In April 2006, the American Society of Landscape Architects (ASLA), headquartered in Washington, DC, dedicated its state of the art green roof. Fortunately, the Nation’s Capital has the advantage of a climate that doesn’t require special consideration for green roof design and maintenance. But for facility managers in places with more extreme weather conditions, it may be a different story.
2005 Green Roof Growth
North America now claims a total of 2,150,000 square feet of green roof space—evidence that acceptance of the concept is growing. A recent CNN article states that green roof installations in the U.S. increased 80% from a year ago. [“Green roofs’ growing more popular,” CNN.com, April 28, 2006.]
The pattern of green roof growth in 2005 resembles a kind of target, with seven of the 10 highest new footage totals installed in the East or Midwest. The rankings for 2005 are:
- Chicago, IL
- Washington, DC
- Suitland, MD
- Ashburn, VA
- New York, NY
- Culpepper, VA
- Austin, TX
- Arlington, VA
- Des Moines, IA
- Ottawa, ON (Canada)
The exceptions in this list are Austin (comparatively warmer than the norm), and Ottawa (decidedly colder). [Source: “What Makes a Roof Green?” by Jason F. McLennan and Peter Rumsey, Environmental Design + Construction.]
The article, “When Is A Roof More Than Just A Roof?” [by Jill Aronson-Korot, Today’s Facility Manager, January 2002] suggests that greening a roof may extend its life by 30% to 60%. Green roofs also help to diminish the urban heat island effect and lower heating and cooling costs. Generally speaking, an urban heat island is an area in a developed environment where the concrete and asphalt absorb heat, increasing the temperatures of surrounding areas. These areas absorb heat during the day, and are slow to cool after nightfall.
Another TFM article, “Green Roofs vs. Cool Roofs,” [by Jill Korot, January 2003] suggests that a one story building with a green roof might cut cooling costs by 20% to 30%. It also cites a study which claims that if all of Chicago’s roofs were “greened,” the city might save as much as $100 million in energy costs per year.
So how can skeptical facility managers determine if green roofs are right for them? Is this option in extreme climates manageable and affordable? What special considerations should be evaluated in a very hot, dry climate, or an unusually cold climate? Finally, by what means can a green roof under demanding conditions be cost effective? The following two examples may prompt facility professionals to consider the feasibility of a green roof in severe climates.
Phoenix, AZ: Optima Biltmore Towers
In order to have a sustainable, living roof, adequate water must be available for irrigation. This is a primary concern in Phoenix, AZ, where David Hovey’s Optima Group of Chicago designed a green roof system for Phoenix’s Optima Biltmore Towers.
The most challenging aspect of the design involved extreme heat and lack of precipitation. In Phoenix, the average summer high is 102.6 degrees F, the winter low, 44.6 degrees F.
Phoenix also lacks significant rainfall. Its wettest month is March, with only 1.07″ of rain on average. In June, the driest month, the city receives less than 1⁄10th” of rainfall—hardly enough to sustain a living roof. Hovey realized that, in this situation, indigenous plants would naturally survive the climate extremes more successfully than the sedum and grasses used in traditional green roof projects.
He investigated a variety of desert plants for the rooftop project and ended up installing those that grow on area mountains. He also used the kinds of plants that thrive in the foothills for the lower levels of the building. Hovey was able to replicate the mountain environment and build a facility that would settle comfortably into the existing landscape.
In Phoenix, a definite advantage to the green roof is the reduction of the city’s urban heat island. Nighttime temperatures in Phoenix are 12 degrees F hotter than in 1948, in part due to the urban heat island effect. [“Planting A Roof,” The Arizona Republic, June 23, 2004.] Hovey’s green roof will help cool the area by absorbing less heat during the day and losing it more quickly at night.
By giving off water, plants also act as natural evaporative coolers. However, due to the extreme heat, green roof plants may give off less than in a temperate area, a fact that should be taken into consideration. Therefore, another means of supplying water to the roof must be found.
Normally, one benefit of green roofs is that the plants absorb rainfall, and thus reduce the deluge of storm runoff into a city’s drainage system. In Phoenix, however, the sparse rainfall means the roof will need all available precipitation—and then some.
Typically, a green roof uses precipitation and stored rainwater for moisture. In Phoenix, the roof will need to be irrigated, an added expense over green roofs in less demanding climates. However, Hovey anticipates low water usage from indigenous plants.
The Optima design calls for “a lightweight pan to hold specially engineered dirt that is lighter than conventional dirt but still contains all the necessary nutrients.” [“High End Living In A High Rise,” Southwest Contractor, March 2005.] The pan will hold and shade rainwater that does fall, keeping it from quickly evaporating, thus reducing the amount that may run off.
The effects of the area’s first green roof will be a test for future projects in hot, arid climates. Consequently, the project will be monitored by Arizona State University.
All results of Hovey’s installations, both positive and negative, have yet to be determined, but the Optima Towers green roof will be worth watching for facility professionals and designers in similar climates.
Northfield, MN: The Carleton College Project
Facility managers in colder climates face a different set of challenges. In Minnesota, for example, green roofs (such as the Phillips Eco-Enterprises Center and the Marcy-Holmes neighborhood, both in Minneapolis) have been in service in the Twin Cities area for several years.
Students and faculty at Carleton College recently conducted an experiment to determine which materials would work best in their area. The Carleton project was especially significant because of the challenge to increase R-value insulation and heat retention (in addition to reducing storm runoff). [For more on R-value, read “When Specifying Insulation, What Should You Know” by Anne Vazquez, TFM, June 2006, page 68.]
Northfield enjoys an average summer high of 86 degrees F but suffers an average winter low of 6 degrees F. And in terms of precipitation, Northfield experiences 4.5″ of rain in the summer—sufficient for a green roof—but averages less than 1″ (primarily in the form of ice and snow) per month in winter.
In sharp contrast to rain, ice and snow accumulate on a green roof instead of running off. So anything in this climate must be built to handle the additional weight of winter precipitation.
The 665 square foot roof used a 6″ soil layer to accommodate the root systems of native prairie plants. For the project, 75 species of plants were tested; selected were those with the strongest likelihood for survival in dry and shallow soil environments.
The experiment looked at the added R-value of the roof created by the plantings and found that the green roof only slightly increased the R-value of the roof’s insulating properties. Thus, additional energy consumption savings proved to be slight.
It also examined how the roof affected the colonization of prairie plants native to Minnesota. Of the 75 species of plants tested, 28 “colonist” species were found to be growing there already. The study determined that these colonist species were more prevalent at the end of the testing period than were the plants that were seeded on the roof. Thus, the plants selected for seeding on the roof, known as the “primary species,” may in fact develop into the secondary species that has a longer dormant period and may take over the roof in the future. This theory, known as succession, may be a condition of green roofs that use native plants.
Based on these results, native plants are the best choice for extreme environments. They will more likely survive conditions that could eliminate imported grasses and trees used in more ornamental green roofs. Also, green roofs in extreme environments generally need to be more functional and less ornamental than those in temperate climates where green roofs often double as pleasant spaces for employees and visitors.
But the fact that places like Minnesota and Arizona can sustain green roofs is encouraging. Based on the lessons learned from these examples, more professionals should evaluate this option in even the most unlikely areas, spreading the greening of roofs to the far reaches of the country, and answering a new, green manifest destiny.
Scalia is project editor with the BOMI Institute, based in Arnold, MD. Questions about this article can be sent to firstname.lastname@example.org; for more on the BOMI Institute, visit www.bomi.org.