Not-So-Secret Green Roof Gardens

Green roofs are a natural cure to problems created by blacktop jungles.
July/August 2002
Chicago City Hall’s living roof is part of the city’s Urban Heat Island Initiative project. The landscape design is based on a formal garden and includes a drip irrigation system fed partially by water collected from the adjacent penthouse roof. Roofscapes contractor: Church Landscape. Landscape architect: Conservation Design Forum. Architect: McDonough + Partners.


Choosing the most appropriate environmentally responsible roofing material for your home may not be easy, but the benefits speak for themselves.

Conventional roofing materials tend to be failure-prone, and most find their way to a landfill within twenty years. Asphalt roofs encourage excessive storm water runoff that washes sediment and other contaminates into our water supply. In addition, roofs, buildings, and roads absorb tremendous amounts of solar energy and reradiate for hours afterward, creating a high-temperature phenomenon called the urban heat island effect, which alters weather patterns and speeds the chemical reaction that creates ozone, the primary component of smog.

Vegetative coverings, known as green or living roofs, can naturally address several major environmental issues—energy/resource conservation, storm water management, climate change, and smog—without taking up even an extra square foot of space.

In Europe, green roofs have been successfully used on homes, office buildings, and parking garages for more than thirty years (more than 10 million square feet of green roofs are installed in Germany each year). Now, several U.S. cities including Seattle, Chicago, and Philadelphia are evaluating the costs and performance of green roof systems for large buildings, and local grassroots groups such as the Northwest EcoBuilding Guild are also exploring ways to bring this technology to homeowners.

The benefits of going green

Green roofs do a lot more than just look good: The plants’ leaves catch dust, the roots filter water, the soil provides noise and heat insulation, and the entire roof provides a habitat for birds and insects. And that’s just the beginning. In urban areas, 75 percent of rainwater becomes surface runoff, and cities spend millions on sewer systems and treatment plants to address these periodic floods. Older cities that use combined systems for handling sewage and storm water cannot handle heavy rains; consequently, contaminates frequently overflow into nearby rivers. Green roofs offer an immediate ­solution. The soil and plant mats retain 75 percent of rainfall, and, even at full saturation, the soil and roots trap sediment from the runoff before releasing it. The retained moisture is released gradually, slowly introducing humidity that helps cool nearby areas.

Nonvegetated surfaces absorb incoming solar radiation and reradiate it as heat. During a typical ninety-five-degree day, for example, blacktopped surfaces can reach 175 degrees or higher. Temperatures in big cities average about 10 degrees higher than in neighboring areas. But computer models performed at the Lawrence Berkeley Laboratory in Berkeley, California, suggest that increasing green space by 5 percent in a city such as Los Angeles would lower summer temperatures by four degrees, resulting in 10 percent less smog and an annual energy savings of $175 million. “Cooling a city could reduce smog more than almost every other pollution-fighting measure,” says Hashem Akbari, a researcher at the lab.

Individually, green roof temperatures rarely vary more than fifteen degrees over a typical day, which helps reduce—or eliminate—air conditioning costs. In Germany, a developer recently used a green roof for a single-story shopping mall. The six-inch-thick roof was sufficient to maintain comfortable indoor temperatures without air conditioning.

Growing your own roof

Green roofs can be divided into two categories: intensive and extensive. Intensive gardens, such as the one recently built on the roof of Chicago’s City Hall, are intended for human interaction. Composed of trees, shrubs, and manicured landscapes, intensive gardens require a minimum soil depth of one foot. Although attractive, intensive roofs add considerable weight (80 to 150 pounds per square foot) and require regular maintenance—just like a regular garden.

Extensive green roofs can also look attractive, but they are built primarily for their environmental benefits. Requiring two to five inches of soil, extensive roofs add only fifteen to fifty pounds per square foot and are more suitable for retrofitting onto existing roofs. Extensive roofs require very little maintenance; once the plantings are established, they require only periodic watering and annual fertilization.

Surprisingly, building a green roof isn’t that difficult. Although a few manufacturers have patented special materials, nearly all roofs are composed of a few basic elements: a waterproofing layer, a root-blocking membrane, a drainage system, a growing medium, and, of course, plants. One company even sells a leak detection system that sounds an alarm the instant water penetrates the membrane.

Most waterproofing layers are made of polyvinyl chloride (PVC), a rubber membrane, or asphalt. PVC isn’t as environmentally friendly as the others, but it offers the best protection against leaks and root penetrations because the seams are heat-sealed. Rubber membranes are a good alternative, though extra care must be taken when sealing seams with tapes or adhesives. Liquid asphalt can provide a seamless surface, but because it’s an organic compound and can become food for various organisms, it must be covered with a layer of poly­ethylene sheeting.

After laying the root-blocking membrane, the soil is applied. Several green roof companies offer lightweight growing mediums, but in most cases the topsoil left over from construction works fine. (You may need to add clay to improve water retention.) Although some extensive roofs can support a healthy plant community with a two-inch base, thinner soil layers place higher physical demands on your plants; green roof designers recommend at least three inches if you’re mixing the soil yourself.

Contrary to what you may think, most green roofs are not made of sod. “Grass is too maintenance intensive,” says David Beattie, an associate professor of ornamental horticulture at Pennsylvania State University. “It requires intensive irrigation and regular trims.” Instead, Beattie suggests succulents and other low-growing plants that are capable of storing water in fleshy leaves, bulbs, or roots. “Think alpine conditions,” says Beattie. “Plants that have a natural ability to survive in regions with shallow soil, little water, high winds, and intense sun are good candidates for green roofing.”

Initially, roofs may need to be fertilized and watered, perhaps once a week, to encourage the seedlings to grow. Within six months, extensive roofs can usually be left on their own. Don’t worry about having to drag the lawnmower up the ladder—green roofs don’t need cutting. Because thin soil layers do not support tall growth, vegetation spreads horizontally.