150,000 square feet of vegetables. Large rooftop farms aren’t new—in Chicago’s South Side, a 75,000-square-foot greenhouse on top of a factory building grows up to 10 million heads of leafy greens each year (and was the largest rooftop greenhouse in the world when it was built). In Brooklyn, a new rooftop farm with a 55,000-square-foot garden and 5,000-square-foot greenhouse is opening this month.