Green roof interest rising
By CAROL KINSLEY
Keynote speaker Barbara Deutsch told attendees at a Green Roof Symposium in Wheaton, Md., last month, “As a developer, I look at a roof as another floor we need to use.” With Washington, D.C., being one of the hottest real estate areas in the country, she’d plant that roof to generate economic opportunity and help the environment. In fact, there’s a “Green Roof Vision” for D,C. called 20-20-20 which hopes to have 20 percent of all roofs covered, a total of 20 million square feet, within 20 years. The expectation is that all new buildings will have green roofs and all new roofs will be green.
There are two kinds of green roofs, extensive and intensive. Extensive green roofs are planted in less than 6 inches of growing media and require little maintenance; intensive green roofs are more a rooftop garden with more soil, more plants — even trees — and more maintenance.
Green roofs are becoming more prevalent, particularly in Chicago where 2.5 million square feet of roof area is covered. Washington is the No. 2 area in the United States, with more than 500,000 square feet of green roof space, 80 percent of that extensive and 20 percent intensive.
Government buildings are going green to demonstrate the civic importance of green roofs. Developers and building owners, particularly of rental real estate, are finding “green sells.” There is a dollar value of being able to look out a window onto a garden and with accessible roofs, new “social space” is created.
Green roofs also can double the life of a roof by protecting the membrane from the elements, which also helps moderate the temperature. The extra layer of insulation provides energy savings also. A study in Chicago, where a heat wave in 1995 killed 500 people, showed that if all the buildings had green roofs, it would cut peak demand for electricity by one small nuclear power plant. Green roofs help meet storm water management goals, reducing volume and rate of runoff, and helping prevent pollutants from being washed from streets and yards into rivers.
“Every seven months, the runoff from U.S. roads is equal to the amount of oil spilled by Valdez,” Deutsch said.
A half inch, even one-tenth inch of rain overwhelms a combined sewer/stormwater system. Green roofs can hold as much as an inch of storm water and in Washington would go a long way toward mitigating the need for a new underground storm water system estimated to cost $2 billion.
A green roof also improves air quality by helping cool temperatures, absorbing air pollutants and producing less ozone. It also reduces noise pollution.
Deutsch said the cost of a green roof depends on whether it’s new construction or a retrofit, the size of the job and the source of material, including freight for hard goods. She said the estimated $7 to $15 per square foot, including a new membrane, was comparable to pavers.
“As the industry becomes more experienced, the cost will come down,” she said, adding her prediction that green roofs would become “just another type of roof system.”
Jack Sullivan of the University of Maryland, speaking as a downpour began at Brookside Gardens, noted that as cities are being “skinned over like a shell” the water has no where to go. He stressed that even on a green roof, water needs somewhere to go, so there must be good drainage.
Sullivan also noted that as much as one-sixth of U.S. energy goes to cool buildings. “On a 95-degree day, the roof can be 158 degrees, but a green roof will be 77 degrees,” he said. Green roofs also lessen heat demand in winter. “And they provide songbird habitat, bringing wildlife back to cities, and help neutralize acid rain.”
Sullivan showed photos of several large green roofs, including a 24.5 acre Millenium Park over underground parking in Chicago, Atlanta’s City Hall, and a 3-acre meadowland in Salt Lake City at the Church of Jesus Christ of Latter Day Saints. The Chicago Center of Green Technology even has its own wetland where water can go when it drains.
He shared tips green roofers are developing, such as planting trees above columns which can support the extra weight, and building up areas with Styrofoam rather than extra substrate.
Ed Snodgrass of Emory Knoll Farms, which grows plants for green roofs, also shared photos, including “rubble ways” in Europe where they let in whatever vegetation comes naturally to attract insects, which draw birds. From China there were photos of sedum growing naturally on terra cotta tiles.
Plants suitable for such gardens have lateral rather than tap roots, are drought and wind tolerant, grow horizontally more than vertically, do not produce wind-blown seeds (for protection of neighbors’ gardens), have low nutrient and maintenance requirements but are long-lived and lightweight at maturity.
Plants can be provided as seeds or cuttings, which are less expensive, as plugs, modules or mats. “The best green roof plants are juvenile, so they have time to adapt,” he said. In Europe there is a well-developed supply chain so contractors can get everything they need locally.
Robert Berghage of Penn State University offered a web site, http://hortweb.cas.psu.edu/research/greenroofcenter/index.html, and a “take home message.” He stressed these points:
• Media are not soils but are analogous to container media.
• A roof is a large pot, wide but shallow.
• Green roofs have physicial, chemical and biological charac-teristics, all critical to the success of the media. You don’t want to have to add substrate every year.
• If it’s not green, it’s not a green roof.
• In order of importance, the membrane is first, second is media, plants are third.
The substrate must support plants as well as retain water. It must have long-term stability, be pathogen and weed-free and non-toxic. And, it should be inexpensive, readily available, environmentally neutral (not requiring a lot of production) and lightweight. “It’s hard to find all these in one,” Berghage said. He recommended a particle size for one-eighth to five-eights inch, to promote drainage, and a dry weight of 32 to 42 pounds per cubic foot.
“It helps if it’s a new building. You can plan ahead,” he added. The long-term goal is to get away from the lightweight requirement, creating more opportunity for local sources, broader plant choices and greater media depth.