Common question asked to roofers:What causes mold on a roof?
In most cases, the black substance that appears to be mold on a roof is actually a form of algae. It grows well in warm, humid climates, so it's more common in areas with tropical climates. Carefully spraying the roof with a mixture of half water and half bleach should kill the algae, but avoid applying it with a pressure washer. High pressure can damage roof shingles.