Check out Larry Hall's raingutter gardening system. I've watched some of his YouTube videos, and I think he does Facebook too. In fact, I think he has a store on the web that sells supplies for this type of gardening.
I prefer to garden in dirt in the ground, not containers, so I don't have much interest in what he's doing; but if I was gardening on top of a parking ramp, I'd definitely give his stuff a look.
