On the rooftop of Public School 118 in Queens, New York, the Department of Environmental Protection (DEP), in partnership with the Department of Education, is testing two alternative roofing surfaces. For a three year monitoring period, data from the study at PS 118 will be collected and compared in order to determine which green, blue, and control surfaces perform best as stormwater management systems. Because all three surfaces will be on the same building, this study will benefit from increased accuracy by comparing the surfaces under similar environmental conditions. The surfaces are known as blue roofs and green roofs; the latter is rather well-known, whereas a blue roof is a fairly new source of alternative energy and conservation.
Blue roofs are non-vegetated systems that detain stormwater and are less costly than their green counterparts. This is how they work: Weirs at the roof drain inlets and along the roof create temporary ponding (damming on a small scale) and gradual release of stormwater. Blue roofs complement long and flat roofing styles, and have wide gutters with a sturdy watertight liner. This design works especially well in highly urbanized areas, like New York City, where less space is available for on-site stormwater detention. Some stormwater may be temporarily stored on the roof while the discharge can be released to a stormwater harvesting or infiltration system, or a portion can be discharged to the drainage system at a relatively slower flow rate. As discussed later in this post, blue roofs are particularly suited to New York’s bid to lessen overflow of sewage into their rivers during storms.
Green roofs, the other surface being tested at PS 118, consist of a vegetative layer that grows in a specially-designed soil, which sits on top of a drainage layer. Green roofs are more costly than conventional roofs, but they are also capable of absorbing and retaining large amounts of stormwater.
In addition, green roofs provide benefits such as absorbing pollution, rooftop cooling, creating bird habitat, and increasing quality-of-life for residents. Depending on the region, cities often provide a Green Roof Tax Abatement from property taxes.
As evidenced by the study at PS 118, New York is utilizing blue and green roofs in a way that other cities have yet to explore. Michael Bloomberg, New York City’s sustainability-minded mayor, has identified stormwater management as a pressing urban issue. He believes the utilization of blue and green roofs could save New York City $2.4 billion over 20 years; without the roofs, taxpayers could end up spending $6.8 billion repairing constantly flooded treatment plants. 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, which helps to reduce the need for more infrastructure, as well as helps to dilute the sewage on its way to treatment.
However, this system is not suited to New York’s dense and growing population. The volume of sewage, when stormwater is present, surpasses the capacity of New York’s 14 wastewater treatment plants. When rainfall flow reaches a certain strength and exceeds capacity, the result is an “overflow” of raw sewage into public waterways like the East and Hudson Rivers. This overflow is highly detrimental to the environment and to its human and animal inhabitants. Under the mayor’s plaNYC 2030, he aims to reduce overflows by 40% and make at least 90% of the city’s waterways hospitable for recreation.
Overall, both blue and green roofs help mitigate stormwater by gathering it as rainfall and releasing it gradually over time, instead of rushing into sewers. They both offer an insulating layer on top of a roof to help trap energy in the winter and reflect sunlight in the summer. Green roofs offer an opportunity for biodiversity and food production; blue roofs can be used for irrigation, cleaning sidewalks or reducing potable water use by filling or toilets. Both would be valuable additions to cities everywhere. It will be interesting to see the results of PS 118′s blue and green roof experiment, and the implications it could have for sustainability activism.