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.