Best Management Practice Fact Sheet 5: Vegetated Roofs

This fact sheet is one of a 15-part series on urban stormwater management practices.

Please refer to definitions in the glossary at the end of this fact sheet.
Glossary terms are italicized on first mention in the text. For a comprehensive list, see Virginia Cooperative Extension (VCE) publication 426-119, “Urban Stormwater: Terms and Definitions.”

What Is a Vegetated Roof?

A vegetated roof (VR) is a best management practice (BMP) that reduces stormwater runoff and pollution. Vegetation and media create a permeable system on a previously impervious surface. The VR intercepts rainfall and filters runoff while reducing the volume and velocity. Vegetated roofs consist of a waterproofing barrier, drainage system, and engineered growing media. There are two types of VRs: intensive and extensive. Intensive vegetated roofs are deeper and heavier, while extensive vegetated roofs are shallower, lighter, and more common (see Figure 1). The type of VR determines the amount of maintenance necessary to maintain the vegetation.

Where Can VRs Be Used?

Vegetated roofs can be used in new building designs where the roof is engineered to support their weight. They can also be used to retrofit an existing roof if it can support the additional weight of at least 15 to 30 pounds per square foot. While VRs are generally used on flat roofs, they can work on shallow, sloped roofs that can withstand the additional weight.

Vegetated roofs are recommended in urban areas where other BMPs may not be possible due to a lack of space. Like permeable pavement, VRs maintain the original function of the space they occupy, while simultaneously removing impervious surfaces. They also moderate the heat island effect in urban areas. They can extend the lifespan of a roof, reduce heating and cooling costs for the building, and create new interactive and functional green space where there was none.

Vegetated roofs can be used in nonurban locations as well. In many cases, VRs are combined with additional BMPs to treat stormwater.

How Do VRs Work?

Vegetated roofs consist of multiple layers, including: (1) a top layer of vegetation, (2) a media layer, (3) a drainage layer, (4) a root barrier, and (5) an impermeable liner which blocks moisture and also serves as a root barrier to prevent roof damage (see Figure 2). Rainfall is intercepted and infiltrates into the media layer, where it is then stored and utilized by selective drought- and heat-resistant vegetation. Typically, roofs can store up to half an inch of rainfall. Rainfall that is over this amount is collected in the drainage layer and discharged away from the roof.

By storing rainfall, the VR increases the residence time of the runoff, which reduces peak runoff entering a stormwater conveyance system. The stored water then returns either directly to the atmosphere through evaporation or indirectly by evapotranspiration through the vegetation growing within the VR.

Limitations

• Vegetated roofs are limited to specifically engineered roofs or to existing roofs that have the additional structural support necessary to withstand the additional weight.
• Because of weight load constraints, only a small amount of runoff storage is available (4 to 6 inches) for the more common extensive roof.
• They require adequate roof access for maintenance.
• To reduce maintenance, drought-resistant plants are recommended. Irrigation may be necessary to protect vegetation during drought periods longer than two weeks. Irrigation is also essential in the plant establishment period.
• Roofs with steeper slopes may require stabilization frames to prevent sliding.

Maintenance

Routine Maintenance
(annual)

• Moderate maintenance is required — usually periodic weeding and watering (depending on vegetation).
• Confirm that plants are growing well; reseed or replant bare areas to prevent erosion and improve filtration.

Nonroutine Maintenance
(as needed)

• An application of a minimum amount of controlled- or slow-release fertilizer may be necessary for vegetation establishment or maintenance. Extreme care should be taken not to overfertilize in order to prevent excess nutrients from leaving the roof in the discharge.

Performance

Vegetated roofs are effective at reducing pollutants mostly through runoff reduction and some biological uptake. An extensive VR is expected to reduce total phosphorus (TP) by 45 percent and total nitrogen (TN) by 45 percent, which includes mass load reductions from runoff reduction. An intensive system is deeper, which provides a longer residence time. Intensive VR systems can improve the expected reduction of TP to 60 percent and TN to 60 percent (VA-DCR 2011).

Expected Cost

Vegetated roofs have relatively high initial installation costs when compared to other alternative stormwater BMPs; however, they provide many benefits. Initial costs depend on site conditions and accessibly of the roof, the roof surface area, and the type of VR system being installed. The initial construction cost of a VR system, not including structural reinforing, is on average $33/ft² (Washington State Department of Ecology, & Herrera Environmental Consultants, 2012).

Additional Information

The Virginia departments of Conservation and Rec- reation (VA-DCR) and Environmental Quality (VA-DEQ) are the two state agencies that address nonpoint source pollution. The VA-DCR oversees agricultural conservation; VA-DEQ regulates stormwater through the Virginia Stormwater Management Program.

Additional information on best management practices can be found at the Virginia Stormwater BMP Clearinghouse website at https://www.swbmp.vwrrc.vt.edu/ (Permanent link: https://perma.cc/WC5L- KCZ8). The BMP Clearinghouse is jointly administered by the VA-DEQ and the Virginia Water Resources Research Center.

