The NYTimes reports on plans for 100,000 square foot greenhouse on the roof of a former Navy warehouse in Brooklyn. Here the plan is for hydroponics. Unlike vertical farming, these things make some sense to me. I say "some" because I've got reservations: in this case I'd assume the economics are based on both a successful hydroponic farm and use of the warehouse. Presumably if one or the other runs into trouble, the whole enterprise becomes a bit dubious. And a dual-function site requires two sets of expertise so perhaps two sets of management and labor, which means it's more complicated than just a site dedicated to one function. But if the people can make it work, it's better use of the area and the resources, as well as reducing the need for transportation of the crops.
See this treehugger piece on a similar project