What are the black stains on my roof?

Ever look up at your roof to find black and dark green matter across the top? It is plant matter growing and feeding on your roof. Gross right? The name of those black streaks is gloeocapsa magma. Typically growing on the north side of your home’s roof, it is a very hardy strain of blue-green algae. This is an airborne algae/fungus. It travels through the air and lands on your roof and starts to grow and grow. When you look at your roof after a rain, the algae looks shiny and it really stands out. Like a plant, it is getting watered, time to grow more.


How does the blue-green algae get there in the first place? Today’s asphalt shingles contain a large amount of crushed limestone in the granules that provide the UV protection for your shingle. Gloeocapsa Magma feeds on the limestone in your shingles. The algae then multiplies and begins to form the unsightly black streaks that stains your roof.

The algae take root on the surface of your shingles helping to dislodge the protective granules on the shingle. As the algae grows naturally, the shingle begins to hold more moisture and heat. This additional moisture and heat aid to amplify the harmful UV rays of the sun. As more and more granules are damaged, the protective asphalt layer of your shingle gets exposed to those harmful UV rays and that is when the real damage begins! The shingle begins to break down leading to premature roof failure!

The damage isn’t just unsightly. Algae covered shingles retain much more heat than clean shingles. Surface temperatures of algae infected shingles can be 10 to 20% higher than clean shingles.  As a result, shingles that are darkened by algae growth can increase air conditioning costs by at least several hundred dollars. This makes your home much more difficult to cool in the summer.

What can you do to avoid the costly and ugly damage to your roof? Trust the professionals at AJ Wells to assist you with your roof inspection, maintenance, and replacements. If you stay on top of algae-darkened shingles, then you should be able to maintain the look of your home without much money out of your pocket. However, if there’s been too much damage to complete a standard maintenance, we offer affordable shingle replacement and repair.


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