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Brush it off with a stiff bristle brush and add copper strips under the cap
with an inch or 2 of copper exposed....

If you agree to put up with the removal of the particles that make
up a shingle and thus shorten its life, then this scrubbing motion
will certainly remove the moss and algae and a portion of the
shingle..


The smart way to remove algae is to not use any scrubbing motion
at all. Just apply a long lasting chemical that will eventually kill
the growth. Then with periodic applications of once every two
years this same chemical will maintain a algae free roof.

http://www.sprayandforget.com/