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Native Green Roofs

green roof research array at the Wildflower Center

With concern growing about climate change and the urban heat island effect, vegetated (green) roofs are gaining popularity as a way to mitigate threats such as these. The Wildflower Center has been conducting studies since 2006 to determine the best ways to build green roofs to take advantage of their environmental benefits.

The Center published a comprehensive review of manufactured green roofs in 2008, revealing why it is important to evaluate green roof capabilities. Among the findings in the Urban Ecosystems article was that 6 different manufactured green roof products varied widely in how well they handled stormwater. In particular, some soaked in little rainwater, while others worked well at absorbing it, allowing time for the soil media to purify the water of impurities before it left a rooftop and hit the ground. In addition, the marked impact of green roofs on a building's cooling costs—a 21 percent drop in some cases—was confirmed when compared to a traditional, tar-based blacktop roof.

Currently, Dr. Mark Simmons and colleagues at the Wildflower Center are investigating how to improve green roofs for use in the Southwest, where their building cooling and other benefits could have a dramatic impact. A new study funded by Huntsman International and others is under way to determine whether a layer on green roofs that traditionally drains water away from rooftop plants can be improved so users in the Southwest can retain more of that water and irrigate less. The stormwater handling capabilities and other features of green roofs will also continue to be evaluated.

On a green roof, the plants and growing medium or soil take the place of the asphalt and gravel or shingles on the roof. The roof includes a waterproofing layer, the drainage layer under study, a growing media and the plants covering the surface. Wildflower Center projects use regionally appropriate native plants, preserving the natural heritage of the region.

layers of native plant green roof research at the Wildflower Center Layers in a Green Roof.

Environmental Benefits
Green roofs can help cities and improve the quality of the air and reduce stormwater runoff while also improving the energy efficiency of the building beneath them. Below is a recap of major green roof benefits:

Reduction of heat island effect - The heat island effect is the difference in temperature between urban areas and the surrounding countryside caused by a lack of vegetation and a large number of reflective surfaces that absorb heat. Wildflower Center research shows that green roofs can be up to 80 degrees cooler than adjacent buildings with traditional roofs.

Reduction of storm water and pollutants - Green roofs retain and filter storm water through vegetation. In areas where rainfall contains significant pollution loads, green roofs should be able to improve the water quality of the stormwater. Green roofs also help air quality by filtering pollutants from the air while adding oxygen to our urban environments.

Increased energy efficiency of building - The cooling and shading properties of a green roof can decrease the amount of heat flowing into a building and can significantly reduce air conditioning requirements.

Wildlife habitat - Particularly in urban areas, green roofs can create valuable habitat.

Aesthetic Value - Green roofs covert typically wasted space into valuable aesthetic and recreational amenities.

Longer lifespan - Plants and other layers on the green roof help protect the roof membrane from extreme temperatures, hail, foot traffic, and direct sunlight. This can prolong the life expectancy of the roof. The initial cost of installing a green roof, which is higher than traditional roof systems, can be partially offset by reduced roof replacement costs in the long run. Results from the first round of tests have been published in the journal Urban Ecosystems. The next round of research currently underway and funded by Huntsman Inc. with additional support from Netafim, Ewing irrigation, Acme Brick, and JDR Enterprises, is looking at optimizing green roofs for our hot climate but simplifying design using recycled and foam products in the growing media.

Live roof temperature data can be found here

Species List from Wildflower Center Research currently being tested