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Associated Renewable
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Green Roofing is Catching Not Only Stormwater, But Also the Attention of NYC and Its Neighbors

By: Nora Prevoznak - Associated Renewable, Inc.

Published: October 5th, 2012


Two years ago on September 28, 2010, Mayor Michael Bloomberg announced the NYC Green Infrastructure Plan to help change the way the City manages its storm water and runoff.  Its goal is to "build green infrastructure demonstration projects on a variety of land use," including the installation of more green roofs across New York City.  The Green Infrastructure Grant Program awarded more than $3.8 million in its inaugural year to projects including green roofs, and other infrastructure features such as porous pavement or rain gardens. This idea to use biological green systems to address urban problems has proven to be smart and cost effective.

The basic green roof consists (bottom to top) of a structural frame covered by a waterproof membrane, an inorganic barrier layer that prevents roots from penetrating into the membrane, a layer of gravel or thermoplastic material to provide drainage, a layer of fibrous material for moisture retention, and, finally, the porous planting medium — volcanic pumice, chipped shale, or even ground up roofing tiles. This planting stratum is mostly inorganic, lightweight, and absorbant, while containing enough mineral content to allow plants to grow." ("Green Roofs are Starting to Sprout in American Cities").

Green roofs work by capturing moisture and evaporating it, while the excess water can drain from the roof to existing drains at existing slopes.   Along with an increase in greener sidewalks and porous parking lots in New York City, these combined surfaces will attempt to eliminate 40 percent of the existing runoff and thereby hope to save $2.4 billion over the next 20 years. This is an alternative approach to spending somewhere of $6.8 billion to fix and improve the 150 plus year-old infrastructure of New York City’s water system.  The city’s old sewer system combines rainwater and wastewater during storms with higher rainfall.  It does not take much to overwhelm the sewers and when there is overflow, the polluted water mix runs into our water streams. 

New York City decided it was more cost-effective to build green infrastructure, including green roofs, than to construct more sewer pipes or storage tanks.  The City is spending $1.5 billion over the next 20 years on green projects that will reduce rainfall runoff, a much smaller amount than the projected $6.8 billion.  To encourage building owners to install green roofs, the NYC Department of Buildings offers a $4.50-per-square-foot tax abatement for green roofs.  Eligibility includes requirements that 50 percent of the roof space is green; 80 percent of the vegetation layer must be covered in live plants; weatherproof and waterproof membrane; an insulation and drainage layer; and a maintenance plan.  

More people and organizations are talking about the added benefits of green roofs for property owners.  Because the vegetation on green roofs lowers absorption of solar radiation, they can substantially reduce annual energy consumption for heating and cooling.  Because of the ‘urban heat island effect’, New York City is roughly 7 degrees hotter in the summer than its surrounding suburbs.  The urban heat island effect develops because built surfaces (in a greater number and density in cities) are composed of a high percentage of non-reflective and water-resistant construction materials.  As consequence, they tend to absorb a significant proportion of the radiation, which is released as heat.   A warmer city leads to more than just discomfort – it increases air conditioner usage and thereby, energy loads.  A recent study conducted by Columbia University and City University of New York looked at 3 green roofs built by the same company in Queens.  They found that green roofs cut the rate of heat gained by 84% in summer and 34% in winter.  This data is very significant.   Added with other energy efficiency technology and conservation measures, buildings can significantly lower their energy costs. 

 One of the largest and most known green roofs is on top of the US Postal Service's Morgan Processing and Distribution facility in midtown Manhattan.  The roof with densely planted vegetation, has flourished since December 2008, two plus years before Mayor Bloomberg's initiative.  Its water runoff is said to have been reduced as much by 75 percent in summer and 40 percent in winter.  There are now many green roofs across the city that are viewed as using a new Green Infrastructure Webmap. 

 New York is not the only city using green roofs to prevent water runoff and decrease energy consumption.  Philadelphia has emphasized the role of green roofs in its ambitious plan to reduce the large volumes of stormwater that overwhelm its combined sewer system. "The Green Roof Tax Credit is available to business owners constructing an addition to their roof that supports living vegetation and includes a synthetic, high quality waterproof membrane, drainage layer, soil layer and light weight medium plants." The City offers a rebate for 25% of green roof costs up to $100,000.  The city hosts green roofs ranging in locations such as the Free Library; Temple University; Philadelphia University; PECO Energy headquarters; Philadelphia Convention Center Parking Facility; Friends Center of Philadelphia; a bus stop shelter green roof; and residential homes and apartments outside of central Philadelphia in South Philadelphia, Wyndmoore, Queen Village, amongst others.

 Cease all subsidies, and there are still long term savings from using green roofs as a solution to water runoff and energy demand problems. Green roofing also has aesthetic benefits in a city like New York, where immense buildings can populate a small built area.   Green roofs reap benefits for buildings and the city, and it is almost a guarantee their prevalence will increase in New York City, Philadelphia, and other urban areas. 


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