Man, that's smart. My grandpa used to do something similar with the water from his roof gutters, but for his vegetable garden. He had this whole system of buckets and a big trash barrel set up. Honestly, I always thought it was a bit much until my own water bill spiked last summer. Now I kinda get it.
Wait, he used the roof water on vegetables he was going to eat? That seems like a bad plan. All that bird poop and whatever else washes off the shingles goes right into the barrel. I wouldn't trust that on my lettuce. Watering flowers or the lawn with it is one thing, but not food.