Lichen on Roofs

Lichens are composite organisms made of a fungus living in partnership with an alga or cyanobacterium. On roofs they form flat, crusty or leafy patches, often grey-green, yellow, orange or white, and they are common on asphalt shingles, wood, slate, stone and clay or concrete tiles. Unlike moss, lichens grow very slowly and can survive long dry periods, so they also appear in drier, sunnier locations.

Effects on the roof

Lichens anchor themselves firmly to the surface. On asphalt shingles, inspectors often find that when a lichen patch is removed, the granules beneath come away with it, leaving a bare spot. Over many years lichens may contribute to surface erosion of porous materials. On old slate and stone roofs, many conservation bodies regard lichen as part of a building's character and a sign of clean air, and advise against aggressive removal unless it is causing a defect.

Removal

• Lichens are difficult to scrape off without damaging the surface.
• Treatments marketed for moss and algae, including diluted bleach or zinc-based products, can kill lichen, but the dead patches often take months to weather away.
• Pressure washing is generally discouraged because it can damage shingles and tiles and drive water under laps.
• Patience is often the safest approach: treat, then let rain and time loosen the growth.

Prevention

Copper or zinc strips near the ridge, reducing shade, and keeping debris off the roof can slow regrowth. Because lichens are so slow-growing, a few small patches on an otherwise healthy roof are usually a low priority compared with structural or leak issues. Heavily colonised shingles on an old roof are more often a sign of age than a cause of failure in their own right. See also roof moss and roof algae.