I gardened for 5 years on my roof in Brooklyn. I experimented a lot. As Margaret says- do not skimp on pot size! What I found to work the best for me: Blueberry bushes, sedums, herbs, heathers. Also it is very easy to keep a water garden; just fill one of those large tubs with water and a potted lily or other water plant. It takes a long time for that much water to evaporate, just make sure your roof can take the weight.