This message was posted by cc0112453, but on the wrong thread - here it
is......

"The trees help in the process because they block out the sunlight. It is a
common problem here in the Pacific Northwest. I asked someone once what to
do about it and they recommended zinc strips. Zinc is a growth inhibitor
for moss mold. I took my life in my hands (I have a very steep, high roof)
and put zinc strips along both sides of the entire length of my house. One
year later the moss had stopped growing but only near the zinc strips. I
then went to a roofing company and asked them what they used. They sold me
a product that has powdered zinc in it and comes in a can that looks like an
over sized container of parmesan cheese. You take a couple of cans up on
the roof and sprinkle their contents along the peak of the roof. The first
rain will wash the powder down the length of the entire roof. You can also
sprinkle it all over if you want or hose it down. The stuff works great.
You still have to apply it every couple of years or so. I think it is
called something like Moss-Off or Moss-Kill. You don't want the stuff for
the lawn, it should say for use on roofs."