Farming on a rooftop

From National Geographic - 

In New York City, farming on a rooftop is not just an idea. Brooklyn Grange farms more than two and a half acres of rooftops in Brooklyn and Queens, and then sells what it produces to New Yorkers. A special soil mixture is used to minimize weight on the roofs and allow rapid drainage during heavy downpours. The farmed rooftops also house chickens and an apiary.