Yes, moss on roof shingles is definitely a bad thing and should not be ignored, even though it may look green and natural. Moss is much more damaging than simple algae staining. Moss acts like a sponge that holds moisture against the shingle surface for prolonged periods, preventing the roof from drying properly after rain or dew. This constant moisture accelerates granule loss, causes the asphalt to deteriorate faster, and can lead to wood rot in the underlying deck if water wicks under shingles. As moss grows, its root-like structures called rhizoids work their way under and between shingles, physically lifting shingle edges, breaking the sealant bond, and creating entry points for wind-driven rain and wind uplift. In freezing climates, moss holds water that expands when it freezes, further lifting and cracking shingles. Extensive moss can also block water flow, causing water to back up under shingles. While a small amount of moss can be removed, widespread moss indicates the roof stays damp too long due to shade, poor drainage, or poor ventilation. It should be removed carefully without pressure washing, which damages granules, and prevention measures like trimming overhanging branches to allow sunlight and installing zinc or copper strips near the ridge to inhibit growth should be implemented.
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