Lichen on Roof Tiles: Should You Worry?

Lichen vs moss: how to tell them apart

Moss is a soft, cushiony green plant that sits on the tile surface and can be scraped away relatively easily once loosened. Lichen is a composite organism (part fungus, part algae) that grows flat and crusty, often in grey, pale green, yellow or orange-toned patches, and bonds much more firmly into the tile surface itself via root-like structures, similar to the black lichen commonly found on patios.

The two can appear on the same roof simultaneously, and it's common to see moss thriving in shaded valleys while lichen spreads across more exposed, sun-facing tile surfaces on the same pitch.

Does lichen actually damage the tiles?

Lichen is generally considered less structurally damaging than moss in the short term, since it doesn't hold as much water against the tile surface as a thick moss cushion does. However, its root structures do penetrate the tile's surface pores over years of established growth, and on softer, more porous concrete tiles this can very gradually affect the surface finish, particularly combined with freeze-thaw cycling in winter.

The honest picture: light lichen coverage on sound tiles is more of a cosmetic concern than an urgent structural one, but established, heavy lichen coverage over many years is worth treating rather than assuming it's entirely harmless indefinitely.

How lichen is safely treated

Because lichen bonds so firmly to the tile surface, similar to black lichen on paving, a biocide treatment with proper dwell time is the effective method, rather than physical scraping, which can damage the tile's surface finish trying to lift firmly anchored lichen off. The biocide kills the organism at the root level, and dead lichen then weathers and washes away naturally over subsequent weeks and months rather than needing aggressive removal.

Pressure washing lichen off a roof carries the same risks as pressure washing for moss removal, stripping protective granules off concrete tiles and risking dislodged tiles, so it's not the method we'd recommend regardless of what's growing up there.

As with moss, this treatment is best carried out from the ground or via pole-fed application rather than walking on the roof, both for safety and because foot traffic on lichen-covered tiles, which can be slippery, adds unnecessary risk to a job that doesn't need it. Paler concrete tiles tend to show lichen patches more obviously by contrast than darker slate, though the organism itself establishes based on surface texture and exposure rather than colour.