One Roof, Two Roofs, Green Roofs, Blue Roofs


Now in his third term (somewhat controversially), Mayor Bloomberg has toted sustainability as a priority throughout his tenure in overseeing one of the most densely populated cities in the world. In addition to creating PlaNYC 2030 and yielding power back to pedestrian traffic in numerous locations like Times Square and Herald Square, the tax credits for green roof construction were also passed under his administration. According to an article by Reuters, the Mayor is taking another swing with an evolved message that adds “Blue Roofs” to the arsenal of tactics to be employed throughout the city.

Bloomberg’s targeting of stormwater management as an urban issue could not be more appropriate.  In many of our cities the problem of our wastewater infrastructure is rarely given the level of importance and urgency that it is due. Like many older cities in the Northeast, New York’s wastewater system has been around for over a century and was constructed around the idea of Combined Sewer Overflows. CSOs funnel stormwater and municipal sewage into the same pipes—a two for one deal that helps reduce the amount of infrastructure that’s built as well as diluting sewage waste on its way to treatment.

The problem is that the growing population of the city has paralleled the growth of impervious surface, meaning that volume of both sewage and stormwater run-off have risen beyond the capacity of New York’s 14 wastewater treatment plants. When rainfall reaches a certain strength (sometimes as little as ½” per hour), the flow exceeds capacity of the infrastructure and is allowed to “overflow” into the East and Hudson Rivers. Raw sewage into public waterways. It sounds shocking, but it is far more common that most of us think and stands in the face of the Mayor’s goal of 90% of the city’s waterways suitable for recreation.

Short of replacing the entire system (a fiscal impossibility for the city any time in the foreseeable future) any efforts to stem the flow of stormwater into the sewage system will relieve the pollution from entering the natural watershed. On the contrary, Bloomberg claims that not only could the city save $2.4 billion over 20 years by utilizing sustainable roof systems but not doing it could force taxpayers to spend $6.8 billion on repairing the flooding treatment plants.

The Rainbow of Options

Layers of green roof

In some ways, Green Roofs and Blue Roofs accomplish similar things. Green roofs consist of growing plants on rooftops and can be divided into two groups: Extensive and Intensive. The former refers to systems that usually have 6” or less of growing medium and often are placed on top of the roof in either trays or bags. Intensive roofs are deeper installations that include full continuous soil over an integrated drainage mat and water proofing. These roofs can be anyway from 8” to multiple feet deep.

Conversely, the label “Blue Roof” has been coined to refer to systems that focus on rainwater collection. By using catchment pools, rain barrels and more discreet water-hungry plantings, the goal of the roof system would be to minimize the amount of storm water that a building site sheds to the rest of the city.

Both of these options help mitigate stormwater by gathering it as rainfall and releasing it gradually over time. They both offer an insulating layer on top of a roof to help trap energy in the winter and reflect sunlight in the summer (reducing the heat island effect felt in urban centers.) The difference is that green roofs offer an opportunity for biodiversity and food production while the water gathered by blue roofs can be used for irrigation, cleaning sidewalks or reducing potable water use by filling washing machines or toilets.

En masse, both would be valuable, productive additions to the top of New York or many other U.S. cities.

A Hard Sell

In my experience, these kinds of roof systems have not yet found a great deal of patronage outside of municipal installations. In laying out the rooftop options of Photovoltaics, Evacuated Solar Tubes or green roofs, PVs always gain the most interest and commitment. Although I have never pitched a “blue roof,” I feel like the results would be the same.

First of all, the entire notion of blue roofs is rather new. I have never seen one installed nor have heard of one being pitched to a client in the city. The concept itself seems a bit half-baked. For an architect, the idea of having standing water on a roof is usually something we try to avoid rather than encourage. After enough time, water has a way of finding its way into just about everything.

The other problem with green and blue roofs is that the benefits are not necessarily less material, but usually less visible and not as concentrated immediately on the space below them. While the benefits I mentioned are real, they are admittedly incremental as an investment with a payback period. Instead, their positive effect is felt more at the urban scale. Adding to biodiversity yields no dollars back to the client, but could bolster urban farming as well as a healthier city ecosystem. While reducing the heat gain of one home is relatively small, covering 20% of the city with green roofs could lower the ambient temperature of the island as much as five degrees in the summer according to one green roof installer. Even stormwater management is a city-scaled benefit that may eventually yield lower taxes but only after a certain critical mass has been achieved.

Unfortunately, too many people may need healthy incentives in order to invest in these new roofscapes. Only so many people are willing to fund a smaller piece of a larger system with minimal return. As of yet, no new details have been released concerning how the effort will be supported by the city. The current subsidy for green roofs is $4.50/square foot up to $100,000 which most likely does not cover more than 20-30% of an intensive green roof system.

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