What Are The Black Streaks On My Roof?

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House roof cleaning with pressure tool. Worker on top of building washing tile with professional equipment. Moss removing with water.

While there are many perplexities for the homeowner about the appearance of the house, one of the most bewildering is the buildup of black streaks on one’s roof. What is this strange occurrence and what causes it? Countless homeowners have wondered about the prevalence of black streaks on the roof and we are here to answer these questions in detail.

Black Streaks are Algae

Believe it or not, the black streaks that are running down your roof are actually a type of algae. This type of algae is also known by the scientific name Gloeocapsa magma. While this type of algae is initially a blue-green color, it changes over time to a darker black color. The texture of the algae also changes, as it hardens in the harsh sunlight.

What Causes Algae?

The algae form on your rooftop because of the composite materials of your roof. The limestone shingles on your roof contain a mineral that the algae find useful in feeding their nutrition.

What Can Be Done to Decrease The Algae?

Replace or Clean the Shingles

The treatment for this formation of algae is of course to replace the shingles, clean the shingles, or install a preventative material. In order to rid the shingles of the algae that already exists, it is necessary to either clean or replace the shingles.

This requires a thorough pressure washing. The most effective treatment for algae that already sits on the shingles is to completely replace the shingles. While this may seem drastic, the unsightly stains stand in noticeable contrast to the shingles themselves. It is recommended that if you have already let the stains develop, that you replace your shingles altogether and proceed with your preventative measures on your new shingles.

Prevent Algae from Forming on the New or Clean Shingles

The preventative process for keeping from developing new algae stains is quite simple, though it does take time to install. You can make a visit to your local hardware store and purchase all the necessary supplies in one trip. Y

ou will be purchasing a special material that is specifically designed to prevent this exact condition. You will purchase either zinc or copper strips made to line the surface of the shingles. You will then place these strips along the top of where the shingles are installed.

This method is effective when the first rain falls after installation. Through a careful chemical process, the rainfall initiates the prevention of algae formation. When the rain falls upon the shingles, it prevents the algae from forming on the limestone shingles.

The chemicals in the zinc or copper, depending upon which one you chose, combine with the rain to form a composition that detracts the algae from feeding on the limestone. It should be noted that this is a preventative measure, rather than a treatment for existing algae. All in all, the formation of these black streaks is entirely treatable and preventable in the future.