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Rooftop Agriculture Offers Urban Storm Water Solution

Rooftop farm
Fresh vegetables are growing sky-high on commercial rooftop farms in New York City. With 1 million lb of soil per acre of roof, the farms absorb significant rainfall, reducing the city’s combined sewer overflow. Courtesy of the New York Department of Environmental Protection 

New Yorkers are reaping long-term environmental benefits as well as an abundance of fresh produce from rooftop farms in the city. 

August 14, 2012—Rooftop farms are sprouting atop New York City buildings, creating a new industry, bringing farm-fresh produce to city dwellers and improving water quality. It’s a winning combination that the New York City Department of Environmental Protection (DEP) is encouraging with its Green Infrastructure Grant program.

The grant program is an outgrowth of the Green Infrastructure Plan and PlaNYC, the city’s master sustainability blueprint. The Green Infrastructure Plan focuses on managing rain-related pollution by using green infrastructure technology to divert storm water from the city sewer system.

Storm-water management is one of the city’s biggest environmental challenges, says Edward Timbers, a DEP spokesperson. “Seventy percent of New York City is serviced by a combined sewer system that conveys wastewater, rain water, and street runoff to the water treatment plant in the same pipe, and we have a high prevalence of impermeable surfaces,” Timbers explains. “A strong storm can overwhelm the sewer system, and the [untreated] excess is discharged into our waterways.” These combined sewer overflows (CSOs) account for the majority of the pathogens in the New York harbor water.

The plan’s goal is to develop green infrastructure that will capture the first inch of rainfall on 10 percent of the impervious areas in the city’s combined sewer watersheds through detention or infiltration techniques over the next 20 years. The DEP estimates that installing such infrastructure as bioswales, permeable concrete, “green” streets, “blue” water-retaining roofs, “green” vegetated roofs, and crop-producing rooftop farms will reduce combined sewer overflows by more than 1.5 billion gallons annually by 2030.

Green-minded entrepreneurs have zeroed in on the opportunity to develop commercial-scale rooftop farms as an investment that can generate a significant return and provide an array of environmental benefits. Rooftop crops reduce heat loss and heat gain, absorb air and noise pollution, reduce greenhouse gas emissions, and even reduce truck traffic on local highways by enabling fresh, healthy food to be sourced locally to New Yorkers.

But from the DEP’s perspective, rooftop agriculture is all about the storm water that the farms can capture and divert from city sewers. Other benefits aside, Timbers says, “The DEP is into rooftop farming for the storm-water control.”

To date, there are at least half a dozen commercial rooftop farms in New York City. The largest is a 45,000 sq ft farm owned by Brooklyn Grange that sits atop two buildings in Brooklyn’s historic Navy Yard. Following two years of success with a 40,000 sq ft rooftop farm in Queens, Brooklyn Grange applied for and received a $500,000 Green Infrastructure Grant to establish the second farm this spring.

The Navy Yard farm is the grant program’s largest project to date and the first to be completed. “We gave the grant because this farm will soak up approximately one million gallons of rainwater, diverting it from the sewer system,” Timbers notes. Brooklyn Grange’s two farms will yield a combined 40,000 lb of lettuce, tomatoes, peppers, kale, and other vegetables this year that will be sold to local groceries, caterers, and restaurants.

The Brooklyn Grange farms each contain about 1 million lb of a special, lightweight growing medium that was hoisted to the rooftop by crane in 1,000 lb bags. Up top, the custom mixture was spread 8 to 12 in. deep over a felt layer that covers a drainage system. Under the drainage system is a root barrier to protect the integrity of the roof.

Before the first bag of soil is lifted off the ground for any rooftop farm, the building must undergo inspection by a state-licensed architect or professional engineer to ensure that the roof can sustain the load, according to the New York Department of Buildings. If the roof doesn’t pass inspection, the farm developer must request drawings for appropriate reinforcement, obtain a permit, and have the work performed. A second inspection is required at completion.

Rooftop hydroponic greenhouse farms—a soil-free operation—have lower load-bearing requirements than soil-based farms, but they also must pass roof and building inspection before construction can begin. Rooftop greenhouses, like Gotham Greens’s 15,000 sq ft structure in Brooklyn’s Greenpoint neighborhood, consume less water than soil-based farms and still divert significant amounts of storm water from the sewer system.

Using a state-of-the art, computer-controlled climate-management system, a solar array, and captured rainwater, Gotham Greens operates year-round, producing 100 tons of lettuce that it sells to New York area restaurants and markets. Demand for the produce exceeds supply, so Gotham Greens plans to construct three more rooftop greenhouses to be open next year, bringing their total production area to 200,000 sq ft.

Supermarket giant A&P will be the first major food wholesaler to enter the rooftop agriculture business, partnering with hydroponic greenhouse builder BrightFarms to construct a two-acre greenhouse on top of a vacant eight-story Sunset Park warehouse on Third Avenue. The Sunset Park greenhouse will capture an estimated 1.8 million gallons of storm water each year.
With rooftops accounting for nearly one-third of New York’s impermeable surface, the DEP is promoting their potential for storm-water management by encouraging would-be farmer/entrepreneurs to apply for seed money to build more sky-high farms next year. By the close of 2012, the DEP expects to distribute up to $4 million in green infrastructure grants.



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