The Reason You Have Those Ugly Black Streaks on Your Roof

The Reason You Have Those Ugly Black Streaks on Your Roof

What are Those Ugly Black Streaks on My Roof?

Although these dark spots and ugly black streaks on your roof may look like dirt, mildew, soot or mold, what they really are is algae. The most common type is known as gloeocapsa magma—also known as blue green algae.

This algae growth usually begins as small spots and then eventually becomes visible streaks. By the time this algae is noticed by the homeowner, it has usually been on the roof at least two or more months.

how to get rid of black streaks on your roofWhy does this algae appear to be black? Gloeocapsa has the ability to form a protective darkly pigmented outer coating that shields the algae from damaging UV rays. This is what makes those unattractive black streaks on your roof!

Although algae can be found on all types of roofing, it tends to be most common on asphalt shingles.

Why do Shingles Get Algae?

Algae spores are airborne. They can be carried by wind, animals and even on your clothing. Because these spores are airborne, they can quickly spread from rooftop to rooftop in a neighborhood. This is also true for townhomes, condos and apartment complexes.

Why does roof algae seem to be more common today that it was 20 years ago? Traditionally, asphalt shingles got their name from being made up of mainly asphalt—an oil based product—and felt material.

To save money, most shingle manufacturers have switched to shingles containing asphalt but mostly made up of fillers such as fiberglass and crushed limestone, which encourages the growth of algae. These algae use the crushed limestone filler as a main food source.

Weather is a Factor in Algae Growth on Shingles

Coastal, warm and humid climates tend to create the friendliest environments for algae growth, although the use of cheaper shingles has made algae growth a problem in almost every part of the United States. Even parts of the country that have cold winter seasons—such as the northeast or Midwest—still have problems with roof streaks caused by algae because they also experience hot and humid summers.

us map of roof algae

Often homes in these algae prone areas will see roof algae on the north side of the house. Why? Because when the sun goes through the sky during the day, the northern part of your house’s roof is shaded for most of the day. This shade allows moisture to accumulate and provides the ideal environment for algae growth.

Gutters draining directly onto the roof and overhanging trees are also contributing factors for roof algae. A shady or damp surface also prevents the sun from drying the roof quickly, encouraging stuff to grow.

Is This Roof Algae Harmful or Dangerous?

Roof algae is not harmful or dangerous in itself, but after a period of time, the algae can cause the protective UV granules that cover roofing shingles to begin to come off, eventually drying them out.

Fungus—which also travels through the air—can combine with algae already present on your roof and form into lichen. This lichen feeds off both the algae and the shingle filler. Unlike algae, lichen can develop roots, which makes it harder to remove than algae alone.

How do You Get Rid of Black Streaks on Your Roof?

Roof algae can be easily removed by a simple roof cleaning.

The important thing to know is that while an occasional cleaning shouldn’t harm your roof, doing it yourself may void any roof warranties or homeowner insurance policies you may have.

cleaning black streaks on your roofAn internet search on how to clean black streaks on your roof will turn up several different homemade formulas as well as commercial brands for getting rid of roof algae.

Most formulas contain chlorine bleach and/or trisodium phosphate (TSP) and water. While lye (sodium hydroxide) is more effective than bleach, it is corrosive and should only be applied using full protective gear.

“Oxygen” bleach, commonly known as the brand name Oxyclean, can be effective and is less environmentally damaging, but will not work as well as formulas with chlorine bleach or TSP.

Remember that even chemicals that are safe for humans can still kill or harm landscaping and vegetation!

Cleaning the Roof Yourself

If you decide to clean the roof yourself, never, ever use a power washer, even on its lowest setting. Not only is using a power washer pretty much guaranteed to void any roof warranties, it can damage shingles by causing the protective coating—or even the shingles themselves—to come off.

Cleaning your roof will require some preparation. Remove any lawn furniture and cover vegetation and landscaping. Overspray is inevitable, and even if the product you use isn’t considered toxic, the runoff from your roof can cause plenty of mess on the ground as well.

Check the weather forecast and plan on doing your cleaning on a cool or overcast day with little or no wind to reduce the chance of the spray going other places besides your roof. A cloudy or overcast day prevents the cleaner from evaporating too quickly.

If you know who manufactured your roofing shingles, check their website for specific cleaning recommendations or products. When using a commercially available roof-cleaning product make sure to follow the manufacturer’s instructions exactly.

