By Katie Navarra
Rooftops in the United States were traditionally only thought of in terms of their structural value. But with a growing interest in sustainability, building owners, architects, engineers and landscape companies recognize rooftops as an opportunity to manage stormwater, reduce the effects of the heat island, improve a building’s insulation value and provide a natural habitat in cities that have become concrete jungles.
A new revenue source and/or tax credits can be added benefits of green roofs depending on the geographic location and local laws.
A green roof, also called a living roof, is a rooftop that incorporates partial or full vegetation. A growing medium is spread over a waterproof membrane and plants are added. Depending on the site, root barriers, drainage and/or an irrigation system may also be a part of the green roof.
Green roofs have been around for centuries, with roots in Northern Scandinavia. The green roof concepts used today were first used in Germany more than 30 years ago. In recent years, green roofs have become increasingly popular in the United States.
“A green roof retains storm water, insulates the building thermally and acoustically, prevents the Urban Heat Island effect, and provides habitat for people and wildlife,” said John Magill, M.S. Horticulture, Green Roof Professional in Illinois. He added that green roofs are also creating a habitat for unusual, rare, ground nesting species that have been virtually been wiped out by cats at grade level.
“Based on the level of maintenance, depth of the planting media, and types of plants, there are two or three types of green roofs: extensive, intensive, and a combination of both,” said José Magan, owner of Accent Urban Design, a landscape and urban design and land planning firm in Homewood, Ill.
An extensive green roof can support 10 to 25 pounds of plant life per square foot, whereas an intensive green roof can support 80 to 150 pounds of vegetation per square foot.
Intensive green roofs typically include grass, shrubs and other plants that require regular maintenance compared to extensive green roofs that are designed to be self-sustaining.
“Even though all new plantings need water until they are established, the use of irrigation is optional and it depends on the climate and particular needs of each green roof,” said Magan.
As with traditional landscape designs, the project owner and designer can take liberty in selecting the look and plant material included; however, “there are two main goals in the design of every green roof,” said Magan. “Protect the integrity of the waterproof assembly, and guarantee that each component of the green roof system works well; if one component fails, then the entire system will fail.”
Before beginning any green roof project, consult with structural engineers to determine the best system for the building.
“Often though, the imagination of the designer is limited by two things; weight and money,” said Magan. “Thicker media green roofs (intensive) are capable of supporting a much wider range of plants, while the thinner and lighter systems are more limited in plant selection.”
Top trends in green roofs
Since designing his first green roof nearly 30 years ago in Cali, Colombia, Magan has watched the green roof industry evolve.
Most recently, he has seen the use of green roofs in combination with solar panels increase significantly. He also sees more green roofs used for urban agriculture.
Urban farms such as the Brooklyn Grange in Manhattan, New York have established lease agreements with landlords in New York City. The Brooklyn Grange leases the rooftop of a six-story building on Northern Boulevard in Long Island City, Queens. The one-acre rooftop hosts a farm that has produced more than 40,000 pounds of vegetables.
A second 1.5-acre site atop a building at the Brooklyn Navy Yard provides additional acreage for growing the farm’s most popular crops. In both cases, the building owner has capitalized on gaining new revenue for income while also receiving tax credits for “going green.”
Magan has also observed an increased use of diverse native plant species in green roof design.
“In many cases, designers choose to limit themselves to plants from the genus Sedum, but many plants have proven themselves in the Midwest on thin (4-inch deep) green roof systems, including Talinum calycinum, Opuntia humifusa, Sempervivums, Alliums (shoenoprasum, senescens, cernuum, stellatum), Koeleria (glauca or macrantha), and many others,” he added.
Older green roof systems in Europe incorporate grasses and other plants. “The possibilities are not close to being fully explored,” he said. “By using taller plants, more solar radiation is absorbed and radiated outwards away from the roof. This is a great positive for reducing energy bills.”
Magill, on the other hand, sees the largest trend as modular green roof systems, including those from manufacturers Liveroof, Greengrid, Firestone, and others.
He has also noticed an increase in retrofit and residential markets using green roofs. These smaller-scale green roofs allow for much more flexibility in design and a closer relationship with the building owner during the design and installation process.
Get involved with green roofs
Interested in getting involved in the green roof industry?
Learn as much as possible. “Read any information available, visit green roofs and walls, talk to professionals and construction people that design and install green roofs, and ask many questions,” said Magan.
Find a green roof and get involved. Magill was drawn to the industry after a rock climbing experience in which he saw the most unusual plants growing on the rock faces. While working on a Master’s degree at Southern Illinois University (SIU) at Carbondale, he learned about a green roof being installed on the one of the campus buildings.
“I became fascinated with the concept of creating plant life where once there was only an extremely uncomfortable environment,” he said. “Through my advisor, Karen Midden, I helped to install and maintain the green roof system for a year (at SIU Carbondale).”
Explore all the options. For professionals interested in the green roof business, Magill suggests deciding which path is the right fit. An individual can work in sales, design, installation or the maintenance of the green roof. Landscape architects are becoming more and more involved as well.
Katie Navarra is a landscape industry professional based in New York. She is also an accomplished author and freelance correspondent with more than 200 articles to her credit. She can be reached via e-mail at email@example.com.