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The Green and Black Stains on Your Roof – Harmful or Just Unattractive?

Roof Algae and Moss

If you have green or black stains running down the roof of your home, it is time to get rid of them! Not only are they aesthetically unappealing, but they can be harmful to your roof as well.

The stains on your roof are caused by two things­ –moss and algae. The most common type of stain-producing plant organism, gloeocapsa magma, is algae and causes the black stains. Moss is also a plant that grows in damp areas, leaves green stains. Although the two are different in appearance, you use the same methods of eliminating them.

Algae require a moisture-rich environment, which is found in coastal or humid climates. If your home is in a shady area that is covered by trees, most likely your roof is vulnerable to algae. Calcium carbonate is found in most shingles, and unfortunately algae love to feed on this. If left untreated, algae are able to slowly deteriorate your roof.

Like algae, moss thrives on moisture and must live in an environment that is constantly damp. Moss is found mainly on the north side of homes, because this part of the roof receives less sunlight. Areas of the roof that are covered by hanging branches or trees are highly susceptible to moss. If moss builds up, it has the potential to create a dam in your roof. This damming will create lateral water movement, which can cause leaks or moisture damage.

Both algae and moss can be treated and prevented through the same methods. To treat the roof we suggest using oxygen bleach, which can be found at your local hardware store. Some suggest using chlorine bleach, but we don’t because it can damage the asphalt, and the runoff could be harmful to your landscape.

Begin by mixing 8 ounces of oxygen bleach per gallon of water to get your mixture. Put the mixture into a pump sprayer to allow equal distribution. Once you spray the roof, allow the mixture to set for 20 minutes. Make sure to do this on a day that is cloudy, as the sun will evaporate the mixture too quickly. While you are waiting, you should cut back any branches within reach to reduce shade. After 20 minutes, take a push broom and lightly scrub your roof. You can tell the organisms are dead once they start to turn a whitish color. After this, rinse your roof with clean water. Make sure to never pressure wash your roof, instead use a hose with a light pressure. Pressure washers will remove granules from shingles, causing the asphalt to be exposed to the sun.

It is important to note that before accessing your roof, it is recommended that you take precautions such as using proper footwear and a safety harness.

To prevent algae and moss, S&K offers zinc strips and algae resistant shingles. Zinc strips are placed at the ridge of the roof and create an environment that moss and algae do not like. This works when rain travels over the zinc strip, spreading a trail of minerals over the roof that is algae and moss resistant. We also offer algae resistant shingles, made by the manufacturer CertainTeed. The shingles have metal granules added to them, which are meant to keep algae away for 10-15 years. When you have your roof replaced, as a courtesy S&K will trim back any branches within reach to reduce shade at your request.

Learn more about the roofing products offer by S&K Roofing, Siding and Windows of Maryland at, or by calling 866-836-7663.

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