Home improvement expert Bob Villa recommends this formula:

  • 1 cup of trisodium phosphate (available at most hardware stores);
  • 1 gallon of bleach
  • 5 gallons of water

Be sure to wear eye protection, slip resistant shoes, protective clothing and rubber gloves. It’s good to have a friend with you for safety’s sake and to easily give you any equipment you might need.

Pour all of these ingredients into a pump-type garden sprayer (the type often used for applying liquid fertilizers) and spray them on the roof using long, even strokes.

It’s best to start at the top of the roof and spray down since spraying water up at your shingles can loosen them. Each strip of shingles that you clean should only be about 4 foot wide. Don’t stand on the wet part of the roof while you are cleaning, because it will be very slippery. You might want to wear a fall-protection harness.

After wetting down the roof with the solution, allow it to remain there for 15 to 20 minutes before rinsing. If you are on the roof at this time, It’s best not to walk around much since the combination of the roof algae and the solution can cause the shingles to be slick.

Once the mixture has been absorbed, rinse off the solution using a regular garden house. Make sure you go slowly and use even strokes. If not, you may end up with random dirty patches that you because you skipped over parts of your roof too quickly.

Hiring a Professional to Clean Your Roof

professional cleaning black streaks on your roof If you don’t want to climb around on a slippery roof or your shingle manufacturer will void your warranty if you try to clean your roof yourself, you should contact a professional roof cleaning service to get rid of those ugly black algae stains on your roof.

Here at Roof Revivers, we only use algae treatment products that are biodegradable and non-corrosive. We never power wash, but apply our eco-friendly treatment using a gentle “garden sprayer” that doesn’t require any rinsing.

The result is a beautifully restored roof with no potential damage to people, pets or property!

Preventing Black Streaks on Your Roof

Can you permanently get rid of algae? The short answer is “no.” Roof algae is similar to having a cold. Like many cold viruses, algae spores are airborne and unavoidable. You can treat the symptoms, but eventually you will have another algae problem, especially if those around you have it on their roofs as well.

But don’t despair! Like washing your hands frequently and eating healthy to prevent catching a cold, cleaning and maintaining your roof can help prevent black streaks on your roof from returning. Here are some ways to keep roof algae at bay:

Maintain Your Roof

Trim tree branches to allow more sunlight to reach the roof and minimize the accumulation of debris. Debris that does begin to accumulate on the roof should be cleared regularly by hand as part of a regular maintenance program. This will provide an environment less inviting the growth of roof algae.

You should also keep gutters and downspouts clean and free of clogs to promote proper water drainage. Never allow gutters or downspouts from an upper roof to drain directly on a lower roof; extend the downspout from the upper roof into the lower gutter.

Gutters draining onto a lower roof not only promote algae growth but can cause water to pool on your roof, which can damage shingles as well as the underlying wood structure.

Install a Copper or Zinc Strip Along the Side of the Roof

Another common solution is to install copper or zinc strips under the row of shingles closest to the peak of the roof, making sure that you leave 2 to 4 inches of the lower edge of this metal strip exposed to the weather.

This means that whenever it rains, the water—which will contain algae killing molecules from the metal—will wash down the roof and kill any algae that may be lurking there.

While copper is better at killing algae, galvanized sheet metal is a less expensive solution. Both copper and galvanized metal are available in different widths and thickness. You should be able to find them at your local home improvement store or online. Many manufacturers make zinc strips specifically made to eliminate roof algae

Although installing copper or zinc strips will help, it’s important to remember that this will not offer you 100% protection. Algae spores still feed off humidity and moisture whether it’s raining or not, but installing these strips will give you an extra layer of protection.

Get New Shingles

Roofing manufacturers have tackled the problem of roof algae by creating shingles containing copper, which is deadly to algae. Replacing the shingles on your roof just because you have black stains is not necessary however—and you should be suspicious of any roofer that suggests roof shingle replacement as the only solution to your algae problem.

If your roof is older or in bad shape and needs replaced anyway, you may want to invest in these copper-infused shingles, but make sure that you get those that have the best and longest warranty. Consumers that have had these shingles installed on their homes have had mixed results. The amount of copper in these shingles is not regulated and varies from manufacturer to manufacturer.


Roof algae can be unsightly, but it’s easy to get rid of, and there are several methods you can do to keep those black streaks on your roof from coming back.

Have questions about those ugly black streaks on your roof? Give us a call today at 614-360-9703 to schedule a free no-hassle roof inspection!