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Cool, Green, or Blue?

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Cool, Green, or Blue? The environmentally responsible urban roof is here. By Larsen Plano

Many city-dwellers are captivated by urban roofs, and once you’ve been on one, it’s easy to see why. You feel a unique sense of calm being apart from the bustling crowds below, and the majesty of the city is much more present when you have the rare chance to gaze for miles in this direction or that.

When you’re standing on your roof, the possibilities seem endless: can we camp up here? New York City residents see in their roof an opportunity to improve their building and their city, especially from an environmental standpoint. As a sustainable building consultant, I’ve spent more and more time on roofs each year, perhaps a good indication of the growing interest in transforming New York City’s rooftops – all 22,425 acres of them.

The Cool Roof

One of the easiest and least expensive ways to make an urban roof environmentally responsible is to turn it into a “cool roof.” During this past summer’s record-setting heat wave, New Yorkers experienced something known as the “urban heat island effect.” Simply put, the temperatures we experience during the day are around 6 degrees hotter than those in surrounding areas, and at night can be as much as 22 degrees warmer. The cause? Our abundance of dark, impervious surfaces such as roofs and pavement, which tend to warm the air, and a lack of vegetated areas, which ordinarily would cool the air. The results are not pretty: more demand for energy to keep us comfortable, higher electric bills, and a rise in air pollution and greenhouse gas emissions.

A cool roof – also known as a white, or reflective, roof – has a light-colored surface that reflects sunlight instead of absorbing it. By minimizing that absorption, the cool roof has a lower temperature than a typical roof on a sunny day and contributes less heat to the atmosphere, diminishing the urban heat island effect.

For just this reason, over the past two summers the mayor’s New York City Cool Roofs Program has covered more than two million square feet of the city’s rooftops with a reflective coating – transforming formerly dark, hot roofs into bright, white, cool ones. The program’s website,, offers a “Cool It Yourself Kit,” with information about measuring your roof, navigating roof warranty issues, and selecting and applying the right reflective coating.

Most coatings cost from 25 to 50 cents per square foot, and a super can easily apply the paint on a small rooftop. As with any flat roof, of course, a cool roof will accumulate grit over time, but an annual cleaning will keep the cool roof doing its job.

Can Your Roof Be Cool?

Unless your roof is topped with a layer of gravel, chances are good that it can be turned into a cool roof. Most New York City apartment buildings have asphalt-based roofs – commonly known as bitumen, modified-bitumen, or built-up roofs. For these roofs, options range from silvery coatings that reflect around 60 percent of the sunlight (but begin to wear off after around five years) to slightly more expensive elastomeric coatings, which reflect over 90 percent when new and will usually last 7 to 10 years.

If your co-op or condo is replacing a roof – an expensive but sometimes necessary event – you can install an intrinsically reflective roof membrane, which requires no coating. Is your building considering a rooftop patio? Reflective roof pavers are also available.

A cool roof, in fact, significantly extends the life of the roof membrane. A cool roof will also transmit less heat to the top floor of a building, and if this space is air-conditioned, installing a cool roof will usually result in energy savings on hot summer days. Remember, though, that nothing improves energy efficiency like roof insulation, which a surprisingly large percentage of older New York apartment buildings do not possess. Roof insulation is fairly easy to install and relatively inexpensive.

The Green Roof

A green roof is essentially a layer of vegetation growing in a light-weight, soil-like medium, which is installed over the roof membrane. The “extensive” type of green roof (and the one most commonly installed) has a carpet of short sedums; the “intensive,” more substantial type has enough soil for shrubs or even trees.

Most green roofs also include additional components separating the roof membrane and the growing medium, such as a drainage layer to allow excess water to drain from the roof and a root barrier to prevent any aggressive plants from damaging the membrane.

