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Most property owners are under the mistaken impression that the black stains on their roof are due to accumulated dirt, tree sap, jet fuel waste, or air pollution. In fact, what you are seeing are colonies of living organisms that feed on shingle material, and while there is a sheath of algae all over the roof, it is especially noticeable in cooler, shady areas on the north and east sides.
For cedar, tile and other composite roofing materials, these are also susceptible to lichen and moss infestation which acts as a moisture trap decomposing the shingle material at an alarming rate (not to mention making the roof look old and neglected). It is imperative to understand the science behind roof algae and lichen infestation, and why ignoring this condition can result in the loss of up to 50% of an asphalt or cedar roof's lifespan -
a very expensive consequence. That is definitely more disturbing than just the lousy curb appeal.
The technical name for what is causing the black stains on your roof is Gloeocapsa Magma, an airborne algae that feeds on limestone filler in your asphalt shingle material. Lichen is a parasite of algae and subsists on it as a food source. It accelerates the damage to the shingles with anchors that embed themselves in the protective shingle granules. With time, this results in early decomposition of the shingle material. The granules dislodge and create voids that expose the shingle mat to weather and UV rays.
Notice the illustration below and you will see the symbiotic relationship that these two spores have with each other ...
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