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

Larry Levine

Posted May 11, 2011

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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!

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Brady Russell - Clean Water ActionMay 12 2011 04:04 PM

Are all countries as parochial as ours? I'm in the middle of this fracking fight in PA and it never ceases to amaze me about how decisionmakers won't trust data that was collected in another place to inform decisions here: even tho it's the exact same technology in use. Sure, there are some differences around the edges, but are the differences that significant?

Similarly, Germany has been deeply, deeply invested in green roofs for years. They green every surface that they can. They find it profoundly effective. So effective that it's virtually verboten not to have a green roof.

But we have to evaluate this ourselves with cumbersome methodologies because we think, what? Water behaves differently in Germany?

The effectiveness of green roofs is proven. Let's move forward on this technology America! Summers in the City are killing me!

Leslie BilleraMay 19 2011 08:10 AM

Thanks for the third party endorsement for green roofs Brady - and for all your hard work!

And many thanks to Larry's fantastic recap on this exciting new research (which, to Brady's point, is not really 'new' but 'new to you, America')...

Part of Greensulate's mission is to educate on green roofs and the astounding array of benefits they deliver for both people and planet. Contact us if you want to learn more about what a green roof is - and what it isn't...

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