New York's Cooperative and Condominium Community

Habitat Magazine Insider Guide



The Greening of Your Roof

When Susan Boyle and her husband bought a six-unit apartment building in Crown Heights in 2001, their goal was to convert the building into a dual-use property – to earn rental income from it and to create an environmentally friendly building. Today, when Boyle and her husband stand on the roof, they can wiggle their toes in the grass and alpine plants they have carefully cultivated.

For Boyle, a “green roof”– usually a flat surface covered with grass and plants, trees, and shrubs – makes practical and aesthetic sense. It increases the life of the roof membrane by protecting the roof’s surface from extremes in temperature, improves the air quality, helps insulate the building, and creates an attractive green space. Having once been an advocate for more bike routes with the nonprofit Transportation Alternatives, Boyle now spends her days with a different ecologically friendly goal in mind – guiding New Yorkers back to nature by getting them to construct lawns on their roofs.

According to the Scandinavian-based Green Roofs Institute, between 2004 and 2005, North American green roofing went from covering 1.3 to 2.5 million square feet. In such cities as Chicago, the movement has been helped along with $5,000 grants from the city council.

But in New York City, some argue that the pace has been maddeningly slow. Part of the problem has been the hodgepodge of regulations that cover green roof construction. The Battery Park City Authority, for instance, mandates that all new construction include a green roof while the city council provides no tax credits or benefits for buildings that do a retrofit.

Still, there are hopeful signs. In October 2005, Mayor Michael R. Bloomberg signed Local Law 86, which requires many of the city’s new municipal buildings, additions, and renovations to achieve a Leadership in Energy and Environmental Design (LEED) rating of “certified” or “silver” and, in many cases, to use energy and water more efficiently than required by current codes. The LEED Green Building Rating System is a nationally accepted benchmark for the design, construction, and operation of high-performance green buildings. Some feel that if the city is requiring green ratings for the buildings it owns, ratings – and incentives – for residential buildings can’t be far behind.

So, how do you go about building a green roof? There are a lot of ways, notes Katrin Scholz-Barth, a Washington, D.C.-based engineer who consults on green roof construction around the United States, but in any approach, a co-op or condo board must have “a clear, defined goal. It’s easy to get lost in the jungle of options,” she says; boards need to know what they want before they start, or the good intentions will go awry.

The first thing to do before considering a green roof is to determine the cost benefits and who will have access. For Boyle, the biggest selling point is that they “immediately double the life span of the roof.” They also improve air quality, control storm-water runoff, and provide an aesthetically pleasing green space.

Practically speaking, there are two types of green roofs: extensive and intensive. An “extensive” one has a shallow soil base, between two and six inches deep, and holds grasses, sedum (a strain of orpine plant), and succulents that can survive in extreme temperatures. An “intensive” green roof has a deeper base – four to twelve inches of soil – and can hold heavier-weight plants and shrubs. Load-bearing capacities vary sharply from building to building: some roofs can handle only 40 pounds per cubic square foot while others can handle up to 100 pounds per cubic square foot. Boards need to “evaluate the existing structure of the building, and know what the load is, then bring in an architect or landscape architect to design what they want to try,” says Scholz-Barth.

While there are several ways to build an extensive green roof, the most common method is to lay down protection boards made of neoprene or fiberglass (so they won’t rot), lay a drainage mat over the boards, and then install the soil system, and layer the plant material on top of that.

When Signe Nielsen, a landscape architect with Matthew Nielsen, designed an 8,500-square-foot green roof for the Battery Park City condominium Tribeca Green two years ago, the developer contracted with the Chicago-based American Hydrotech ( to provide a soup-to-nuts extensive garden. The company weatherproofed the roof with its patented membrane and laid down the protection boards, drainage mats, and soil systems for the plants, offering a guarantee on all its work. The total cost: $15 a square foot. For boards that want to shop around for different suppliers – there are plenty via the internet – the New York-based nonprofit Gaia Institute has created a lightweight soil system that weighs only 10 pounds per cubic foot (many buildings in New York City can handle between 40 and 100 pounds) and can hold up to twice its weight in water.

Managing the water load is a key component of a green roof, says Scholz-Barth. For a commercial building, the structure must be able to control storm- water runoff. For a residential one, capturing rain in an extensive green roof means reducing the chances of leaks. It also means you can filter the excess water more slowly into the city’s sewer system, and thus reduce the amount of excess water sent untreated into Long Island Sound and the East River.

One of the pitfalls of building a green roof, however, is that residents think they will automatically be able to use it as a lawn, kicking around a soccer ball, or sunning themselves on warm days. The grass and the sedum need time to take root. After that, some limited access may be available, but most extensive green roofs in New York City include pavers for walking around the garden and for keeping people off the greenery. Such roofs can’t handle heavy foot traffic; the benefits come with creating a green space people can enjoy viewing, one that also helps insulate the building – keeping it cooler in the summer and warmer in the winter, thus reducing the building’s energy costs – and helping to improve air quality.

While some boards may be tempted to jump into designing a green roof right away, the smartest way to handle such an installation is to fold it into the costs of roof repairs. An extensive roof can run from $8 to $10 a square foot in cost, with repairs, depending on the situation, running to a similar amount. If the roof is in good condition, the board just needs to check the warranty to ensure the green rehab will be covered. If the surface does need repair, building a green roof at the same time makes sense.

This spring, the Gaia Institute plans to introduce a policy paper to the New York City Council, advocating for tax credits or benefits for retrofitting New York City rooftops to green. Such organizations as the Sustainable South Bronx, which have planted 10 extensive green roofs, have been working with the Gaia Institute, tracking the control of storm-water runoff and the improvement in air quality (less particulate matter) because of the green roofs. It’s been a long process to get the city council interested in financially supporting the idea, but green advocates are hopeful.

“For green roofs to gain traction, there needs to be a lot more facts about the benefits,” admits Nielsen. Once the grass and plants are established, usually over a two-year period, the green roof only needs to be weeded occasionally and watered over long dry days. Other than that, the “lawns” on the roof are pretty much self-sustaining, points out Nielsen, who adds: “The benefits go to the wider urban environment.”

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