

In many cities, rooftops that can handle the soil, water, and plant loads are being put to productive use, attacking food insecurity and building communities in the process, said Leigh Whittinghill, Michigan State University, at the Green Roofs for Healthy Cities conference in Philadelphia. Still, there are some major barriers preventing more widespread rooftop farming, namely just finding enough low-cost, structually-sound roofs to plant; obstructive municipal zoning; and issues with “managing fertility.” There are also challenges with fertilizer runoff and economic profitability.

Rooftop Agriculture Isn’t Easy

Ben Flanner, the founder of the Brooklyn Grange, a 40,000 square foot rooftop farm in NYC that grew 15,000 pounds of produce last year, said it took a while to find a “landlord with guts.” He struggled, cold-calling lots of building owners. “The big issue was finding a space we could afford,” given urban agriculture quickly becomes un-economic if rents are more than $1 per square foot. Flanner understands that building are worth millions so some landlords would be worrying about possible damage. “But it’s about risk and return.” With more successful projects, the risks will go down, and so will the costs.

“Educating the local building department is really important,” said Brendan Shea, Recover Green Roofs, who just worked on adding a 1,500 square foot rooftop herb and vegetable garden to a restaurant. “It’s about building permitting agencies and making them feel comfortable, that we aren’t damaging the building.”

Mark Morrison, FASLA, Mark Morrison Landscape Architecture, who did his first green roof in Moscow in the 1970s and works on lots of diverse rooftop spaces (restaurants, hospitals, and community gardens), said the issues relate to policy. “We need policy changes.” He pointed to his Visionaire greenroof project in Battery Park City, where there’s a “strong authority” that didn’t want to see plants on roofs, so he had to make design changes to hide the roof produce. Keith Agoada, Urban-ag, agreed, adding that “commercial farming is often illegal.” Rooftop farmers often need “special use permits” to get around out-dated regulations meant to encourage densification by keeping farmers out of the city. There are also complications with adding greenhouses on roofs, which are “technically another floor,” so farmers need “legal and design workarounds.”

There are structural challenges as well: Brad Rowe, Michigan State University, says roofs can only grow so much in 5-inches of soil, largely because they can only handle so much weight. For Lisa Goode, Goode Green, who designed Eagle Street Farms in NYC, roof weight and waterproofing are major issues. Morrison said “structural engineers” are then very important to the process of evaluating a roof. Engineers can either evaluate roof load by finding the average load over a roof or just identifying a capped load for every square foot. The second option prevents designers from creating deeper 24-inch areas for fruit trees. “If you find an engineer who won’t provide an average range over the roof, find a different engineer,” said Morrison, with a wink.

Managing Soil Fertility

Keeping the soils healthy and productive may be challenging. Rooftop structural soils are different from ground-bed soils, argued Shea. “The media doesn’t hold compost well.” Furthermore, some structured roof soils may “not be ideal for growing vegetables.”

Meanwhile, at the Brooklyn Grange, where there’s 10 inches of soil, “every inch counts,” but some 300 square feet is dedicated to composting made up of leaf scraps, coffee grounds, wood shavings. Flanner said their next roof farm will have a freight elevator, which will be used to bring up manure.

Both Goode and Shea mentioned that it’s also particularly hard to maintain high levels of nutrients after rain, not to mention that fertilizer-rich runoff can present problems. Goode said “with rooftop farms, it’s very hard to keep nutrients in the soil.”

Morrison thought that laying out the garden the right way was key to maximizing fertility. “If you are gardening in small areas, it doesn’t have to be aesthetically pleasing. You can over-maximize.”

Agoada said the tricky part is finding local waste streams and managing fertilizers with small amounts of space. He explored hydroponic solutions, including liquid nutrient sources, as well as agriculture / aquaculture combined systems that continually reuse the whole cycle of resources and nothing is wasted. “It would be good to even include animals on the roof.”

Dealing with Runoff

It turns out that urban farms don’t store all stormwater and have runoff problems, which can be made worse through the use of fertilizers. On the Brooklyn Grange, a new bioswale will be added to the roof, which will be compared to another area that simply has a drain.

