The setting is a 900-square-foot area on top of an historic building in Brooklyn, New York, where the owner recently installed a new roof in anticipation of adding the greenery. She had the roofer raise the skylights several inches and bring waterproofing up the sides of the roof's walls. Architect Inger Staggs Yancey directed the project.
An engineering assessment showed the roof structure could handle 55 pounds per square foot, enough for a full scale rooftop farm. Green roofs need a minimum of 2.5 inches to 3 inches of growing media, but vegetables need up to 7 inches of material. The owner opted for a low-maintenance green space with a few planters for vegetables.
Most of the roof is covered with sedum, what Yancey says is a standard roof plant with extremely high drought tolerance.
The bottom layer is a root barrier, often very thick plastic sheeting. Several inches of gravel is placed along the perimeter to keep roots away from vertical elements, such as skylights or parapets. That's followed by a moisture-retention layer that retains some moisture to nurture plants and allows excess to drain away.
Filter fabric is next. It retains the dirt or growing medium while allowing water to pass through. With those steps taken, the plants can be brought in.
"It’s kind of like putting together an intricate jigsaw," Yancey says.
Costs range from $18 per square foot for a basic green roof to as much as $50 per square foot for more elaborate installations.