I saw a programme many years ago about a Japanese supermarket where fresh greens were produced IN the supermarket hydroponically.
What happened to that idea?
Still Rooftops are a good idea now we have good waterproofing membranes and someone has just invented a very "light" soil mix. This means that roofs not structurally strong enough to take heavy soil can use this new (patented?) very light soil mix. I think it was a Yank who invented it