Multiple benefits, new technologies are building up green roofs.
By Sonja Hegman
Green roofs are gaining in popularity as more people acquire knowledge on the subject. Awareness of green roofs as an eco-friendly concept is also being nurtured in the design and architectural communities. And building owners are not as dismissive of green roofs as they may have been only a few years ago.
“It doesn’t take much to overflow the water drains in cities,” says Ed Snodgrass, owner of Emory Knoll Farms in Street, Md. “As we become more urbanized, water has nowhere to go. Cities have rivers flowing down the middle of them. Green roofs are a way to decrease impervious areas in cities.”
Green roofs also cool the environment, says Jay Hudson of Rehbein Turf in Blaine, Minn. He added that green roofs enhance roof life, absorb stormwater; and regulate, release, clean, and filter the water while providing beauty for the area. Rehbein Turf has installed two green roofs in the Twin Cities—one, a retrofit for a private residence, and the second on a new building for a Minneapolis company.
Europe is seen as the leader in green roofs, says Jon Crumrine of Teufel Nursery in Portland, Ore. “Because it is more densely populated than the United States, Europe hit the threshold for pain in their sewer infrastructure at an earlier point than the United States,” he says. “This encouraged the development of ways to reduce stormwater runoff from the impervious surfaces created by new development—including green roofs.”
Today, many large U.S. cities, burdened by ever-expanding development, have hit the wall in terms of sewer infrastructure capacity. “It’s really, really expensive to add to or replace sewer systems,” Crumrine says. “It’s less expensive to find ways to reduce what goes into them.”
But in the United States, there are a number of factors that hinder green roof implementation.
In Germany, for example, people are much more eager to green their public and private urban spaces, says Charlie Miller, principal at Roofscapes Inc. in Philadelphia. Another factor, he says, is the emphasis on using smaller contractors. Much of their equipment is tailored specifically to small-scale construction, and smaller-scale contractors can be found at work everywhere, Miller said.
Finally, there is the willingness of most Germans to accept limitations placed on the use of their property. This translates to a business environment where government can require property owners to take responsibility for managing stormwater runoff on site.
In contrast, in the United States an emphasis has long been placed on large-scale public works, Miller said. “A lot of political momentum is behind large public works projects, and Americans generally see these projects as providing jobs and helping the economy,” he said.
Cost is also a deciding factor when looking at green roofs. Public incentives and regulations that will promote the installation of green roofs must be implemented for decades before the benefits can be fully realized, Miller continued.
In the United States it is difficult to implement programs with such long time lines. The encouragement and implementation of green roofs, and other low-impact development measures, would cost less in the long run and would establish an approach toward urban development that would be sustainable. “Instead, the tendency will be to promote more-expensive, short-term solutions,” Miller says.
Snodgrass agrees that price is a major factor for implementation of green roofs in the United States. He added that the ability of a building to support the weight of a green roof is another factor. Because of this, few retrofits are being installed. Green roofs are mostly being seen in new construction, he said.
Snodgrass said he believes that the green roof market will increase in the United States, but several things must occur: more installers are needed, government has to act to pick up or reimburse the added expenses, and stormwater management polices must be in place. Also, green-roofing companies can start when, for example, a greenhouse and contractor merge services. There are few companies like this now, he said.
The current popular market for green roofs is with colleges, universities, and government buildings. “These places are more likely to install a green roof because their buildings are more long term,” Snodgrass said. “The longer the intention of the building, the more likely a green roof will be installed.”
Miller, from Roofscapes, installed his first green roof in 1998. “It has been our observation that for the succeeding five years, the market grew about 50% each year,” he said. “2005 marked the beginning of an acceleration in market growth. I would say that projects built or contracted for construction doubled in 2005 compared to 2004. The overall market, however, has a long way to go.”
Benefits of a green roof are generally divided into public and private sectors. With a private building, the owner and perhaps surrounding observers benefit from the green roof. A public building with a green roof provides more green space to the population at large.
Jon Crumrine of Teufel Nursery noted that among the most common green roof plants are succulents. He says these types of plants are often used for extensive green roofs or lightweight green roofs. This also greatly limits the plant palette. However, the objective of the extensive green roof is environmental rather than aesthetic.
Incentives for green roofs in the U.S.
In the United States, various incentive programs could be implemented to promote the use of green roofs.
Dhani Narejo, a product manager at GSE Lining Technology in Houston, says these incentives could include: tax breaks, new zoning laws, grants, engineering assistance, and insurance assistance for builders.
Miller added that the most successful incentives for green roofs often involve the use of floor area bonus trades for green roofs. In this approach, a developer is granted a zoning exception that allows an increase in the size of the building footprint or to add floors. The developer increases the value of the building, which then offsets the cost of the green roof. The municipality in turn gains a larger tax base.
Another approach, which has not been tried, he says, is mitigation trading. The concept of mitigation trading is to identify stormwater control goals for a community.
“Owners of buildings that are ideally suited for green roofs could install large and efficient green roofs and ‘sell’ some of this mitigation value to nearby property owners for which green roof construction is impractical or too costly,” Miller said. “We would also like to see municipalities change their stormwater regulations to emphasize control of the type of storms that green roofs are best at controlling—for example, high-intensity but short duration rainfall events.”
Use of geosynthetics
Some geosynthetic products are being tailored for use in green roofs, Narejo said. For example, geotextiles and drainage geocomposites are the same products used currently in other civil and environmental applications. However, other products, for example geomembranes, will be quite different because of unique requirements of roofs.
Miller says innovations of the last 20 years have leaned toward developing approaches to install lightweight landscapes on buildings. Thick and heavy plaza landscapes—usually with trees and pavement—have been around for 60 years in the United States and are familiar to everyone.
“However, the advent of landscapes weighing only a fraction of old plaza designs and offering a convincing natural environment on lightly built structures opens up a new area for design innovation in contemporary architecture,” Miller said. “This new approach would not be possible without the use of high-performance fabrics and geocomposites.”
Geosynthetic materials used in extensive green assemblies are diverse, including: waterproofing membranes, root-barrier layers, capillary fabrics, soil-reinforcement textiles, geocomposite drainage layers, and water-retention layers.