Greening Your Roof: You Can Do It, and It'll Be Great. Here's How, and Why…

A "green roof" — generally a flat surface covered with some combination of grass, plants, trees and shrubs – makes practical and aesthetic sense:

• It increases the life of the roof membrane by protecting the surface from temperature extremes.

• It helps insulate the building

• It improves air quality

• It creates an attractive green-space amenity.

According to the Scandinavian Green Roof Association, North American green roofing went from covering 1.3 to 2.5 million square feet between 2004 and 2005. And the pace will likely accelerate now that New York State, following the lead of such cities as Chicago, has begun a tax-abatement program to encourage the creation of green roofs.

The abatement, for which co-ops, condos and other residential buildings can apply for beginning Jan. 1, 2009, totals $4.52 per square foot of approved green roof, up to either $100,000 or a building's tax bill for the applicable year, whichever is less. According to the law's Assembly Memo, this abatement would offset about 35% of the cost of installing a green roof — "a growth medium and a vegetation layer of drought-resistant, hardy plant species" that provides an insulating layer to helps reduce energy consumption, among other benefits.

Planting the Seed

So how do you go about building a green roof? Practically speaking, there are two types of: 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" has a deeper base — four to 12 inches of soil — and can hold heavier plants and shrubs. Load-bearing capacities vary from building to building, generally ranging from only 40 pounds per cubic square foot to a much sturdier 100. 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 engineer and green-roof consultant Katrin Scholz-Barth (above, at the green-roofed Montgomery Park Business Center in Baltimore, Md.).

While there are several ways to build an extensive green roof, the most common is to lay down protection boards made of neoprene or fiberglass (which unlike wood and other materials do not rot), lay a drainage mat over the boards, install the soil system, and layer plants atop that.

When Signe Nielsen of Mathews Nielsen Landscape Architecture, designed an 8,500-square-foot green roof for the Battery Park City condominium Tribeca Green, the developer contracted with the Chicago-based American Hydrotech to create an 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, at $15 a square foot.

You may be able to reduce that cost by separately purchasing the soil system yourself. Among the many suppliers, the New York-based nonprofit Gaia Institute has a lightweight system that weighs only 10 pounds per cubic 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. Capturing rain in an extensive green roof reduces 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.

Access Issues

One of the pitfalls of a green roof, however, is that residents think they will automatically be able to use it as a lawn, for kicking around soccer balls or sunning themselves. But the grass and the sedum need time to take root before you can grant even limited access. Green roofs can't handle heavy foot traffic, and most extensive green roofs in New York City include paved walkways that allow for strolling while keeping people off the greenery.

While you may be tempted to jump into designing a green roof right away, the more practical approach is to fold it into your next roof repair. But if your roof is in good condition and you decide to green it anyway, check your warranty to ensure that the green rehab will be covered.

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 roof lawns are mostly self-sustaining, says landscape-architect Nielsen — who notes that along with all the private benefits they give your own building, you're also benefiting "the wider urban environment." But be careful! You could make other buildings go green with envy!

Adapted from Habitat January 2007. For the complete article and more,  join our Archive >>