The Dirt

Steven Peck, Hon. ASLA, kicked off the Washington, D.C. meeting of his organization, Green Roofs for Healthy Cities, by stating the U.S. green roof market grew 30 percent in 2010 despite the challenging economy. A survey of corporations involved in green roof design and development found that 8-9 million square feet of green roofs were added last year. Much of this growth occured in cities like Chicago, New York, and Washington, D.C., which offer significant ”public policy support” for green roofs.

Up to 30 percent of many cities’ surfaces are rooftops, meaning there are “huge opportunities for expansion.” He believes adding green to roofs is increasingly critical given urban boundaries continue to expand at a rate of 1 percent per year, meaning “we are destroying nature and wildlife habitat at a rapid rate.” Some of that nature can be restored on roofs.

Furthermore, green infrastructure projects create lots of local jobs. “Green roofs are highly labor intensive and you can’t export these jobs.” Making the case for increased public investment, he said adding green roofs to 1 percent of U.S. buildings would cost $9 billion and create 190,000 jobs. For 5 percent of buildings, the cost would rise to $48 billion and create almost a million jobs. Finally, if 10 percent of all buildings had these green infrastructure systems, the cost would be $96 billion but almost two million jobs would be created.

According to Peck, green roofs have many financial benefits. His calculations show their average stormwater mitigation benefit is $4.26 per square foot. He cited studies showing how increased greenery in urban areas reduce crime, which is expensive for communities and governments. Rooftop greenery increase property values. “A view of a green roof improves property values for nearby buildings by 11 percent; in comparison, a nearby park leads to a 20 percent increase.” These systems also act as sound absorbers. While the benefits are hard to quantify, a large building near an airport was able to reduce significantly reduce noise by creating a large-scale green roof, creating a nearly one million dollar benefit. Lastly, Peck said rooftop solar panels work better if integrated with green roofs systems: their efficiency is increased by 5-25 percent. 

At the conference, the U.S. General Services Administration also presented findings from a Congressionally-mandated research project on the costs and benefits of green roofs on government buildings in the D.C. metro region. Ken Sandler, Office of Federal High Performance Green Buildings, GSA, said Arup, PennState, the University of Toronto, and Columbia University did the research, which involved a review of literature and case studies. Some findings: green roofs retain 1.5 inches of stormwater runoff. They can reduce acidity in rain but are “mixed on other nutrient pollution.” These systems can increase biodiversity, particularly birds, bees, butterflies, wasps, and beetles, if they include native plants. “The more diversity of plants, the better.” In addition, these systems reduce the urban heat island effect but “more precise data and research is needed.” On energy, they can reduce solar gain by 84 percent, and reduce energy use by 6 percent in the summer and around 1 percent in the winter (Sandler noted these are conservative figures). On health and productivity, he said “there isn’t enough data.”

According to Sandler, the most important benefit of green roofs is their ability to increase rooftop longevity by 25-60 years, “effectively doubling the lifetime of any roof. Any added costs of building or maintaining a green roof over time are made up by the increased longevity of the roof.”   

In one omission in their study, the government didn’t explore whether green roofs are carbon neutral and how much green house gases they sequester over their lifetime. A lifecycle analysis of the primary materials that go into green roofs, along with the process of construction and installation, also needs to be done to determine the overall carbon footprint of these systems. For example, some green roof manufacturers are using lots of PVCs, said one attendee.

Learn more about Green Roofs for Healthy Cities’ upcoming national conference, CitiesAlive 2011.  

Image credit: ASLA 2010 General Design Honor Award. Neuva School, Hillsborough, CA. Andrea Cochran Landscape Architecture / Marion Brenner