Why Mold, Mildew, Moss, Fungus and Lichen Grow on Roofs

For some of us, the black streaks or stains we see on asphalt shingles is something relatively new. When we were younger we didn’t see this eyesore, waiting to make our home look unsightly, inviting more fungi and plant matter to attach itself to the shingles and cause damage if left alone. These black streaks are called gloeocapsa magma, or GM, more commonly called mold or fungus. These cyanobacteria are bacteria that get their strength to survive through photosynthesis and the limestone filler found in the composition of shingles.

In the past, shingles were constructed using felt and asphalt. Now that shingle composition has changed to mostly fiberglass and limestone, the GM blue-green algae thrives, using the limestone as their source of food. Why is it black and only found on the north- and east-facing sides of the roof? GM can form an outer coating, protecting it from UV rays. When there is no light or low light, GM has a blue-green color. GM’s other self-protective factor is that it enjoys the cooler, shady areas of the roof where moisture is usually present.

There is very little information as to the damage GM will cause to roofing materials. Some state it is only an aesthetic problem while others state it causes damage. Both the American Roof Manufactures Association (ARMA) and Canadian Asphalt Shingle Manufacturers Association (CASMA) state that GM is only an aesthetic issue, giving a home poor curb appeal. One thing is certain, however, roofs with GM can attract other life forms that do cause damage.

When algae and fungus get together, they form something new, lichen. Lichen is a photosynthetic fungus that has roots that wrap around the granules, feeding on the nutrients of the GM and the filler in the shingles. Once on the roof, lichen cannot easily be removed. Even if it dries out, it can come back to life when rain or moisture appears. Scrubbing or power washing the lichen will only do more damage.

Another organism growing on your roof is moss, which is a plant, not a fungus or algae. Moss aides in shortening the life span of your roof because it retains moisture and in cold climates can freeze, causing damage to the shingle and granules.

When you’re trying to decide whether to clean your roof or have a professional perform the cleaning, keep a few things in mind. Roof cleaning is not a DIY project. Some homeowners might want to tackle this project but without proper training and equipment, the result could be a disaster.

ARMA, CASMA and the asphalt roof manufacturers state that shingles should not be cleaned using a high-pressure washing system. Professional roof cleaners have special equipment, and most will clean the roof in the same manner as suggested by roof manufacturers: a low-pressure chemical treatment. A roof cleaning can remove black streaks or other discoloration for a fraction of the cost of replacement.

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