Should mosses be removed from rooftops?

Many people are concerned about mosses growing on their rooftops. On the other hand many people are enthusiastic about intentionally having a living surface on our rooftops. See the website for the greenroof industry and search on "moss".

Some roofing materials promote moss growth than others. (See photo: mosses, mainly Dicranoweisia, on a west-facing cedar shake rooftop in western Oregon.) Cedar shake roofs are more likely than asphalt and metal roofs to have moss growing on them. Mosses are also better suited to growing on the north-facing parts of roofs, especially those areas shaded by trees (Oregon State University Extension and Experiment Station Communications).

Moss holds itself to its substrate using root-like structures called rhizoids. Roofing professionals say that these structures grow into tiny cracks in the roofing materials, accelerating their degradation, and eventually causing leaks. The moss may also hold moisture on the roof, allowing decay-causing organisms to get a foothold. Moss could grow under the edges of shingles, loosening them, and causing leaks in that way (Oregon State University Extension and Experiment Station Communications; Niemiec & Brown 1993).

Some homeowners have also observed that their cedar shingle roofs are damaged by sun exposure, which cause the shingles to crack and curl. The areas of the roof covered by moss may be protected from this kind of damage.

Scientific study on this subject is very sparse. Perceptions of the damage that mosses can cause to roofs has not been directly confirmed by scientific research, although there seems to be a consensus among roofing professionals that they are detrimental to rooftops.

To find out about methods of removing moss from rooftops, please go to the Methods of Control section.

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