Like a giant sponge, a green roof will absorb rain rather than allow it to flow unimpeded into the sewer system. The greenery absorbs some of the water, and the rest evaporates. Indeed, Columbia University researchers have found that the natural cooling caused by evaporation from a green roof is similar to a 70 to 85 percent reflective cool roof, and of course a green roof does not wear off. A green roof will also help rid the air of pollutants.

Beneficial but Expensive

So, with all these benefits, why aren’t green roofs more common in Gotham? I talked about the issue with Don Sussman and Hanna Packer of Town and Gardens, a Manhattan landscape firm that has designed and constructed green roofs for the Diane Von Furstenburg Headquarters and a site operated by the social service organization Common Ground, among other venues (I have worked with T&G on several green roof projects).

According to Sussman, cost is an undeniable factor in New York City, where just about any building-related work is complicated. In his experience, the typical price is $20 to $30 per square foot of vegetative area, and installing the materials also tends to be costly. A $4.50-per-square-foot property-tax abatement helps, but hiring an engineer or architect to file with the Department of Buildings can eat into those savings.

Think Before You Plant

For Common Ground’s site, at Brook Avenue and 148th Street in the South Bronx, Town and Gardens designed and constructed a green roof safely enclosed by a high chain-link fence, with minimal walkways and a few benches. This is both an environmentally responsible creation and a peaceful retreat.

Still, Sussman cautions a co-op or condo board to consider its goals carefully before installing a green roof. Generally, a vegetative roof does not work well if a board primarily wants an amenity. A complete green roof is not intended for frequent traffic, only for weed-pulling and other maintenance.

Josie Rivera, who maintains her co-op’s green roof at 1347 Bristow Street in the Bronx, reports that she keeps a lock on the roof door and goes up only once or twice a week to pick weeds, although, she admits, “Sometimes I do bring a book.” Not surprisingly, Hanna Packer of Town and Gardens insists that weeding “be done by a professional who knows what is actually a weed!” She has seen too many supers and residents pull up plants that had been placed only months before.

Installing the Green Roof

If a board does opt for a green roof, it’s best to call on experienced designers and contractors. One critical concern: the load-carrying capacity of the roof. According to Sussman, “Saturated growth medium weighs as much as a layer of concrete of the same thickness – around 25 pounds per square foot for four inches.” In order to meet code, any building should be able to handle that weight, but poor roof installation or maintenance can result in reduced capacity, especially when the supporting structure is wood. So inspect your roof’s structural condition before going ahead.

Examine the roof membrane for leaks. Town and Gardens strongly recommends that buildings go the extra mile and install a detection system that will enable any membrane leak to be pinpointed within millimeters.

The Blue Roof

Every New Yorker notices that rainstorms tax our sewer system. But lakes at crosswalks are just the beginning. When a large amount of rainwater enters the sewers, NYC’s sewage-treatment plants can be overwhelmed, causing them to release untreated sewage straight into our rivers. Known as Combined Sewer Overflows, or CSOs, these discharges amount to an astounding 27 billion gallons each year.

A blue roof alleviates CSOs in much the same way a green roof does, by retaining water during a heavy rain. But instead of vegetation absorbing the moisture, a special roof drain goes to work, restricting the flow rate. After a rainfall, when water pools on a roof, controlled flow devices allow a steady, slow stream to enter the sewers over many hours, rather than as an intense surge all at once.

A blue roof costs considerably less than a green roof. Figure around $5 per square foot for a second membrane (highly recommended) and labor.

A Final Word About Roofs

Any of these roofs – white, green or blue – would make a beneficial contribution to our urban environment. Indeed, the opportunity to improve our roofs would seem obvious, yet when “roof” comes up on the agenda many boards find themselves almost paralyzed. The safest choice often seems to be “keep doing whatever we did the last time.” This paralysis is understandable: the stakes can seem impossibly high. After all, a reliable roof is critical to the integrity of a building. But the possibilities can be both affordable and beneficial – for a building and for this invaluable urban environment we all share.

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