My understanding of roof runoff is that it varies based on where you live and your roof.

Some folks let the first few minutes of rain go into the ground and then collect the runoff. Some (and I don't advise this without lab testing at least once) do not treat the water collected this way before drinking it. Others treat with filters or chemicals, and I'm sure there are situations where roof runoff cannot be treated economically.

I would suggest either using roof runoff for non-potable purposes or springing for a lab test of both the first water that comes off the roof and water that comes off after five minutes or so of a good rain, to see what you're up against.