What Are Those Black Streaks on Your Roof?

Is the roof on your home looking a little worse for the wear? Learn tips for identifying when you need to replace your roof.

What Are Those Black Streaks on Your Roof?

10 March 2016
 Categories: , Articles


Are there black or green-black streaks running down your roof? No, it's not the color from your shingles running, and it's not soot from pollution, either. Those black streaks are caused by a specific type of algae-like bacteria called Gloeocapsa magma. Often referred to as "roofing algae," these organisms do more than make your roof look a bit ugly. They also lead to shingle damage, increasing your chances of needing repairs or a premature roof replacement. Read on to learn more about what causes roofing algae growth and how you can fight back.

What causes roofing algae to grow?

Roofing algae is more likely to grow on some types of shingles than on others. It is particularly common on lower-end, inexpensive roofing shingles that are made using limestone as a filler material. They hold onto moisture for a long time, and since Gloeocapsa magma likes moisture, it thrives on their surfaces. Higher-end shingles made with less limestone can also grow algae, though, particularly in very moist climates. Roofing algae is quite common in areas where the weather is often humid.

Another factor that contributes to roofing algae growth is shade. Shade slows down the rate of evaporation, so a shaded roof stays moist for longer after a rainstorm, giving algae time to grow. Plus, roofing algae grows best with limited sunlight.

How can you get rid of roofing algae?

To get rid of the algae in the short term, visit your local home improvement store and purchase a few gallons of trisodium phosphate solution. These solutions are usually sold in the roofing supplies section and are sometimes labeled "roof cleaner" or "algae remover." You'll also want to purchase several gallons of plain bleach and rent a pressure washer.

Wait until a shady day to clean your roof. This way, the mixture won't evaporate as quickly and will have more time to kill the algae. Read the back of your trisodium phosphate solution bottle. It should tell you how to mix it with bleach and water in your pressure washer. Prepare your solution in your pressure washer chamber according to these instructions.

Wear goggles, gloves, and long sleeves to protect yourself. Once your solution is mixed, attach an extender wand to your pressure washer and climb up your ladder with the pressure washer in hand. Make sure you have a helper on site to hold the ladder steady. Aim for the peak of the roof and keep spraying until that area appears to be saturated. Slowly work your way down the roof, ensuring you saturate each section. Move your ladder as needed so that you are sure to soak the entire roof.

As you're spraying, you should see some of the streaks disappearing. Some of the algae won't be rinsed away immediately, but it should be rinsed away over the next few weeks when it rains. At this point, your roof should be free of streaks.

How can you keep roofing algae from coming back?

To keep algae from growing back, you'll need to take steps to keep your roof dryer and less shaded. If the roof is overhung by tree branches, trim them to reduce shade. If your gutters are damaged and this is keeping your roof from drying properly, have them repaired.

If you think your roofing algae is mainly attributable to a humid climate or shingles that simply harbor moisture for too long, then there's another solution to consider: zinc strips. A roofing company can attach long strips of zinc along the peak of your roof. When it rains, zinc ions will flow down the roof. Gloeocapsa magma does not grow in the presence of zinc, so your roof should stay streak-free.

If the prospect of pressure washing your roof sounds daunting, consider hiring a roof contractor to complete this task. They may be able to install zinc strips during the same session.

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how to identify when your roofing needs replaced

How's the roof on your home looking? The roofing on your home should look nice and it should keep out any rain or snow. When your roofing begins to fail, you will find the sandy particles in your gutters and possibly on the ground below. There are several signs that will alert you to the need of a roofing replacement. Our blog will show you what signs to be aware of so that you can quickly identify the problems before they spread from the outside of your home to the inside of it causing far more damage.

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