Roof Moss

Moss is a common growth on roofs in cool, damp climates such as the Pacific Northwest of North America, the British Isles, northern Europe and New Zealand. It favours shaded slopes, north-facing slopes in the Northern Hemisphere, areas under trees, and rough surfaces that hold moisture, including asphalt shingles, wood shakes, concrete and clay tiles, and slate.

Why it matters

Moss acts like a sponge. It keeps the roof surface wet for long periods, which can speed decay of wood shakes and the weathering of shingles and porous tiles. Thick cushions of moss can grow under the lower edges of shingles and tiles, lifting them and allowing wind-driven rain to enter. Dislodged clumps wash into gutters and downpipes and block them. In freezing climates, water held in moss can freeze and damage porous surfaces.

Removal

• Physical removal. Brushing or gently scraping moss off by hand, working downslope so units are not lifted, is described by environmental groups as the most environmentally sound approach, though it can be hard work and needs safe access.
• Chemical treatments. Products based on zinc sulfate, potassium salts of fatty acids and other ingredients are sold as moss killers. Some zinc products are toxic to aquatic life, and runoff should be kept away from ponds, streams and vegetation. Local rules on roof runoff may apply.
• Avoid pressure washing. High-pressure water can strip granules from shingles, damage tiles and force water under laps.

Dead moss often loosens over time and can then be brushed off more easily.

Prevention

• Trim overhanging branches to let in sun and air.
• Keep the roof and gutters free of debris.
• Fit zinc or copper strips near the ridge, whose runoff discourages regrowth (see zinc and copper strips).
• Choose materials suited to the climate when reroofing.

Working on a wet, mossy roof is dangerous; fall protection or professional help is often advisable.