Could you use a mild bleach and water solution to first kill the mildew and fungi then somehow either shop vac your waste water or attach some kind of flexable hose to the down spouts and collect it in buckets? If you use bleach, which will kill all of that crap, you will want to neutralize it so it does not react with the new top coating of that white, elastermeric, rubberized roof gunk you will be applying with a roller. I would use minimum pressure and just let the bleach do the work. Then follow the roof coating manufacture's suggestions. Oh yea, get before and after pictures because if you pull this one off without a hitch, everyone in Florida will be after you to do theirs! I've never done one of these but that never stops me from trying something new. I would try it once. I say go for it!