Green Roof Implementation in Washington, DC: A Stormwater Management Tool for an Impervious Urban Environment

Abstract

Green roofs have potential environmental and economic benefits of great consequence for our major cities if implemented at a broad scale. These benefits are beginning to be realized in commercial new construction and retrofitting, where green roofs are coming to be seen as a cost effective method of stormwater control and minimization of thermal heat gain in buildings. In Washington DC, green roofs are being explored as a tool to mitigate water quality problems across the city, and the benefits of retrofitting large buildings with green roofs are being examined and quantified. Here we explore the potential for green roof retrofitting in the residential sector, with a focus on the existing flat roofed architecture of residential neighborhoods in the District of Columbia, in particular the rowhouse. Our findings show that through green roof implementation on residential rowhouses in the District, it is possible to obtain an annual reduction of stormwater discharge volume within the Combined Sewer System area, where stormwater combines with sewage and released into the District’s waterways after most rainfalls. Our evaluation shows a 12.18% reduction (279 million gallons) in contaminated discharge within the Combined Sewer System area. While this reduction will not change the number of contamination events, it will have a positive effect of water quality in the District of Columbia. We approach residential green roof retrofitting as Landscape Architects and so this report includes the topics relevant to designers and their clients such as sustainability issues, construction materials, and economic obstacles. Our analysis also includes an assessment of the role residential green roof retrofits can play in stormwater management in the District, with a particular emphasis on how residential green roofs may be incentivized by the government. Green roofs can be made more cost-effective in the residential market with government incentives for homeowners to mitigate the first costs of retrofitting row houses.