In 2008, Multnomah County's Central Library became the first library in Oregon to feature an eco-roof. Constructed with living plants and simulating the processes that occur in nature, eco-roofs (also known as green roofs) absorb rain water 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. Its palletized structure represents the next evolution in eco-roof designs.
Frequent backstage tours allow you to see the roof from windows overlooking the roof. The 20-minute tour begins at the first floor Welcome Desk.
Registration required; register online, in the library or by calling 503.988.5234.
Green roof benefits
Green 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 stormwater can exceed pipe capacity in combined sewer overflow areas, causing untreated sewage to enter the Willamette River. Green 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 a green 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. Green 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 global warming.
- 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. The selection of sedums and grasses (PDF, 2.4KB) will provide swaths of color that will change with the seasons, providing a field of green with tinges of red 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.
The combination of a large rooftop area and palletized 2" growth medium allows for a wide variety of vegetation on the Central Library eco-roof. This plant diversity will increase knowledge of what will thrive on rooftops in this area as well as provide a beautiful and functional addition to a heavily used local landmark.
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 Project funded by two grants that account for 90% of the costs, from the Department of Environmental Quality and the Green Investment Fund (which is supported by the City of Portland and Energy Trust of Oregon, Inc.) The remaining 10% costs will be offset by a significant storm water credit from the city of Portland and an anticipated 6 - 8% energy savings during the winter, and up to 50% energy savings during the summer.
Structure: The Central Library eco-roof was constructed using 24” x 24” palletized soil. The use of pallets allowed for easy installation.
Soil Characteristics: The soil is 4” 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 of Green Roof: Total green roof assembly = 15–20 pounds per square foot (saturated).
Irrigation System: The irrigation system is used only for hot, dry summer months when the plants need additional water to maintain health. Calculations will be made carefully to ensure that the minimum amount of water is used exactly where and when it is needed.
Portland State University professor Graig A. Spolek and the City of Portland are partnering with Multnomah County to provide storm water monitoring of the Central Library eco-roof. The objective is to measure how much rain is being held back from the sewer by the eco-roof, and to identify energy savings realized from a reduced need to heat and cool the building. Measuring the performance of a green roof is important to assessing what benefits can be realized from installing green roofs on other buildings in the area.
Project led by Commissioner Maria Rojo de Steffey for the Board of Multnomah County Commissioners:
Chair Ted Wheeler
Commissioner Maria Rojo de Steffey
Commissioner Jeff Cogen
Commissioner Lisa Naito
Commissioner Lonnie Roberts
Multnomah County Facilities and Property Management
Multnomah County Sustainability Program
Multnomah County Library
Green Investment Fund:
City of Portland Office of Sustainable Development
City of Portland Water Bureau
Energy Trust of Oregon, Inc.