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

By Nancy B. Solomon, AIA

Anatomy of a green roof

Like all roofs, the most important characteristic of a green one is its ability to keep water out of a building. It does this with several protective layers: waterproof membrane, drainage system, fabric filter, growing medium, and vegetation. Depending on the particular system and project, a layer of insulation may be installed below the membrane, a root barrier may be added above the membrane, a wind barrier may be placed above new plants, and irrigation may be installed as part of the roof system.

The waterproof membrane is the most critical layer. “Vegetation can always be replanted,” says Lambert, “but you’ve got a big problem if the building gets a leak.” After installing the membrane, he recommends waiting long enough before planting to see if there are any holes. “Do a flood test. Try to make it leak. You want to make sure the materials and labor are top-notch,” he cautions. Different green-roofing companies promote different membranes—from multi-ply modified bitumen to PVC—often depending on what their sister roofing company sells for a conventional roof.

 

20 River Terrace, Battery Park, New York City An intensive green roof on the 17th floor (above) and an extensive one on the 27th floor are tied to a gray-water system: Roof runoff and other building wastewater, collected in a basement cistern, will irrigate the midair gardens. In a typical building, roof runoff is not reused. The project was designed by the New York office of Cesar Pelli & Associates and the New York landscape and urban design firm Balmori Associates in accordance with Battery Park City Authority’s Green Residential Guidelines.

A root barrier is usually specified when tall plants or small trees are included in the landscape design. It may not be needed when low-growing vegetation with very fine hairlike roots is planted. In this latter case, however, a root barrier may still be installed to protect the membrane from larger plants that could take root if seeds migrate from other areas by birds or wind.

A drainage system is required below the soil layer to handle excess water due to very heavy precipitation. The system essentially consists of an elevated air space into which water can collect and, once it reaches a certain level, flow off the roof—typically through interior drains. As with membranes, the drainage layer varies from manufacturer to manufacturer: One, for example, relies on a plastic tray whose profile resembles an egg carton, while another installs a layer of gravel. Some drainage systems are designed so that a portion of the collected water can be saved and recycled back into the soil during arid periods. In comparing drainage systems, architects should be cognizant of how much water they can hold for how long a period, and how much they weigh. A very fine cloth filter is placed between the growing medium and drainage system so that only water can pass through.