Lots of good advice in this thread, especially the rinse suggestions.
You can NOT over rinse, but you can under water, with a disaster on your hands.
Even the best roof cleaning job will be ruined if you kill your customers plants!
Water, water, and water some more, keep all plants wet during cleaning if chemical drips on them.
TRY and have a helper on the ground, but if not,, either tarp, or take a water hoes up on roof with you.'