Sidebar: Growing Food Locally: Integrating Agriculture Into the Built Environment
Installing an intensive green roof or a greenhouse on a rooftop shouldn’t be approached casually. While some buildings are structurally overbuilt enough to support the extra load from such uses, many are not. Before proceeding with any rooftop farming or gardening operation, especially the addition of a greenhouse, a structural engineer should evaluate the building.
Big-box retail and newer warehouse buildings are particularly challenging, according to Taylor Keep, P.E., of Arup. “The roof designs of big boxes usually minimize spare load capacity,” he said. Arup’s New York office studied retrofitting its building to produce food in an effort to better understand this sort of building modification, which the company believes will be increasingly needed. Adding a structural system to support a greenhouse and other rooftop growing features would cost about $100/ft2 ($1,080/m2) with careful design and economies of scale. They suggest that, whenever possible, new loads should be placed above existing structural columns to keep costs down.
Zabar added an entire steel superstructure to support his rooftop greenhouses in Manhattan. Though he didn’t provide cost figures for the structural reinforcement, it was clearly very expensive. Adding structural modifications to the cost of the greenhouse itself—typically about $35/ft2 ($380/m2)—can make rooftop farming very expensive.
