Look around at roofs, not just in Florida. It is black algae and prevalent on most roof shingles with a little age. Asphalt is a natural deterrent but the amount of asphalt used in the manufacturing process has changed. Copper has been found to be a natural deterrent and the best shingles are now infused with copper but these shingles are much more expensive. Google black algae and roof shingles, there is a lot of information out there for your reading pleasure.
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