City Green Building
Green Roofs

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,
November 2005

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.

Seattle Incentives, Technical Guidelines and Research

Incentives
Seattle currently has an impervious surface reduction credit that lists green roofs and roof gardens as acceptable strategies. See Appendix B of the Flow 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
As of January 21, 2007, the new Seattle Green Factor requirements (for 30% equivalent plant coverage on commercial developments in Neighborhood Commercial (NC) zones) can be met in part through use of green roofs. Learn how at the Green Factor design and permitting page.

Technical Guidelines
Technical guidelines for building green roofs in Seattle are currently in development, and will be posted here when complete. Meanwhile, please find links to existing resources, design and case study examples below. 

Ongoing study and modeling for stormwater code development
Green roofs have the potential to reduce roof runoff and delay peak flows during storms. Seattle Public Utilities (SPU) is attempting to quantify the stormwater benefits, to help us credit green roofs in our city stormwater code.

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. In a few years this model will be further updated 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 possibly the new 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.

As a result of stormwater modeling, Seattle currently has a trial Stormwater Code Exception Policy for the South Lake Union area, that allows green roofs to partially satisfy stormwater code.

Green Roof Monitoring & Evaluation

Green Roof Case Studies

General

Seattle
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.  

Green Roof Design & Research Resources

General

European

Academic

Cities with excellent Green Roof Websites

Design Guidelines

Last Updated: February 12, 2007