If you look at the north facing roof any single family house built with asphalt shingles in the past ten to fifteen years, you will most likely see black stains flowing down the shingles. In some cases there are additional colors in the stains like reds, greens or gray. What is growing on your roof is a complex set of living organisms that is adapting to a new food supply, your roof. These roofs can be seen in Purcellville and adjoining areas. Depending upon conditions of humidity, shading, adjacent trees or vegetation, color, age and slope of the roof, the stains may be minor or major in nature.
What’s growing on these roofs is a combination of algae, lichens, fungus, moss and bacteria. These organisms will only grow on areas of the roof shaded from the sun. In addition, the organisms need moisture and food to grow. The main reason that the organisms don’t grow on east, south or west areas of a roof is that the sun usually heats those areas above the boiling point of water which destroys the cells of the organisms. (In addition, the ultra-violet light from the sun is a strong disinfectant, and will destroy organic cell DNA.)
The reason that these organisms are on your roof in the first place is that the modern asphalt shingle contains limestone and/or marble dust as filler that increases shingle weight to provide resistance to wind damage. Chemically, limestone is calcium carbonate and identical to the stuff that you put on your lawn to make your grass grow better. It turns out that limestone is cheaper that asphalt and other non nutritive stones. The fungus that is eating your roof, Gloeocapsa Magma, loves limestone, as evidenced by the way it attacks old tombstones and statutes made of limestone. The fungus has developed a symbiotic relationship with the other organisms on your roof, in that, the lichens and moss form a protective coating that absorbs water and shields the fungus from any remaining sunlight.
So what can you do about it? Well it’s a two step process as shown the TV program “This Old House.” The procedure uses zinc metal strips to kill the lichen and fungus long term and bleach and water for immediate treatment.
For more information about how tackle the problem, here are several websites: