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Roofing technology developed in Germany is starting to take root in North America

By Nancy B. Solomon, AIA

Engineered soil and hardy plants

When it comes to living roofs, not all dirt is the same. In fact, the medium in which the vegetation is planted must be carefully engineered for the system to achieve its expected life. It would be a big mistake, for example, to install a soil that is more than 20 percent organic: “The plants will look great for the first two years, but then roof health will deteriorate because organic soil disintegrates quickly,” says horticulturist Ed Snodgrass, who studies and grows green-roof plants on his family farm, Emory Knoll Farms, in Street, Maryland (www.greenroofplants.com). Again, each manufacturer offers its own special blend that considers weight, water capacity, and compaction over time.

The depth of the growing medium and species of plant distinguish the two basic types of green roofs—extensive and intensive. Extensive green-roof systems have very shallow beds, in the order of 3 to 6 inches, that support only very hardy, low-growing vegetation. These are the most practical and economical green roofs, as they are lightweight—weighing little more than a traditional ballast roof—and require little to no maintenance or irrigation.

 

Earth Pledge Foundation, New York City Earth Pledge Foundation, a nonprofit environmental organization based in Midtown Manhattan, promotes the construction of green roofs in New York City through its Green Roofs Initiative program. The foundation’s own 600-square-foot intensive roof garden (below), used for promotional events, lunch meetings, and informal entertainment, was designed by Balmori Associates and completed in 2002. Principal Diana Balmori experimented with perforated plastic screens at the roof perimeter (above) to moderate wind. They were not installed, however, due to budget limitations. Photography: © Earth Pledge

With deeper beds, intensive systems can grow many more varieties of plants—and even small trees. They can be much more aesthetic than extensive roofs, easily lending themselves to a gardenlike atmosphere. But the additional soil and taller vegetation translate into higher roof loads, so additional structural support is often necessary. This type of roof landscape typically requires regular maintenance—including watering, fertilizing, and mowing—that adds to ongoing operation costs. And nutrients from the fertilizer, which is water soluble, can slowly make their way to local water bodies, where they can add to the ecological havoc.

A lightweight growing medium can easily be lost to the wind until stabilized by mature plants. A mesh made out of a biodegradable material, such as jute, is recommended to cover the dirt temporarily. Alternatively, in some proprietary systems the plants are grown in plastic trays or sodlike mats on grade, so that they are fairly mature and secure in the growing medium before these components are lifted into place.

Rooftops can be harsh environments. The plants that survive the best are those that have adapted to similarly harsh locations. Imagine the handful of plants you would find growing on a windswept rock outcropping high up on a mountain, scorched by intense direct sunlight in the summer, frozen by snow in the winter, and drenched by rain in the spring. “That’s what you want to plant—not roses,” explains Perry. Fortunately, such hardy plants can be very attractive, and they come in many different colors, not just green.

By and large, the plants that seem to fare best on U.S. roofs are the same ones promoted in Germany: succulents, with varieties of sedum being the most common. Because of their particular metabolic processes, succulents are very adept at conserving water and can therefore survive drought conditions. And, when planted in the appropriate region and soil conditions, many do not need ongoing fertilization because the nutrients from their dropped leaves are simply reused. “It’s self-sustaining,” observes Perry.

But conditions—in terms of temperature fluctuations, light, rain, wind, maintenance, occupant access, to name a few—do vary from roof to roof, necessitating one type of plant over another. Priorities also vary: In the U.S., for example, there is a lot of pressure to use native plants on roofs despite the fact that, according to horticulturist Snodgrass, “we don’t yet have any proven natives.”

Whether drought-tolerant low-growing vegetation or thirstier larger shrubs, all plants need sufficient water to get up and running. Often temporary, above-soil irrigation systems are installed for the first growing season. “It’s used as an insurance policy,” explains Lambert. If aesthetics is very important, a permanent irrigation system may be installed about an inch below the soil to deliver water to the roots as needed.