Morrison said “I don’t love extensive roofs,” which are made up of shallow sedum layers. “Haven’t we seen sedum done before?” With deeper pits, up to 4-inchs for tomatoes, farmers can grow anything that can be eaten. Those deeper, “volumetric” soils may also hold more water better. Futhermore, the addition of bioretention or permeable pavement systems to urban farms could also help capture and cleanse farm runoff.

Economic Profitability and Social Value

Low profitability is associated with high social value, argued Goode. Projects like Eagle Street Farm, which are a “hub of education and interest,” are unique. “There’s no rent, or profit sharing. What is sold supports the project.” She added that “farming is a tough numbers game. It’s a tough sell, but educating children in cities about how food is grown is worth any size project.”

The Brooklyn Grange is moving ahead profitably and even expanding because it’s big and efficient. Flanner said “it does require significant scale. You have to figure out how many square feet you need. Then you have to be really scrappy.” For example, the Grange has a beehive, totes, jaring. “We monetize social capital.” Given products are sold locally, they also save by excluding packaging. “That’s 10 cents per unit that we don’t have to spend.” Bicycles are used to distribute rooftop food grown locally, which makes us “cheaper than rural distribution, with their trucks, fuel, and labor costs.”

Rowe at Michigan State University thinks the economics of rooftop farms only make sense in dense environments like NYC. “It doesn’t make sense in Detroit where there’s lots of available open space.” There, he said farmers, protect against vandalism by guarding their spaces.

Another model for economic profitability may be to go high-tech. “With high-tech hydroponics, farmers growing herbs, grasses can do well,” said Agoada. With these sytems, it’s important to create a large enough space, also to recoup the “soft costs.” “Economies of scale are very important. You still need the same staff for a small space.” Also, cutting out the middle man is a way to improve profits. At farmers’ markets, rooftop farmers can sell their produce directly, and turn basic products like basil, which may go for $2.25 a bunch, into pesto, which can easily go for $5.00 a container. “This is a way to add dollars without adding cost.”

Agoada made an interesting argument when asked whether rooftop farm businesses should donate a share of their produce to food banks. He basically said no, arguing that a share of pre-tax profit can be given to a cause but the role of the commercial rooftop farmer is to maximize profits, selling heads of high-end lettuce for $2.50 a pop. Socially-focused non-profits need to be the ones that can spread access to low-cost produce, combatting food deserts in the process.

Maximizing Yields

“We are constantly experimenting with planting schemes,” said Flanner. Tomatoes, herbs, and lettuces are set within rows but large spaces, the areas between rows, are also used. “We utilize every square foot.” Instead of calculating yields monthly, the Brooklyn Grange just calculates aggregates at the end of the year.

The grange maximizes vertical growing, mixing taller and shorter plants. “The challenge is time and efficiency. If crops are all over the place, they are harder to harvest.” He said some mixes that work very well are tomatoes with basil.



Scaling Up

“We’ve gotten feedback from restaurants. The quality of our rooftop produce is on par with those grown at conventional farms,” said Flanner. Another competitive advantage: rooftop farm produce is really fresh, particularly when it’s delivered to the restaurant the same day it’s picked. So, for commercial rooftop and non-profit producers doing things correctly, there’s no quality issue preventing rooftop farming from scaling up.

But Agoada thinks the future of rooftop farming is tied to subsidies. “Green roofs get lots of subsidies” and rooftop farms should get them, too. If cities like NYC, Philadelphia, San Francisco, and elsewhere are serious about rooftop farming, they will need to incentivize their development.

There are also more ambitious concepts being considered: Parking garages with millions of rooftop square feet could be ideal places for farms. Agoada’s consulting firm did a study evaluating the potential of parking garages in San Francisco. In another idea, the roofs of big box stores could be transformed into productive food landscapes. The roofs could be leased out to local farmers, and then the produce could be bought back by the stores. One audience member said roofs in D.C. will soon be growing medical marijuana. Inner-city school and hospital rooftops could also be ideal places for non-profit organizations.

Image credit: (1) Brooklyn Grange / Cyrus Dowlatshahi, (2) Brooklyn Grange / Brooklyn Grange