"S. Barker" wrote in message
...
what you need is a strip of galvanized metal all along the ridge or just
under the ridge shingles. The galv prevents the algae from growing. It
won't remove what's already there, however.

[snip]

Been there, done that -- South Florida's heat/humidity overpowers the zinc
strips. Instead of having solid mold/mildew you get mold/mildew with
streaks.

I'd suggest that unless you have a HOA that's uptight about mildew on roofs,
you just accept it as a fact of life and let the GAF shingles do whatever
they're capable of. IMHO, over the course of years you'll have more shingle
damage from walking around on it spraying chemicals than if you just let it
age gracefully -- especially true for others with concrete or barrel tile
roofs.