In 2008, Multnomah County's Central Library became the first library in Oregon to install an eco-roof. Constructed with living plants and simulating the processes that occur in nature, eco-roofs (also known as green roofs) absorb rainwater like a meadow in a natural setting.
The Central Library eco-roof provides a unique opportunity to showcase sustainable green roof technology in the context of a beloved historic landmark.
Tours are available from April to October and allow you to see the eco-roof from overlooking windows. The 20-minute tour begins at the first floor Welcome Desk.
Registration required: register online, in the library, or by calling 503.988.5123.
Eco-roof technology has been used in countries around the world for decades. Currently, eco-roofs are gaining recognition in the U.S. for the environmental, economic and social benefits they provide. These include:
- Reducing rainwater runoff: Eco-roofs commonly reduce total annual runoff volume by 70 percent. This is especially important locally, where storm-water can exceed pipe capacity in combined sewer overflow areas, causing untreated sewage to enter the Willamette River. Eco-roof technology reduces the frequency of such overflows. Eco-roofs intercept and delay rainfall runoff in several ways:
- Capturing precipitation in the soil and plant foliage, then releasing it back into the atmosphere (evapotranspiration)
- Absorbing water in the root zone and soil
- Slowing the velocity of direct runoff as it infiltrates through the layers of vegetated cover
- Reducing energy costs: On warm summer days, a city can be 6 to 8 degrees warmer than its surrounding areas. Scientists call this the “Urban Heat Island Effect.” Higher temperatures increase demand for air conditioning and increase air pollution and smog.
- Extending the life of a roof:The added material levels of an eco-roof protect the roof’s structural elements from temperature extremes, doubling or tripling roof life. During hot summer months, rooftop temperatures can reach 175 degrees; such temperature extremes, along with ultraviolet radiation and wind, shorten roof life.
- Increasing vegetation and wildlife habitat: Urban areas typically have very little green space for displaced birds, butterflies and other wildlife.
- Reducing air pollution: Eco-roofs filter dust, smog, particulates and other harmful pollutants, and bind them in the soil where they are broken down into useful plant nutrients. This is particularly important in urban areas where a variety of pollutants become trapped in the air.
- Filtering the air: It is estimated that 1,000 square feet of eco-roof can remove 41 pounds of airborne particles per year. Additionally, eco-roofs absorb carbon dioxide, a cause of climate change.
- Improving fish habitat: Fish benefit from cooler streams made possible by lower stormwater temperatures created when runoff water is delayed or absorbed into the eco-roof.
Plants on the Central Library eco-roof
Map of plants (PDF, 361KB)
Over 17,000 plants were used to create the Central Library eco-roof. All were carefully selected using several criteria: appropriateness to rooftop conditions, ability to develop vigorous root systems, low maintenance, cost, local availability, drought tolerance, value to wildlife, and aesthetics. Sedums and grasses provide swaths of color that change with the seasons, providing a field of green with tinges of red, yellow, orange, and purple all year.
Plants support the health of an eco-roof in several ways:
- They provide vegetative coverage and subsurface roots that hold soil in place, preventing soil erosion caused by wind and water.
- They help to reduce temperatures on the surface of the roof.
- They reduce runoff by absorbing water through their leaves and roots, allowing it to slowly enter the storm sewer system or to be released through evaporation.
Completed: September 15, 2008
Location: Downtown Portland. Stormwater discharges into the Willamette River.
Square footage: 7,188 square feet. (Total roof area: 18,145 square feet.)
Cost per square foot: $25
Structure: The Central Library eco-roof was constructed using 24” x 24” soil trays. The use of trays allowed for easy installation and access to roof-top equipment.
Soil characteristics: The soil is 6” deep and consists of an engineered, light-weight blend of inorganic and organic components. Zebra, a water retentive additive made from natural cornstarch, is included in the soil mix.
Weight: Total eco-roof assembly = 15–20 pounds per square foot (saturated).
Irrigation system: The irrigation system is weather-based and only applies water as needed. A maintenance program monitors to ensure that the minimum amount of water is used exactly where and when it is needed.
Multnomah County Board of Commissioners
Multnomah County Facilities and Property Management
Multnomah County Sustainability Program
Multnomah County Library
Green Investment Fund: Multnomah County received $60,000 in GIF grant funding for this project.
City of Portland Bureau of Environmental Services
City of Portland Office of Sustainable Development
City of Portland Water Bureau
Energy Trust of Oregon, Inc.
Oregon Department of Environmental Quality
For more information
Alan Proffitt, GRP, Multnomah County Facilities Project Manager, 503.988.4218
Carleton Hart Architecture, PC — Lead Architect
Macdonald Environmental Planning — Landscape Architect
Munro & Associates — Mechanical Engineer
KPFF Consulting Engineers —Structural Engineer
Snyder Roofing of Oregon LLC — General Contractor
Teufel Landscape — Landscaping Subcontractor
B&K Sheet Metal — Sheet Metal Subcontractor
Beaverton Plumbing — Plumbing Subcontractor
Waco Scaffold & Equipment Co. — Scaffolding Subcontractor
TremCo — Roofing Membrane System, Drainage Mat
Columbia Green Tech. Inc. — Tray system