Green Roofs in Seattle: A Survey of Vegetated Roofs and Rooftop Gardens

This report, Green Roofs in Seattle, summarizes finding from a 2009 inventory of green roofs located within the city limits of Seattle, Washington.  The inventory was completed as a result of a collaboration between Seattle Public Utilities, the Seattle Department of Planning and Development, and the Green Futures Lab at the University of Washington. In part, the research was designed with the intention of gaining insight into Seattle’s growing green roof industry and its trends, to create a baseline understanding of this emerging marketplace, and to gauge the actual use of green roofs as a Green Stormwater Infrastructure tool.

Information about existing green roofs in Seattle was collected using a combination of online research, phone interviews, and site visits. Designers, property managers and/or owners of all known green roofs were contacted and asked to provide certain data and background information on their project.

As described in the report, when Seattle receives significant rainfall during the winter months, the quantity and quality of runoff from streets and other impervious surfaces has a serious impact on the health of marine and aquatic life, and ultimately on human life. The filtering capacity of green roofs may reduce pollutant loads in receiving water bodies. “Ecosystem Services plus Economic and Health Benefits” are explained in the report. 

According the survey, the green roof installations found in Seattle include: 

  • Extensive green roofs or “eco-roofs with a soil depth of 2-6” account for 42% of Seattle’s green roof area.
  • Rooftop gardens or intensive green roofs account for 41% of Seattle’s green roof area  These roofs are installed with soil depths of 6-36” to support more lush landscaping.
  • About 7% of Seattle’s green roofs have a combination of extensive/intensive assemblies, with varying soil depth and plant selection and varying degrees of human access. 
  • Lidded structures or at-grade green roofs comprise a separate category of green roof. They are predominantly turf or park areas with trees on top of major infrastructure such as reservoirs. They are often very large, and account for 12% of Seattle’s total green roof area. While they may not fit the definition of a green roof, they do provide planted, pervious coverage over impervious infrastructure.

 Some of the predominant survey results are: 

  • As of December 2009, the total area of existing green roofs, including roof- top gardens, ultimately contributes nearly 5 acres of pervious surfaces to the city’s total roof surface area of 13,150 acres.
  • Green roofs tend to be eliminated from early plans due to initial costs and the uncertain availability of material and maintenance warranties. 
  • To date (2009), most green roofs have been custom-designed to fit the specific requirements of each building, however there are a growing number of companies offering complete or partial green roof assemblies which relieves the responsibility of design and sourcing from architects and landscape architects.

 

The Seattle Stormwater Code, adopted by City Council in late 2009, requires storm-water filtration and retention projects to implement “Green Stormwater Infrastructure to the maximum extent feasible” including green roofs. 

The use of green roofs also meets the requirements of Seattle’s “Green Factor.” The Green Factor contains landscaping requirements are for 30% equivalent plant coverage in Commercial and Neighborhood Commercial zones and 50% coverage in multi-family residential zones.

In addition, the City of Seattle offers a Floor Area Ratio Bonus in its building code. Developers may build an extra 3 sq. ft. per foot of green roof they construct without additional permits.

 

 

 

Publication Date: August 2010

Author or Affiliated User:

  • Annika McIntosh

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  • Best practice

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