The only rain water I would worry about is in puddles where cars have been...where oil may be mixed in. Otherwise, dogs are pretty durable and drink all kinds of water. Yeah, there is a small risk of giardia, but I bet that's pretty rare. Dogs are designed, it seems to handle a lot of gross stuff....from road kill, to eating poo, to drinking semi stagnant water. They have sure been a successful species and must have drunk water that had a little birdie or deer poo mixed in on occassion. LOL. I probably wouldn't try to collect water from the roof because that may have some chemicals in it from the roofing material. I'm more afraid of chemicals than stuff from other animals.