There are three approaches.
Mechanical, that is clean it off. Some people use power washers. I
suspect it will do more damage to the roof than the mold is doing.
Chemical, Sum suggest harsh chemistry like bleach. That will kill it
and it will likely also wash off on it's own after it has been bleached, but
other than a weak solution I would worry about roof damage by the bleach.
Then there are commercial chemicals that you spray on. I did try one of
those and it worked, it killed the moss/mold and it was washing off. The
product indicated it will need to be reapplied from time to time, but I will
not know about that for a while as my roof was just replaced due to hail
The use of copper/zinc materials are also suggested. These are metal
devices that are put on the roof for long term protection.
Many roofing materials come with mold/moss resistance built in and that
should last 5-15 years depending on what the add and your local conditions.
Joseph E. Meehan
26 + 6 = 1 It's Irish Math
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