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Green Roof

Green Roof Research at the Environmental Protection Agency

While the city of Denver, Colorado is famous for its mile-high elevation, employees working in the Environmental Protection Agency's regional headquarters there are thinking slightly higher. The LEED-certified office building they occupy downtown is topped with a "green roof," some 40,000 plants growing in a network of two-foot by four-foot modular trays. The roof is the focus of a collaborative research project exploring the practicability and environmental benefits of cultivating green roofs in such high, semi-arid climates.

"Even before we moved into the building, we worked with the developer to help design a workspace that that could serve as model for minimizing the environmental footprint of the typical office building found in an urban setting. Our green roof is the most conspicuous-and I think the best-part of that effort," explains EPA regional scientist Patti Tyler, the project manager for the green roof research project.

Tyler serves as the Regional Science Liaison (RSL) for EPA's Region 8 (the intermountain states of Colorado, Montana, North and South Dakota, Utah, and Wyoming). As such, she provides an important, scientist-to-scientist link between her region and the Washington, DC-based EPA Office of Science Policy, a science policy and management arm of the Agency's Office of Research and Development.

RSLs from across each of EPA's ten regions work directly with counterparts in the Office of Science Policy to form a network of scientists that collaborate in order to keep communications about the latest research findings and outcomes flowing to the locations where they are most needed.

One way the Office of Science Policy manages and supports the RSLs is through its Regional Applied Research Effort (RARE) Program, which provides $200,000 to each region to develop an annual research topic. Each topic is submitted as a research proposal for extramural funding to a specific lab within the Office of Research and Development. Once approved, proposed research is conducted as a joint effort, allowing regional staff to leverage the specialized, technical expertise of lab scientists and engineers in pursuit of solving local environmental challenges.

Green Roof Vegetation Growing Over Drip Irrigation

That's where the green roof project comes in.

A collaborative team of EPA researchers from Region 8 and the Agency's National Risk Management Laboratory, together with partners from Colorado State University (Department of Horticulture and Landscape Architecture), the Denver Botanical Garden, and the City of Denver are conducting a comprehensive set of studies to quantify the environmental benefits of maintaining a green roof atop EPA's Denver offices.

While installing green roofs has been gaining in popularity in temperate areas marked by relatively ample rainfall such as Europe (green roofs are particularly popular in Germany) and the Northeastern U.S., developers, architects, and building managers have much less experience installing them in places with low rainfall and the solar extremes presented by high altitudes. EPA's green roof is the first of its kind in the state of Colorado.

One of the research program's primary objectives is focused on what makes a green roof green: live plants. "We need to know how to measure the biological performance of the roof: figuring out what plants do best in our semi-arid climate, and how they yield the desired environmental benefits" explains Tyler. To help, Jennifer Bousselot, a Colorado State University doctoral student in horticulture, is building a profile of the combination of plant species and management techniques best suited to keep the vegetation alive in the harsh growing conditions presented by the hot, dry climate of an inner-city rooftop.

The horticulturalist has plenty to work with. The vegetated roof covers approximately 20,000 square feet, spread across three terraced levels. The plant community is composed 40,000 individual plants, primarily of six different sedum species, along with grasses, perennials, and other groundcovers selected in accordance with the U.S. Department of Agriculture's hardiness zone classification for Colorado. The plants are growing in modular trays filled with four inches of engineered growth medium.

As you would expect, all that green offers an attractive, welcoming patch of nature in a sea of the more typical tar and asphalt roofs perched high above the concrete roads and sidewalks of the city. Local birds, bees, and butterflies have definitely taken notice. "In addition to some serious science, one of the great things about the project is beauty of the urban habitat that's been created," says Tyler.

One of the terraces has been outfitted with patio furniture, giving EPA staff the opportunity to enjoy a mid-morning coffee break in the company of busy honey bees or the occasional red-shafted flicker (Colaptes cafer collar) taking refuge among the plants. The roof has also become a destination for visiting architects and tour groups from the Denver Botanical Garden.

As satisfying as it is to know the roof above you is helping to keep the local honey bees in business, the real attraction for EPA and others dedicated to environmental management is the green roofs potential to deliver measurable environmental benefits.

The green roof research team is closely monitoring how much less stormwater runoff the EPA building contributes to the watershed during rain events or when snow accumulated during colder months starts to melt. Rain and snowmelt absorbed by the plants and the growth medium they sit in should significantly reduce the amount of water that runs off the building, flowing into the Denver sewer system or flowing toward the South Platte River. To measure just how much less, the team has installed monitors on both the EPA building and an office with a comparably-sized roof across the street.

The results will provide important insights into how effective a green roof is for stormwater management, an increasingly important and expensive activity for many American municipalities. Working with the City of Denver, EPA hopes to be able to offer the data necessary to add the installation of green roofs to the list of "Best Practices" water managers can offer to mitigate the environmental impacts of stormwater runoff from developed areas, improving the health of the nation's impaired waterways such as the South Platte.

Of course there is another type of green that is making living roofs increasingly attractive., particularly with property managers: money. As energy costs for heating and cooling large workspaces continues to rise, anything that makes buildings more efficient is attracting attention. Here, too, green roofs have something to offer.

The research team is measuring the difference between a typical roof, topped with dark roofing materials, and their roof. "We are working to quantify the potential for living roofs to absorb solar radiation instead of adding to the urban island heat effect caused when thousands of sun-baked roofs collectively raise ambient air temperatures across the city," explains Tyler. A set of weather stations and monitors on the EPA building and the one across the street measure ambient temperatures, solar radiation, and wind.

Preliminary results show dramatic decreases in temperature just below the canopy of plants, suggesting less heat available to be absorbed into the interior of the building. The next step is more analysis to directly quantify how the temperature differences translate into energy savings.

The EPA's green roof and its associated research are starting to get the attention of the neighbors. "It's such a natural for EPA to better understand the benefits of green roofs. Now that our collaborative research project is up and running, we're eager to share the story," says project manager Patti Tyler. Officials from the University of Colorado at Denver recently paid her a visit to gather information about a green roof of their own.

Habitat for birds and bees. Reducing stormwater runoff. Energy savings. There seems to be much promise in green roofs. Quantifying how much that promise offers in terms of real world environmental management practices and cost saving is at the heart of EPA's research into green roofs. It's just part of thinking slightly over a mile high.

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