Conscious Choice: Gardens in the Sky

July 2000

Gardens in the Sky

by Claudia M. Lenart

Despite the lure of our increasingly fast-paced cyber-existence, an ancient undercurrent flows. People yearn to return to their green roots. In densely populated urban centers, most of the green space has been paved over in favor of concrete. But that hasn’t stopped us. There are plenty of flat rooftops, and this resource offers vast potential for greening our cities.

"I think rooftop gardening is going to become more popular as land becomes more precious," says Liz Serritella, co-owner of Old Town Bed & Breakfast and a new rooftop gardener. "I love to dig in the dirt. We had gardens before, and when we moved I couldn’t garden anymore," says Serritella. Last spring, she put in two penthouse gardens to the delight of guests as well as herself.

Serritella found that her rooftop gardens required an initial investment: before installing a rooftop garden, gardeners need to check with an engineer to ensure the structural safety of the roof. She also needed to install a wooden deck and buy new lightweight containers for plants. And rooftop gardening is trickier than ground level gardening, due to high winds, sunnier conditions, and an increased need for water. But Serritella found that there was a lot of information available on container gardening. Lightweight planters and lightweight planting medium are readily available at garden centers. One trick she learned from her research was to line the containers with styrofoam, to insulate and help the plants over winter.

Rooftop gardens are definitely more complicated than digging a backyard plot. But in areas where that backyard plot doesn’t exist, they offer an untapped potential for greening and cleaning up the air we breathe and the peace that can be found communing with nature in a garden. They also offer the exhilarating feeling of looking at the world from above.

"Flying into O’Hare, you see acres and acres of bare roofs. They all could be green and that could make an enormous difference," says David Yocca, principle of Conservation Design Forum, a landscape design group that utilizes landscape architects, ecologists, and botanists to design sustainable environments. Conservation Design Forum has been involved in designing "new town" communities including Mill Creek in the far western suburbs and Coffee Creek in Chesterton, Indiana, where the roof on one of the community buildings will feature a putting green.

The firm also is in the process of greening up the 38,800 square-foot roof at Chicago’s City Hall, as part of a five-city U.S. Environmental Protection Agency, Urban Heat Island Project, whose goal is to reduce smog by lowering the temperature in congested cities.

The Chicago Plan

Just like dark clothes, dark rooftops absorb the sun’s rays, heating buildings and increasing temperatures in congested cities by as much as eight degrees. Hotter buildings increase the use of air conditioning, which leads to higher energy use and more pollution from the burning of fossil fuels. Meanwhile, higher temperatures outside cook the pollution, creating smog.

Other cities in the Urban Heat Island project, Baton Rouge, Louisiana; Houston; Sacramento, California; and Salt Lake City, plan to use reflective roof surfaces to deal with the problem. Chicago, however, has decided to build an innovative "green" roof to reflect the sun’s rays. Plants on the roof will cool the air by evapotranspiration, a process in which plants secrete water through their pores and as the water evaporates it cools the air. Plants also take in carbon dioxide, release oxygen, and filter the air to some extent. According to computer models conducted by the Lawrence Berkeley National Lab, which is participating in the heat island project, widespread use of reflective surfaces and green roofs could reduce summer temperatures in cities by several degrees.

As Yocca points out, "the technology has been established for many years. Green roofs are quite common in European countries, where they have had problems with flooding." In fact, rooftop gardens aren’t a new idea at all. The first historical accounts of rooftop gardens date back to 500 B.C., when the Hanging Gardens of Babylon were in place. During the Renaissance, Pope Pius II had a roof garden built at his summer residence in Pienza, Italy. In more recent history, settlers on the American Plains built sod houses to insulate themselves from temperature extremes.

The technology that has made green roofs a realistic solution for today’s structures was developed in Germany, where some cities use them to combat flooding problems caused by stormwater runoff. According to Theodore Osmundson, landscape architect and author of Roof Gardens: History, Design & Construction, 43 percent of German cities offer financial incentives for building green roofs.

Green roofs are starting to catch on in the states, says Matt Carr, Garden Roof product manager for American Hydro Tech, a company that manufactures roofing membranes, including the one at City Hall. Three years ago, says Carr, "we were working with two or three green roof jobs. Today, we’re overseeing about a hundred green roofs throughout the states. I think Chicago is going to be one of the leaders in the country," Carr adds. "They’re really looking to green up the city."

Green roofs are a step forward from the old style of rooftop garden associated with penthouses. Most of those rooftops feature a deck over the roof and lots of large, lightweight containers that hold plants. While those gardens are better than a bare black rooftop, they are high-maintenance affairs that offer fewer environmental benefits than the newer green roofs.

