Credit: 100RC

Learning from the Planet’s Largest Multi-System Green Roof

In 2007, the Five Borough Technical Services Division of the New York City Department of Parks and Recreation installed its first green roof system atop their headquarters on Randall’s Island. Nine years later, this green roof is one of the largest in New York City and the planet’s largest multi-system green-roof, covering 35,000 square feet. The 100 Resilient Cities team visited the Five Borough Green Roof and spoke with Artie Rollins, Assistant Commissioner for Citywide Services, to learn from this living laboratory for innovative green roof design, and to learn about the manifold benefits green roofs provide.

Assistant Commissioner Artie Rollins explains the various systems that coexist on one of NYC’s largest greenroofs

The Five Borough green roof comprises 30 distinct growing systems – it’s the only roof of its kind with different plant systems side by side. The soil on the roof is lighter than traditional organic soils and includes up to 80% lightweight inert material made from Styrofoam. The roof is full of native plants and sedum that can withstand extreme heat, wind, and rain.

As a part of New York City’s resilience strategy, the City set an ambitious goal to lower greenhouse gas emissions by 80% by 2050. The City’s Parks Department has been working towards this goal in numerous ways – over 1,000 buildings in the department are moving to LED lighting in the next year. Assistant Commissioner Rollins went into detail on the numerous benefits of green roofing, including:

  • Reducing the heat island effect: The surface temperature of a black rooftop on a hot summer day could be as hot as 160 degrees Fahrenheit. Green roofs reduce the surface temperature to about 85 degrees.
  • Energy efficiency: Air conditioners on a conventional rooftop would be taking in 160 degree air during the summer, but on a green roof, they have to work less, as the temperature pre-cooled by 60-80 degrees. Green roofs keep buildings cool enough that in the early and late summer, buildings don’t need to turn on their air conditioning. In the winter, green roofs help the higher floors of buildings retain heat.
  • Roof protection and longevity: Green roofs protect rooftops from UV rays, and roof expansions and contractions. Conventional rooftops have a lifespan of approximately 20 years, but green roofs have two to three times the lifespan, saving building maintenance costs.
  • Water Quality Improvement: Green roofs lower a building’s storm water runoff by 50-90%, preventing rain from leaving the facility and entering the sewer system. Green roofs filter 95% of the cadmium, copper, and lead (in addition to 30% of the nitrogen and phosphorous) in storm water, preventing environmental contamination.

Green roofs are on the rise throughout the 100RC Network. In fact, Toronto is the first city in North America to mandate the construction of green roofs on new development. Buildings with floor areas of at least 21,527 sq feet (200 sq meters) are required to create green roofs – the City now has almost 450 green roofs and has an “eco-roof incentive program” to subsidize the cost.  Chicago‘s City Hall has a 38,000 square foot green roof that saves the city $5,000 USD per year on utility bills. Singapore’s Nanyang Technological University has a curved green roof that doubles as recreational space, and also harvests rainwater for irrigation.

Nanyang Technological University Green Roof (Source: INHABITAT)

Green roofs are a cost effective intervention that can bolster a city’s flood resilience, lower temperatures, and boost energy efficiency. The NYC Five Borough Green Roof is a model for how city governments can leverage their own assets to build city resilience, and inspire others to reclaim green space.