Seattle Public Utilities and the Department of Planning and Development have recently completed a report on local green roofs. The report provides a snapshot of industry trends, project locations, and the total square footage of green roofs in Seattle today. This information will help us support the expansion of green roofs as a green building strategy.
Seattle Green Roof Report | Seattle Green Roof Inventory |
Green roofs extend the life of roofing materials by blocking ultraviolet radiation and moderating temperature extremes. They can also slow stormwater runoff, reduce building heating and cooling costs, cool and clean the air, and provide habitat and attractive greenery in urban environments.
On this page, you'll find:
|Planting the Zoomazium roof,|
|Six months later...the roof in full bloom|
|Photos: Ryan Hawk, WPZ|
What Is A Green Roof?
Green roofs, also called vegetated roofs or eco-roofs, are part of the green building toolbox. They consist of plants in a growing medium (special soil mix) installed over a building’s waterproofing membrane. Green roofs are of two types:
Intensive green roofs
Intensive green roofs are what most people would think of as a traditional “roof garden". The growing medium is deep, greater than eight inches, usually contained in pots or planters, and the vegetation ranges from kitchen herbs to lawns and flowers to shrubs and small trees. An intensive green roof is often park-like and accessible.
Extensive green roofs
With a growing medium of two to six inches depth, extensive green roofs are thinner and lighter. They are most often engineered systems beginning with a drainage layer of materials that can range from simple gravel ballast to specialized molded plastic sheets with water-collecting cups. Above the drainage layer is a geotextile root barrier and a special lightweight soil mix often low in organic content. The plants chosen for extensive green roofs are usually drought-tolerant varieties such as sedums, so that established green roofs will not need irrigation. Extensive green roofs are usually not accessible except for maintenance.
Download Cascadia Region Green Building Chapter's Green Roof Fact Sheet
Seattle Incentives, Technical Guidelines and Research
Seattle currently has an impervious surface reduction credit that lists green roofs and roof gardens as acceptable strategies. See Control Technical Requirements Manual. Additionally, the LEED green building certification program offers a point for a green roof. Given the proven marketing value of LEED certification, this too could be considered an incentive for green roofs.
Seattle Green Factor
Seattle Green Factor landscaping requirements (for 30% equivalent plant coverage in Commercial and Neighborhood Commercial (NC) zones and 50% coverage in multi-family residential zones) can be met in part through use of green roofs. Learn how at the Green Factor design and permitting page.
Ongoing study and modeling for stormwater code development
Green roofs have the potential to reduce roof runoff and delay peak flows during storms. SPU is helping to standardize green roof modeling methods through use of the Western Washington Hydrologic Model (WWHM), commonly used by stormwater design engineers. The WWHM model has been modified to include a green roof/eco-roof modeling element, which has been calibrated using monitoring data from Portland. We are updating this model using local monitoring data. See Eco-roof Stormwater Modeling Memo.
SPU is currently collecting rainfall and runoff flow data on several city buildings (the Zoomazium, Ballard Branch Library, Ross Park Shelterhouse starting in fall 2007, and Fire Station 10 starting 2008). SPU is also conducting limited grab-sampling of runoff water quality to see if new green roofs are likely to add nutrients to stormwater draining to streams, lakes and Puget Sound.
There are an increasing number of green roofs in Seattle, many at publicly accessible facilities. Projects are listed below with links to relevant information, where available. See Seattle's Green Roof Plant Lists and Soil Specifications for information relevant to all the following projects.
Cities with excellent Green Roof Websites
For more information
about green roofs, contact:
Sustainable Urban Designer
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