Online Resources

Charles River Watershed Association – https://www.crwa.org/hs-fs/hub/311892/file-643219745-pdf/Our_Work_/Blue_Cities_Initiative/Resources/ Stormwater_BMPs/CRWA_Green_Roof.pdf

Low  Impact  Development  Center  – www.lid-stormwater.net/greenroofs_home.htm Perma- nent link: https://perma.cc/K5PR-P5XD

Michigan State University – www.hrt.msu.edu/greenroof/ Permanent link: https://perma.cc/QSA6-2VW3

U.S. Environmental Protection Agency – www.epa.gov/owow/NPS/roofcover.pdf

Virginia Stormwater BMP Clearinghouse – https://www.swbmp.vwrrc.vt.edu/ (Permanent link: https://perma.cc/WC5L-KCZ8)

Companion Virginia Cooperative Extension Publications

Daniels, W., G. Evanylo, L. Fox, K. Haering, S. Hodges, R. Maguire, D. Sample, et al. 2011. Urban Nutrient Management Handbook. Edited by M. Goatley. VCE Publication 430-350.

Gilland, T., L. Fox and M. Andruczyk. 2018 Urban Water-Quality Management - What Is a Watershed? VCE Publication 426-041.

Sample, D., et al. 2011-2012. Best Management Practices Fact Sheet Series 1-15, VCE Publications 426-120 through 426-134.

Acknowledgements

The authors would like to express appreciation for the review and comments provided by the following individuals: Dan Nortman, lecturer, Virginia Tech; Jon Vest, Extension agent, Virginia Tech; Lia Doumar, undergraduate student, biological systems engineering, Virginia Tech; and Richard Jacobs, conservation specialist, and Greg Wichelns, district manager, Culpeper Soil and Water Conservation District.

References

Virginia Department of Environmental Quality (VA DEQ). 2011. Virginia DEQ Stormwater Design Specification No. 5: Vegetated Roof, Version 2.3. https://www.swbmp.vwrrc.vt.edu/wp-content/uploads/2017/11/BMP-Spec-No-5_VEGETATED-ROOF_v2-3_03012011.pdf.

Washington State Department of Ecology, and Herrera Environmental Consultants. (2012). PugetSound Stormwater BMP Cost Database.

Glossary of Terms

Best management practice (BMP) – Any treatment practice for urban lands that reduces pollution from stormwater. A BMP can be either a physical structure or a management practice. Agricultural lands use a similar, but different, set of BMPs to mitigate agricultural runoff.

Detention time – See residence time.

Erosion – A process by either physical processes, such as water or wind, or chemical means that moves soil or rock deposits. Excessive erosion is considered an environmental problem that is very difficult to reverse.

Evaporation – process by which water changes from liquid to gas and is “lost” to the atmosphere.

Evapotranspiration – When water is released by a plant and evaporates from leaves and soil.

Extensive vegetated roof – A type of vegetated roof with a media depth of 4 to 6 inches; vegetation is composed of drought-resistant plants whose only water source is usually rainwater.

Filtration – a treatment method that removes pollutants using straining, sedimentation, and similar processes.

Green roof – See vegetated roof.

Heat Island – this is an effect, observed in urban areas, of elevated ambient temperatures, which occurs due to storage of heat in the mass of concrete. This mass takes longer to cool than surrounding areas, producing the observed effect.

Impermeable liner – A material designed to retard seepage from ponds and wetlands.

Impervious surfaces – Hard surfaces that do not allow infiltration of rainfall into them; not pervious.

Infiltration – The process by which water (surface water, rainfall, or runoff) enters the soil.

Intensive vegetated roof – A vegetated roof with soil depth ranging from 10 inches to 4 feet. Vegetation can be composed of shrubs and trees in addition to other plants. Irrigation is generally necessary.

Media, media filter bed, filter bed – The mix of soils and other constituents that filters runoff, removing some nutrients, while also supporting plant growth.

Nutrients – Substances required for growth of all biological organisms. When considering water qualities, nutrients of greatest concern in stormwater are nitrogen and phosphorus because they are often limiting in downstream waters. Excessive amounts of these substances are pollution and can cause algal blooms and dead zones to occur in downstream waters.

Peak runoff – The highest amount of water flowing off a surface during a storm event.

Pervious – A ground surface that is porous and allows infiltration into it.

Residence time – The average time it takes water to travel through a treatment system such as a VR. Residence time can also be called detention time.

Root barrier – Protects the impermeable liner from root puncture. It must be either a dense inorganic material that inhibits root penetration or contain a root repellant ingredient, such as copper.

Stormwater – Water that originates from impervious surfaces during rain events; often associated with urban areas. Also called runoff.

Stormwater conveyance system – Means by which stormwater is transported in urban areas.

Stormwater management – The management of runoff from pre- to postdevelopment, often using stormwater treatment practices and BMPs to manage quality and control release into receiving bodies of water.

Stormwater treatment practice – A type of BMP that is structural and reduces pollution in the water that runs through it.

Vegetated roof – A roof designed and constructed to support living vegetation; also known as green roofs.

Vegetated roof media – A composite of inorganic and organic materials that supports plant growth and filters runoff.