The focus of this seminar, was on our need for rain barrels, and how to go about making a rain barrel. In the literature they handed out, the only real mention regarding roofs (other than as being used as a catchment area), was in the publication, "Rain Barrels, A Homeowner's Guide", Author: Gerard Waston, (Hillsborough County Water Department, Water Conservation Staff),http://www.swfwmd.state.fl.us/publications/files/rain_barrels_guide
On page 23 of this publication,
"What about water quality,
The quality of rainwater shedding off a roof or gutter system is more than adequate for plants. In fact, it can be beneficial due to the lack of chlorine and a lower PH level then utility-provided water. As rainwater sheds off a roof or other catchment area, it can carry with it some levels of algae, roofing materials, or bird droppings. These are not a concern for nonpotable uses or occasional contact by the homeowner."