Blue Roofs and Green Roofs at PS118

Although water quality in New York Harbor has improved significantly over the last few decades, many parts of the waterfront and its beaches are still unsafe after it rains. The main culprit is our antiquated sewer system, which combines sewage from buildings with dirty stormwater from streets. When it rains, as little as one-twentieth of an inch of rain can/will overload the system. In fact, 27 billion gallons of untreated sewage and stormwater are discharged into the harbor and regional waterways every year.

Roofs comprise almost 1/3 of New York City’s total horizontal impervious surfaces- using them to hold and control rain water would yield an immediate benefit.

To achieve this, the NYC DEP (Department of Environmental Protection) is testing two alternatives to conventional rooftop surfaces side by side- Blue Roofs and Green Roofs, on the roof of PS 118 in Queens.

Blue roofs are non-vegetated source controls that detain stormwater. Weirs (a small overflow dam) at the roof drain inlets and along the roof can create temporary ponding and gradual release of stormwater. Blue roofs are less costly than green roofs. Coupled with light colored roofing material they can provide sustainability benefits through rooftop cooling.

Green roofs consist of a vegetative layer that grows in a specially-designed soil, which sits on topof a drainage layer. Green roofs are more costly than conventional roofs but they are capable of absorbing and retaining large amounts of stormwater. In addition, green roofs provide sustainability benefits such as absorbing air and noise pollution, rooftop cooling by reducing UV radiation absorption, creating living environments for birds, and increasing the quality-of-life for residents.