Why are my shingles discolored?

Shingles become discolored for several reasons, ranging from biological growth to aging and manufacturing variations. The most common cause of dark discoloration is black algae, Gloeocapsa magma, which appears as black streaks or stains running down the roof, especially on north-facing or shaded slopes. Algae feeds on limestone filler in asphalt shingles and leaves dark pigmented streaks. Green discoloration is usually moss or algae growth that holds moisture and appears in shaded, damp areas. White or light discoloration can be due to heavy granule loss exposing lighter asphalt underneath, or due to lichen growth which appears as white or light green patches. Rust-colored stains may come from rusting metal flashing or fasteners washing onto shingles. New roofs may show color variation due to different manufacturing batches or shading, where bundles from different lots have slight color differences that create a patchy appearance. Aging causes overall fading as UV bleaches pigments and granules wear. Chimney soot, pollution, or overhanging tree sap can also cause localized discoloration. While black algae streaks are mostly cosmetic and do not immediately affect waterproofing, they indicate moisture and can accelerate granule loss if extensive. Moss-related discoloration is damaging and should be removed. If discoloration is due to manufacturing defect, it may be covered under warranty. Cleaning with oxygen bleach can remove algae stains.