September 15, 2000
Most Seattle residents are concerned about preventing mossy green stuff from growing on their roof, so you wouldn’t expect the city of Seattle to purposely install a roof on which plants are encouraged to grow. But using a “green roof” system is exactly what the architects for the new Justice Center and City Hall are planning.
Green roofs, also called living roofs or sometimes, eco-roofs, are complete roof systems of vegetation, soil, drainage and a waterproof membrane. Unlike roof gardens of container plantings that need lots of watering, green roofs are a continuos hydrological system across the surface of a roof that is designed to absorb and slowly release rainwater. They can be paired with systems to infiltrate or capture remaining runoff, creating a complete on-site stormwater management system.
|Where to learn more|
There are a variety of manufacturers of green roof systems or components. Some companies provide just design services using other company’s components, while some will design, manufacture, and install your roof. You can learn more through the resources listed below. |
For general information check out the Web site at www.greenroofs.com.
For suppliers try www.roofmeadow.com, www.zinco.de, www.hydrotechusa.com, www.garlandco.com.
To get involved with the EcoBuilding Guild’s Eco-roof Project, e-mail: firstname.lastname@example.org or visit www.ecobuilding.org.
Underground parking garages and earth-sheltered structures are the closest thing to green roofs that most people are familiar with. The lawns and parks planted on top of these structures are similar to green roofs but are connected directly the surrounding ground and tend to be heavier.
Green roof systems can range from very lightweight systems with two to three inches of soil and low, succulent plantings, to deeper (12-inch), heavier systems that support a wider variety of plantings and some public space. The city of Portland’s Bureau of Environmental Services, which has built two demonstration projects and has more planned, is using the lighter systems and refers to them as “eco-roofs.”
Green roofs are not entirely a new phenomenon. When you explain green roofs to someone, they frequently have a story about a funny building on some nearby island that has had a sod roof for years. Like many environmental building practices, green roofs have been used in Europe for many years. According to one source, approximately 10 percent of Germany’s roofs are some variety of a green roof.
More recently, cities like Chicago and Portland have begun experimenting with eco-roofs to alleviate environmental problems such as stormwater run-off and the urban heat island effect.
Now, Seattle is embracing the concept by including green roofs on the first two buildings in the new Civic Center downtown: the Justice Center and City Hall. According to Dunkin Thieme of NBBJ, including a green roof in the plans for City Hall helped NBBJ meet many of the city’s environmental goals for the project. Green roofs aren’t just for major public projects. The Seattle Chapter of the Northwest EcoBuilding Guild is starting an ambitious project to install as many as a dozen test eco-roofs on Seattle garages. The project will evaluate the performance of a variety of eco-roofs systems in terms of cost, environmental performance, and a variety of other factors.
The primary benefit of green roofs is stormwater management. Green roofs can retain 50-60 percent of the total annual runoff volume of a roof, according to Roofscapes, Inc., reducing the need for stormwater retention. Green roofs also slow water velocity, help return water to the hydrological cycle through evaptranspiration, and naturally filter water to improve water quality. These qualities make green roofs superior to other stormwater management options that require scarce and expensive land, and which are hampered by the low infiltration of our native glacial till soil.
These benefits have enormous appeal to local governments in Portland and Seattle which are struggling to respond to the listing of salmon as a threatened species. Many governments pass these benefits on to developers and property owners. Portland’s Bureau of Environmental Services is planning to offer reduced stormwater fees to encourage property owners to install eco-roofs. Seattle Public Utilities is interested in offering green roofs as one alternative for controlling stormwater flow.
The U.S. Green Building Council offers points in its LEED (Leadership in Energy and Environmental Design) system for installing a green roof to help reduce the urban heat island effect, another environmental benefit.
Other benefits accrue directly to the property owner through energy savings and extending a roof's service life. Green roofs provide insulation and thermal mass, which help conserve energy and reduce temperature swings that can damage roof membranes. The planted surface of a green roof protects the roof membrane from damage from ultraviolet radiation and wear and tear from maintenance staff.
Additional environmental and community benefits include the preservation and replacing of greenspaces, added wildlife habit and improved air quality. Green roofs improve livability of the urban environment by buffering noise, reducing glare and offering a pleasant, aesthetic alternative to acres of asphalt roofing.
Green roofs are a versatile tool for landscape architects in creating a variety of looks for different projects. For example, Chicago’s City Hall will have a formal garden green roof. The most common installation is a low maintenance (1-2 times a year) meadow look, which creates a pleasant connection with nature in an urban setting.
A green roof system is more expensive than a traditional roof. Depending on the roof system you are comparing it to, a green roof currently costs 30 to 60 percent more. Green roofs are sophisticated systems that involve more materials and more disciplines than a standard roof system.
These costs may be offset, however, by reduced need for stormwater facilities, energy savings and a longer roof service life. A green roof can be a cost-effective way to achieve the environmental and aesthetic goals of a project. The stormwater management and water quality benefits of a green roof will also make them an important tool to developers for responding to the salmon listing.
Market-based green building rating systems like LEED and the Master Builders Association of King and Snohomish Counties’ Built Green are an effort to build demand and create a strong market for buildings with environmental features such as green roofs.
Like any roof system, the primary function of a green roof is to provide moisture protection for the structure below. The waterproof membrane and drainage systems assure that green roofs live up to this standard. Like most roof systems, the keys to guaranteeing a waterproof roof are careful detailing and high quality materials. If you are still nervous, at least one green roof company sells a leak detection system that notifies you immediately of water penetrations.
Weight is also a concern with green roofs, but not as much as many people think. Thinner eco-roof systems using lightweight soil weigh 10-15 pounds per square foot fully saturated which is comparable to other systems such as gravel ballast. Thicker systems with more soil weigh closer to 25 pounds per square foot. These thicker systems are most appropriate for new construction, but eco-roofs can be retrofitted onto many buildings.
How steep a roof can be and still host a green roof is also a question.
The newness of green roofs, at least in the U.S., is the biggest barrier to use. Although we know many of the benefits, they haven’t been fully quantified yet. And there are only a limited number of local installations to help property owners and architects evaluate the costs and benefits.
Patrick Carey, an architect and chair of the EcoBuilding Guild’s Eco-roof technical committee states, “Of all of the roofing options, eco-roofs seem to have the smallest ecological footprint but we are playing devil’s advocate and critically evaluating every aspect of eco-roofs.”
Ultimately, what separates green roofs from traditional roofs is that they provide a variety of beneficial functions where traditional roofs provide only protection while creating environmental problems.