New Study Rethinks NYC's Old Assumptions: Green Roofs May Be the Single Most Cost-Effective Way to Clean Up Waterways

As I’ve described in other blogs here, and here, New York City has a nearly 30 billion gallon-per-year problem with raw sewage overflows that’s going to take decades of sustained public and private investment to fix.  Some of the best investments we can make come in the form of “green infrastructure” -- things like green roofs, parks, roadside plantings, and porous pavements that provide space for rainwater to seep into the soil or be taken up by plants, before it overwhelms our overburdened sewer systems. 

New York City has a tax credit that covers about 25% of the costs of installing a green roof.  But, anecdotally, it seems that there are so many bureaucratic hurdles involved that hardly anyone has taken advantage of it.  Perhaps that’s because, while the city is generally keen on green infrastructure, it hasn’t really been convinced that green roofs, in particular, are worth the money.

A new study could change all of that.  Last week, ConEd released a study by Columbia University researchers, which found that a quarter-acre green roof on a ConEd building in Long Island City captured rainwater for a mere 2 cents per gallon annually. 

ConEd green roof, Long Island City

The new study suggests the city may be over-estimating the per-gallon cost of using green roofs to control stormwater runoff by a factor of 22 to 166 times.  Comparing their findings to those in a 2008 city report, the Columbia researchers concluded that, even on the higher-end of their cost estimates, their result “changes the ranking of green roofs from least cost-effective in the PlaNYC report to most cost-effective of the stormwater interventions considered in that report.”   

And on a city-wide scale, the benefits of green roofs could be enormous.  The report concludes:  “Assuming New York City has at least 1 billion square feet of roof area that could in principle be greened, we estimate that annual stormwater flow to the City’s wastewater treatment facilities would be reduced by at least 10 billion gallons.” 

And then, of course, there's the insulation and associated energy savings that vegetated rooftops provide -- not to mention the way vegetation works as natural air conditioning for the outdoor environment, critically important in the face of global climate change.

Based on these new findings, the city may want to re-think how it prioritizes green roofs within its overall Green Infrastructure Plan -- and get to work improving the green roof tax credit and streamlining the process for building-owners to install them. 

The city should give green roofs a closer look.  I think they’ll like what they see!

About the Authors

Larry Levine

Senior Attorney, Water program

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