Rooftop disconnection (RD) is one of the simplest means of reducing stormwater from residential lots. RD takes roof runoff that has been collected in gutters and piped directly to streets, storm drains, and streams and redirects it away from impervious surfaces to landscaped areas (figure 1). Rooftop disconnection is a very sustainable best management practice (BMP) because it controls pollutants in runoff near their source. Redirected runoff from downspouts is infiltrated, filtered, treated, or reused prior to draining into a stormwater conveyance system.
Rooftop disconnection can be used in either commercial or residential areas with a lot size larger than 6,000 square feet. It can be used in any soil; however, caution should be applied to applications in poorly draining soils, such as clays and silts.
A variety of different means of disconnecting runoff from impervious surfaces is used in RD. If sufficient land area with good soils is available, simply disconnect the rooftop runoff, creating a wide, shallow flow across the ground surface. The flow path can be enhanced with compost to encourage infiltration.
In other cases where space is limited, rooftop disconnection can be accomplished by diverting runoff into other best management practices, such as bioretention, a cistern, a tree planter, or a dry well.
The most common of these practices is simple disconnection. This easy method can often be accomplished by cutting the downspout and redirecting it horizontally onto a splash pad and to a pervious area (such as a lawn) to infiltrate.
Adding a compost-amended filter path involves tilling the soils in the flow path (about 10 feet wide) to a depth of 6 to 10 inches, and adding 2 to 4 inches of compost in the top layer while maintaining the low point of the channel. This provides enhanced infiltration of runoff through the flow path.
Adding a rain garden in series can reduce runoff and provide additional treatment, and it can effectively eliminate most of the rooftop runoff. Refer to “Best Management Practices Fact Sheet 9: Bioretention” (VCE publication 426-128) for further information on this practice. Best Management Practices Fact Sheet 8 (VCE publication 426-127) focuses on dry wells.
The same limitations as for simple RD apply, plus:
A rooftop disconnection is a small practice whose performance is difficult to measure. Most of the treatment expected is due to runoff reduction, which reduces the mass of pollutants.
Therefore, performance is directly affected by soils. In sandy soils, with high infiltration, an RD is expected to reduce total phosphorus and total nitrogen by 50 percent. In less infiltrative soils (such as clays or silts), this is reduced to 25 percent for both total phosphorus and total nitrogen (VA-DCR 2011).
The cost of installation for RD can vary significantly. For simple RD, the costs of materials and labor are rather inexpensive and can be done for less than $100 per downspout. The addition of compost-amended flow paths and rain gardens will likely increase costs significantly. Costs must be estimated on a site-by-site basis. However, many of these practices are in use by homeowners and appear to be performing well. Maintenance for all RD practices is similar to other home landscaping tasks and can be performed by the homeowner.
The Virginia Department of Conservation and Recreation is the state agency that controls nonpoint source pollution and stormwater. The VA-DCR regulates urban stormwater through the Virginia Stormwater Management Program.
Additional information on best management practices can be found at the Virginia Stormwater BMP Clearinghouse website at http://vwrcc.vt.edu/swc. The BMP Clearinghouse is jointly administered by the VA-DCR and the Virginia Water Resources Research Center, which has an oversight committee called the Virginia Stormwater BMP Clearinghouse Committee. Committee members represent various stakeholder groups involved with stormwater management.
Chesapeake Stormwater Network – http://chesapeakestormwater.net/all-things-stormwater/rooftop-disconnection-design-specification.html
City of Philadelphia – http://phila.gov/water/ Stormwater/pdfs/WQ_Interpretation_fo.pdf and http://scribd.com/doc/13322624/Stormwater-Management-Guidance-Manual-Ver-20
City of Portland, Ore. – http://portlandonline.com/bes/index.cfm?c=43081&a=322320
City of Richmond, Va. – http://richmondgov.com/DPU/documents/BMPQuantity_Rooftop_Disconnection_Inspection_Checklist.pdf
City of Toronto, Ontario – http://www.riversides.org/rainguide/riversides_hgr.php?cat=2&page=39&subpage=41
StormwaterPA – http://www.stormwaterpa.org/5-8-1.html
Pennsylvania Department of Conservation and Natural Resources – www.dcnr.state.pa.us/cscp08/Horner%20presentation.ppt
Virginia Stormwater BMP Clearinghouse – http://vwrrc.vt.edu/swc/
Benham, B., and J. Lawrence. 2010. Stormwater 101: An Introduction for Master Gardeners. Presentation at Master Gardeners Meeting, Winchester, Va. http://water.rutgers edu/Rain_Gardens/RGWebsite/misc/Virginia_Tech-Rutgers_Rain_Garden_Wichester_Stormwater_101_benham_040109.pdf.
