choose the right replacement roof. Certain roofs will reduce your likelihood of coming into contact with these streaks.

Repel the Streaks

If these streaks are on your roof or a roof near you, you’ll notice these black streaks steer clear of certain areas of your roof such as the pipes and chimney flashing.

This repellence comes from the relationship between blue-green algae and copper. According to the Canadian Asphalt Manufacturers’ Association, the galvanized properties of copper inhibit algae growth. Shingles with a concentration of copper granules over 10% will keep black streaks from forming on any roof.

AWS Roofing


At Advanced Window Systems LLC, we offer shingles with a 13.25% concentration of copper. Our replacement roofs and shingles will make the fear of black streaks on your roof seem like a thing of the past. Peace of mind is very important, so stop stressing over your roof. All AWS installations come with our lifetime warranty which allows us to always take care of our customers.

In Conclusion

Every homeowner wants to preserve the visual dignity of their home by making sure their curb appeal only increases over time. These black streaks on roofs are actually blue-green algae which aesthetically damage your house, so your best bet is getting a new roof.

Be careful when dealing with companies who promise a cleaning. These actions will most likely leave you with a big bill and a ruined roof.

Have any other questions about copper granules and black streak prevention?  Advanced Window Systems, LLC  is happy to help. Contact us on  Facebook and Twitter too!

Sources: 1 and 2.

Written by Alex Buonocore 07/30/20


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