By Frank Fortino
As previously reported, part of the Climate Mobilization Act requires installation of sustainable roofing zones in new construction and for buildings undergoing certain alterations. Local Law 94 of 2019 defines a “sustainable roofing zone” as roof assembly areas that include installation of “a solar photovoltaic electricity generating system, a green roof system, or a combination thereof.” Developers that are exploring green roof options can learn from the New York City Department of Parks & Recreation, which maintains more than 200,000 square feet of green roofs across the five boroughs.
Green Roof Systems
A green roof consists of a thin layer of vegetation, installed on a traditional roof. Design specifics vary, but basic components include the following:
- Root barrier (to prevent plant roots from growing through the roof)
- Water retention/drainage
- Growing medium
Green roofs can be classified as either intensive or extensive.
- More than 6 inches deep, which allows greater plant variety. Similar to traditional rooftop gardens, with social and recreational uses. Require more maintenance. Typically more expensive to install.
- Shallower, with growing media less than 6 inches deep. Lighter weight systems that mainly offer ecological and economic benefits. Fewer layers than intensive roofs. Require less maintenance. Lower installation costs.
Guidance from NYC Parks
Since 2007, NYC Parks has maintained a green roof at the Five Borough Complex on Randall’s Island. Currently encompassing 45,000 square feet, this space is the nation’s only known green roof featuring dozens of unique systems side by side. The space serves as a laboratory for green roof design and construction. The systems vary in type, as well as depth, of growing medium and in plant selection.
New Yorkers who are interested in learning more about urban green roofs can schedule a tour with NYC Parks. In addition, the Greenbelt Native Plant Center publishes a list of plants suitable for green roofs in New York City.
Benefits of Green Roofs
Beyond the new mandates for green roof installation, these systems offer a number of bottom-line benefits for property owners:
- Roof longevity. Green roof systems reduce temperature variations on a roof, which creates stress due to thermal expansion and contraction. Green roofs also protect the roof surface from ultraviolet rays. As a result, these systems may extend the usable lifetime of roofs by up to 60 years.
- Energy conservation. These systems also reduce energy usage for heating and cooling. Green roofs insulate the upper floors of a building in the winter. During the summer, green roofs help cool the building through shade and the evaporative process by which plants release water into the atmosphere.
- Property value. Green roofs improve the beauty of buildings, creating green space and potential recreational areas—factors that make a property more attractive to residents and commercial tenants.