The green roof planned for City Hall includes a seamless membrane made of hot, rubberized asphalt designed to last the lifetime of the building. It incorporates German irrigation technology, called a floradrain, which retains water and reduces the need for irrigation of green roofs. During a typical rainfall, this roof would retain 50 to 70 percent of the rainwater, says Carr.

Next, a layer of gravel and lightweight soil will be spread in thicknesses ranging from three to thirty inches. This will be planted with 20,000 plants, from shallow-rooted sedums and ivies to shrubs, hawthorn, and crabapple trees, says Jessica Rio, spokesperson for Chicago’s Department of Environment. Many of the plant species will be of native origin, because natives are hardier and can withstand adverse conditions like drought and high winds. Most native prairie plants have root systems that are too deep for roof gardens, but species that are found in hilltop prairies have shallower root systems, says Yocca. Groundcover plants, including sedums, mosses, and grasses can tolerate both an abundance and a scarcity of water. "These plants will go dormant and turn brown during a prolonged drought, but then they’ll green up again as soon as it rains," says Yocca.

The roof won’t be open to the public, but workers at City Hall and other neighboring buildings will be able to enjoy the view from indoors. Local scientists will enjoy a unique opportunity to monitor the effect of a green garden on climate. Infrared satellite photos will also measure the effectiveness of the green roof, and the black tar rooftop on the Cook County building next door will serve as a standard for temperature comparisons.

A green roof on top of City Hall is only part of the solution, of course. The city intends to install other green roofs at strategic spots throughout the city. But for rooftops to have any impact on reducing the city’s temperature, private building owners will need to follow the city’s example. City officials hope their innovation will start a trend. So does Yocca. "People often think green roofs will cause problems. So it helps to have an example," he says.

The city’s green roof will cost about $1 million, which will come out of funds the city won in a franchise lawsuit against ComEd. (The utility is paying Chicago $25 million a year for four years.) Money from ComEd also will be available in the form of grants for private building owners who want to install a green roof or reflective roof surface. The city hopes these grants will help inspire building owners to give it a go.

In fact, a green roof can be a relatively small-scale investment. The green roof at City Hall is a large-scale undertaking known as intensive greening, which includes areas of deep soil depth, a diversity of plants and a park-like design. A smaller-scale version, known as extensive greening, requires only a few inches of soil planted with low growing plants, such as sedum. Carr says most of the projects he’s seen have been intensive. But he sees any green roof as an asset to the local environment. He expects more new buildings, especially public buildings, will utilize green roofs as government incentives become available.

Environmental benefits are the primary goal of roof greening. Green roofs reduce stormwater runoff, insulate buildings leading to lower energy use, clean the air, and control local climate, lessening the formation of smog. But there are economic benefits, as well. Rio says the city expects to save $4,000 per year in cooling and heating the building due to the insulating capability of a green roof. In addition, the soil and plant cover on a green roof protect the roof membrane from the elements, which leads to lower roof maintenance and a longer lasting roof. "Green roofs can last fifty to a hundred years as opposed to a fifteen-year roof," says Yocca. Rooftop gardens can also add to the desirability of a building, allowing a property owner to charge higher rents.

Green Roofs to Greenhouses

Rooftop gardens aren’t the only option to green up rooftops. Roald Gunderson, an architect who has worked on Biosphere II, promotes the use of rooftop greenhouses. Gunderson has designed a sustainable, solar greenhouse design for cold climates. Just like gardens, greenhouses get crowded out of urban landscapes. But urban rooftops are more available and less expensive than land at ground level. And rooftop greenhouses offer many of the same benefits as rooftop gardens. While rooftop gardens clean the air outside, greenhouses clean the air inside buildings. They also insulate buildings, leading to lower energy use. "A solar rooftop greenhouse is actually more insulative than a garden and is actually able to heat the building," says Gunderson.

One of the biggest advantages of a greenhouse is the production of locally grown, organic food year round. "Chicago easily has about fifty square miles of flat roof," says Gunderson. "Even ten square miles would offer a tremendous opportunity for year round food production."

Gunderson also believes greenhouses provide an opportunity to bring the outdoors inside, where we spend most of our time. "We spend 94 percent of our time indoors, yet all things in a building are dead," says Gunderson. "Greenhouses offer the potential to take us back to the human ecology we lost... when we entered the industrial age."

"To be in a garden on a roof is the ultimate experience of being up high, for here a garden is not supposed to be," says Osmundson. But rooftop gardens can offer another high, as well: the satisfaction of being part of the solution on a balmy summer day.

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