In the heart of Montreal, Canada, on 3 football fields (15,000 square meters) of roof-space sits a mega-greenhouse.

Large-scale roof-top greenhouses are growing in popularity, especially in urban areas. They are excellent for high-value crops or crops which require chilled transportation. This one produces 12,000 kilos of tomatoes & eggplants per week. 

And because of the tightly controlled environment, there are more crops per annum & more options around energy,fertilizer & water inputs. 

This greenhouse has a water system that collects & reuses rainwater, resulting in savings of "up to 90 percent" compared to a traditional farm. 

https://www.thedailystar.net/backpage/news/worlds-biggest-rooftop-greenhouse-opens-montreal-1953081