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O.K.  So you have black streaks on your roof ?  So what you say ?  It just looks bad, but is it really causing any damage ?  This roofs owner noticed the black streaks last year, didn't look to bad so let it go. Well, this summer it got worse looking, and on closer inspection, moss was growing at the edges of the shingles. The home owner called Great Lakes Roof Cleaning for a quote, and on closer inspection we noticed that the moss has actually started to grow under the shingles, lifting them up off the roof. This will lead to an easy point of migration for moisture under the shingles.  The reason the moss was able to start to grow was because of the gloeocapsa magma ( black streaks ) that was allowed to take hold. It is holding moisture on the roof longer, which in turn makes for a great enviroment for lichen and moss.  For home owners that would like to get the most life out of their roofs, it is a smart financial decision to have their roof cleaned as soon as they notice the black streaks appear.