Moss can grow on shingles and other roofing materials if they are kept moist by poor sunlight conditions and bad drainage. Once it grows, moss holds even more moisture to the roof surface, causing rot, and its roots actually work its way into the deck causing deterioration. Algae also grows in damp, shaded areas on asphalt shingle roofs. Besides creating an ugly black-greenish stain, it can retain moisture, causing rot and deterioration. Trees and bushes should be trimmed away from the house to eliminate damp, shaded areas, and gutters should be kept clean to ensure good drainage.