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

By Nancy B. Solomon, AIA

The main draw for green roofs in Germany is, undoubtedly, their environmental benefits. Top on the list is their contribution to storm-water management. In highly developed, densely populated regions, the sewer infrastructure can be overwhelmed by the amount of water that quickly flows off conventional roofs during a heavy downpour. A green roof slows down, reduces, and even cleanses storm-water runoff. “It’s like having a giant sponge on the roof,” says Brian Lambert, marketing manager with the Garland Company, a roofing manufacturer based in Cleveland.

This permeable surface absorbs and retains water, allowing much of it to evapotranspire, and only very slowly releasing the rest to the ground—at which point it has a chance of reaching an aquifer rather than simply disappearing down a pipe. According to an industry rule of thumb, notes Roger Schickedantz, AIA, associate partner at William McDonough + Partners in Charlottesville, Virginia, a green roof with low-growing vegetation can absorb about 70 percent of the rainwater it receives. And air pollutants that often get swept up with the precipitation, including nitrogen and phosphorous, are filtered out by the vegetation before they can pollute groundwater or streams.

Another major factor is a green roof’s contribution toward alleviating the heat island effect, an ever-increasing problem in major urban centers. Exposed blacktop roofs are notorious for reaching very high temperatures in the summer and raising the ambient air in the process. Plants counteract this phenomenon. In addition, vegetated roofs help moderate a building’s internal temperature, purify the outside air, and provide habitats for plants, insects, and birds.

Green-roof proponents list other benefits, as well. Although more expensive up-front, green roofs may be more economical over time: Stewart Comstock, water-resources engineer at Maryland’s Department of Environment, in Baltimore, estimates that, in five to seven years, operational savings in terms of energy consumption and maintenance costs will offset the construction premium associated with a simple green roof with low-growing vegetation. In addition, living roofs provide aesthetic and psychological relief to a concrete-and-asphalt urban jungle.

Transplanting to North America

As green-roof technology has improved and our own environmental challenges loom ever larger, government leaders and clients are slowly realizing that the benefits of a living roof can also apply on this side of the Atlantic.

In fact, green roofs are gradually being added to the regulatory toolbox for storm-water management in the United States. Maryland’s Department of Environment, for example, recommends that local jurisdictions consider it a permeable surface. Taking heed, Anne Arundel County now equates green roofs with grassy surfaces when calculating the amount of water that a new development must retain and treat, says Merrill Plait, chief engineer for the county’s Office of Planning and Zoning.

 

Chiropractic Health Center, Luzerne County, Pa. Although most often applied to flat planes, green roofs can also be installed on sloped and even curved surfaces (left and below). Here, plastic lumber battens were attached at the steepest sections to prevent soil slippage. Fabric mesh held soil in place until plant roots took hold. Water drains freely, although slowly, from green roof to eave. The extensive roof was planted with a type of sedum (far below). Van der Ryn Architects of Sausalito, California, designed the center.

With regard to the urban heat island effect, one highly visible experiment with green roofs is taking place in Chicago. Concerned about the public health threat due to the increasing summer heat differential between downtown Chicago and its outlying suburbs, Mayor Richard M. Daley encouraged the Chicago Department of Environment to look into possible solutions. This effort has resulted in a 20,300-square-foot green roof on top of Chicago’s City Hall [record, February 2003, page 79]. The pilot project, completed in 2001, will test various plant types and green-roof systems for this region. The design team included Weston Solutions, an environmental engineering company based in West Chester, Pennsylvania; McDonough + Partners; and Conservation Design Forum, a landscape architecture and planning firm in Elmhurst, Illinois.

Ever pragmatic, U.S. building owners are being motivated not just by environmental gains: “Longevity is one of the places where our clients see benefits,” observes Russell Perry, AIA, managing partner at McDonough + Partners. To understand why, consider the three conditions that typically damage a conventional, black-membrane roof: brittleness caused by the sun’s UV radiation; thermal shock due to temperature differential between top and bottom layers; and punctures resulting from pedestrian traffic or dropped tools. The vegetation and soil layers of a green roof protect the membrane from all three.

Assuming the membrane is properly installed at the beginning, it should last much longer than an exposed roof membrane. No one knows for sure, as green roofs have not been in place long enough, but some in the industry have suggested a doubling or even quadrupling of the membrane’s life, thereby estimating a life expectancy of 20 to 40 years.