Daniels, W., G. Evanylo, L. Fox, K. Haering, S. Hodges, R. Maguire, D. Sample, et al. 2011. Urban Nutrient Management Handbook. Edited by J. M. Goatley. VCE Publication 430-350.
Gilland, T., Fox, L., Andruczyk, M., French, S., and Swanson, L. 2009. Urban Water Quality Management: What Is a Watershed? VCE Publication 426-041.
The authors would like to express appreciation for the review and comments provided by the following individuals: Brian Benham, associate professor, biological systems engineering, Virginia Tech; Jon Hathaway, Biohabitats Inc.; Thomas Bolles, Extension agent, Virginia Tech; and Adria Bordas, Extension agent, Virginia Tech.
City of Portland (Ore.). Bureau of Environmental Services. 2009. Portland Stormwater Management Manual. http://portlandonline.com/bes/index.cfm?c=47952&.
Virginia Department of Conservation and Recreation (VA-DCR). 2011. Draft of Virginia DCR Stormwater Design Specification No. 1: Rooftop (Impervious Surface) Disconnection, Version 1.9. http://vwrrc.vt.edu/swc/NonPBMPSpecsMarch11/VASWMBMPSpec1DISCONNECTION.html.
Best management practice – Any treatment practice for urban lands that reduces pollution from stormwater. A BMP can be either a physical structure or a management practice. A similar but different set of BMPs is used to mitigate agricultural runoff.
Bioretention – A BMP that is a shallow, landscaped depression that receives and treats stormwater with the goal of discharging water of a quality and quantity similar to that of a forested watershed. Bioretention devices typically consist of vegetation, soils, an optional underdrain, and an outlet.
Cistern – A storage tank designed to store rainwater for later use. Also known as a “rainwater harvesting system.”
Clay – Soils with a particle size smaller than 0.002 millimeter (mm), according to the U.S. Department of Agriculture’s (USDA) soil classification system.
Compost – Vegetative or organic matter that has been allowed to fully decompose, leaving a rich, organic medium that can be mixed with soils.
Compost-amended flow path – The practice of restoring soils within the flow path (with redirected roof runoff from RD) using compost (see “soil restoration”).
Dry well – A small, underground structure that disposes of stormwater through infiltration. Usually consists of a hole lined with gravel.
Erosion – The movement of soils and rock through weathering from water and wind.
Flow path – The path water takes as it flows over land; in the case of RD, after it exits the downspout.
Impervious surface – A hard surface that does not allow infiltration of rainfall into it; not pervious.
Infiltrate, infiltrated – The act of water entering soils (see “infiltration”).
Infiltration – The process by which water (surface water, rainfall, or runoff) enters the soil.
Media, filter media – The topsoil that supports plant growth. Bioretention media typically has a high sand and low clay content and a low phosphorus content.
Outlet – The point of exit of water from a downspout or other BMP.
Pervious – A ground surface that is porous and allows infiltration.
Rain garden – Often used interchangeably with bioretention, however it typically refers to a less formal design and installation process. Typically implemented in residential areas by homeowners.
Roofshed – The area of the roof that drains to a single downspout. The boundary is determined by the roof and the roof ridge lines.
Rooftop disconnection – RD redirects runoff from streets, storm drains, and streams onto landscaped areas and away from impervious surfaces.
Sand – Soils with a particle size larger than 0.05 mm, according to the USDA’s soil classification system.
Sediment – The soil, rock, or biological material particles that are formed by weathering, decomposition, and erosion.
Silt – Soils with a particle size between 0.002 and 0.05 mm, according to the USDA’s soil classification system.
Soil restoration – The technique of using compost to amend soils to improve their porosity and nutrient retention. The restored soils are less compacted and can replicate natural, forested.
Stormwater – Water that originates from impervious surfaces during rain events, often associated with urban areas; also called “runoff.”
Stormwater conveyance system – Storm drain or stream that is used by more than one property upstream from it.
Sustainable – The ability of the system to endure and remain productive over a long time.
Tree planter – An ultra-urban, small BMP that is a bioretention system designed to exist inside a concrete box or tree planter (see “bioretention”).
Underdrain – A perforated pipe in the bottom of a bioretention system designed to collect water that does not infiltrate native soils.
Virginia Cooperative Extension materials are available for public use, re-print, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences; Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
August 18, 2011