Green Roof for the Center for Urban Ecology

Green Roof for the Center for Urban Ecology

The Center for Urban Ecology, in Washington, DC, staffs the Natural Resources and Science Division for the National Capital Region. It’s appropriate that this unit of the National Park Service has a vested interest in utilizing sustainable technology that reduces and/or mitigates the adverse impacts created by impervious surfaces in an ultra urban area. Currently, 37 percent of the nation’s capital is covered with impervious surfaces such as roads, buildings and parking lots. Serious impairment to streams usually begins when the amount of imperviousness in a watershed reaches 10 percent. Planners are attempting to utilize a number of tools to protect watersheds from the pressures of increased development but one that is often overlooked in the United States until recently is a “green” roof. This technique can be utilized on structures with flat or sloped roofs to mitigate environmental degradation related to the ever increasing amount of impervious surface and provide for additional green space.

The Center for Urban Ecology has taken action to reduce the anthropogenic impacts on the environmental attributable to their facility. A 7000 square-foot lightweight extensive green roof has been installed for its ability to detain stormwater, improve insulation properties and conserve energy, prolong roof life, reduce ambient air temperatures and thereby mitigate the urban heat island effect, fixing dust and particulate matter, and provide habitat for birds, bees and butterflies.

The idea for a green roof for our facility came about after an Exchange Team sponsored by the U.S. Environmental Protection Agency and the National Park Service and comprised of leaders from agencies and non-profit organizations with an interest in urban watershed management in the Potomac River watershed, visited with watershed professionals in Germany. Countries such as Germany have tackled similar urban watershed issues as experienced here in the nation’s capital. Germany has long been viewed as a leader in sustainability. For example, policy tools such as the “impervious surface tax”, which requires owners to pay for cleaning stormwater runoff attributable to their property, have propelled a national trend toward green roofs. By planting grasses and succulents on roof tops, stormwater is absorbed, cleaned and then discharged slowly into the natural environment. It’s estimated that over five percent of commercial and residential buildings in Germany have green roofs.

Green roofs are fundamentally divided into two categories, intensive and extensive. Intensive green roofs usually consist of a thick soil or substrate capable of supporting root growth for larger shrubs and trees. They are heavy systems and thus can have major structural implications for the building. Extensive green roofs normally have a thin layer of soil or substrate. They are lightweight systems with minimal structural implications for the building. One of the functional constraints of retrofitting a building with a green roof is the weight-bearing capacity of the existing roof structure. For this reason, lightweight extensive green roofs (soil media usually two to four inches thick) are becoming increasingly popular for their ability to provide environmental benefits and extending roof life. However, before you can even consider a green roof for an existing structure a professional structural engineer must conduct a structural analysis of the roof truss system to ensure that the structure can withstand the extra loading. We were initially disappointed to learn that a section of the building with 2 X 12 wood trusses would not support a green roof, but were delighted to hear that the larger part of the building with steel web joists were strong enough to support the additional loading.

The green roof consists of a wood deck overlying the steel web joists, a rigid insulation board that is tapered so it provides positive drainage to the gutters, and an 80 mil. PVC roofing membrane. The plants and soil media are contained in 2 ft X 2 ft plastic flats that contain 3 inches of planting media. The flats are placed right on top of the roofing membrane. The planting medium consists of 75 to 85 percent inert expanded slate with 15 to 25 percent organic material. This inert material is important to the long-term success of the green roof. Over time an all organic soil will decompose and erode. To eliminate the burden for the continuing loss of soil a durable lightweight planting medium is used to prevent compaction and maintain the necessary pore space for water and oxygen retention. Green roofs are a long term investment, so you don’t want to design something that will require ongoing maintenance, like replenishing soils and plants.

A compelling sustainable design argument for green roofs is that they prolong the life of conventional roofs by protecting them from ultraviolet radiation and extreme temperature fluctuations—the two primary sources of roof membrane degradation. On a conventional asphalt roof, for example, annual temperatures may fluctuate 170 degrees. Green roofs, however, dramatically reduce the temperature differential. A study conducted by the Chicago Department of Environment found that on a 100-degree day, the surface temperature of a blacktop roof reached 165 degrees Fahrenheit, while a green roof was only 85 degrees. Not only do green roofs save money by lengthening the life span of roofing membranes multiple times, they also yield short-term savings by insulating buildings and reducing energy use. Research shows that green roofs can cut energy costs in half in summer and by 25 percent in winter.

Ed Snodgrass of Emory Knoll Farms, the nursery that provided the plants, has been testing and cultivating green roof plants for the past five years at his nursery north of Baltimore, Maryland. “The Germans have been testing and building green roofs for the past few decades,” says Snodgrass. “But their climate is milder than ours, and some plants that succeed in Germany often wither under the hot summer conditions here in the United States.” Snodgrass specified the soil and selected the plant palette for the Center for Urban Ecology. twenty species of plants were selected from the sedum, phlox, thyme, and sempervivum genera, and roughly 14,000 plugs were installed at eight plants per flat. Certain plants were selected for their aesthetics and year-round interest. However, the overarching factor guiding plant selection was long-term viability.

A green roof is easier realized than commonly believed. The weight of an extensive green roof is similar to that of gravel ballast so that structural upgrades are not always necessary. An extensive green roof with three to four inches of planting media will do well to reduce stormwater runoff, enhance the energy efficiency of a building and add green space. The relative design simplicity of a green roof and their significant benefits offer great potential for the many rooftops in US cities. As managers become more familiar with the technology and performance of green roofs they’ll be able to get past their resistance to install green roofs because of unsubstantiated fears that they will leak or be too heavy with the additional loading. The U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) green building rating system provides incentives for the use of green roofs. The Sustainable Sites section encourages the use of green roofs as an integrated component of stormwater management and additional benefits can be gained for reducing the urban heat island effect.