Green Roofs

According to the Sustainable Facilities Tool of the U.S. General Services Administration, “Planted roofs - also known as vegetated roofs or eco-roofs - use plants as a technology to help bring the natural cooling, water-treatment and air filtration properties of vegetated landscapes to the urban environment.  These systems consist of vegetation (plants), growing medium (soil) and a waterproofing membrane, overlying a traditional roof. There are two primary types of planted roofs: extensive and intensive. Extensive planted roofs consist of small succulent plants, while intensive planted roofs have greater plant diversity, including native vegetation, bushes, or trees.”

What is a Green Roof?

Green roofs are partially or completely covered with a growing medium and vegetation planted over a waterproofing membrane. They may include additional layers, such as a root barrier, and drainage and irrigation systems. Extensive green roofs typically have a depth that ranges between 4-6 inches. In addition to providing onsite stormwater management, green roofs mitigate urban heat island impacts, reduce building energy costs, decrease noise pollution, and improve air quality and mental wellbeing.

Buildings with roof slopes between 0-9 degrees are ideal for green roofs and the least expensive roof to construct and maintain. Roof slopes 9-30 degrees are suitable for green roofs. They may require additional erosion control which can increase construction costs. Roofs with slopes greater than 30 degrees are not considered suitable for green roofs.

Benefits of a Green Roof?

• Stormwater Management: Most urban and suburban areas contain large amounts of paved or constructed surfaces which prevent stormwater from being absorbed into the ground. The resulting excess runoff damages water quality by sweeping pollutants into water bodies. Green roofs can reduce the flow of stormwater from a roof by up to 65% and delay the flow rate by up to three hours.
• Energy: Green roofs reduce building energy use by cooling roofs and providing shading, thermal mass and insulation.
• Cooling: Make roof surfaces 30-40% cooler.
• Heat Flux: Reduce heat flux from roof to building by up to 72%.
• Biodiversity and Wildlife Habitat: Green roofs provide new urban habitat for plants and animals, like birds, bees, and insects, thereby increasing biodiversity. 
• Urban Heat Islands: Cities are generally warmer than other areas, as concrete and asphalt absorb solar radiation, leading to increased energy consumption, heat-related illness and death, and air pollution. Green roofs can help reduce this effect.
• Stormwater: Reduce the stormwater runoff rate from a roof by up to 65%.
• Roof Longevity: Green roofs are expected to last 40 years or more, twice as long as conventional roofs. 
• Aesthetics: Green roofs can add beauty and value to buildings.

Green Roof Mapping Tool for Northern Virginia Urban Core Jurisdictions

The Northern Virginia Regional Commission has launched a green roof mapping tool that depicts the green roof potential for buildings within the region’s urban core (Arlington County, Fairfax County, Cities of Alexandria, Fairfax and Falls Church).  

The map shows the following elements of a green roof: 

• Event volume capture capacity
• Annual volume capture capacity
• Annual stormwater interception value
• Annual energy savings
• Annual CO2 offset
• Annual Air Pollutants Removed

In the five jurisdictions, there are 541,248,585 square feet of potential usable green roof. That square footage of green roof has the potential to capture 8.7 billion gallons of rainwater on annual basis and offset annually,119,345 tons of CO2 with projected potential annual energy savings of $146.1 million dollars.

Potential green roof area was determined using the degree of building roof slope which was calculated from a digital surface model (DSM) and building footprint data. The Center for Neighborhood Technology Green Values Calculator was utilized to determine the green roof effect on stormwater runoff. Pollution reduction values were calculated based on findings from green roof reports from the Center for Neighborhood Technology and U.S. General Services Administration.

Stormwater is a major source of pollution in local streams, rivers, and the Chesapeake Bay. Green roofs are just one element that can help reduce stormwater runoff in communities. Efforts to reduce stormwater runoff are critical to the region achieving its required pollution reduction to the Chesapeake Bay.

Other Online Resources

U.S. General Services Administration: https://www.gsa.gov/governmentwide-initiatives/federal-highperformance-green-buildings/resource-library/integrative-strategies/green-roofs

U.S. EPA: Using Green Roofs to Reduce Heat Island: https://www.epa.gov/heatislands/using-green-roofs-reduce-heat-islands

Whole Building Design Guide: https://www.wbdg.org/resources/extensive-vegetative-roofs

Study Finds Green Roofs Make Solar Panels More Efficient: https://www.greenroofs.com/2021/08/28/study-finds-green-roofs-make-solar-panels-more-efficient/ 

Sydney, Australia Study:  City of Sydney, Australia.