Green Roofs Aren't Perfect—Yet

Roofs have been changing color for years now—from black and red to green, white, and blue—but the shift is happening slowly. In the greenest European cities, like Copenhagen and Stuttgart, Germany, green roofs cover at most 25 percent of the available real estate. In the United States, where the concept gained popularity more recently, the numbers are smaller.

Still, the ideas behind installing plants on a roof, painting it white to reflect heat, or using it to manage storm-water runoff are compelling—white roofs have been plugged by New York City Mayor Michael Bloomberg as a simple and cheap environmental fix. Almost anything is preferable to a black tar roof, but if green roofs are going to become standard, the idea needs to be refined in order to live up to its full potential.

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After all, not all green roofs are created equal. I was surprised when I stepped onto my first one to find that it resembled a grubby doormat in texture. The plants grow in shallow plastic containers tiling the entire surface. They feel functional and tame, and couldn’t double as a rooftop oasis where a person would want to while away an afternoon. They also work best when the plants are growing and green, and achieving that state is more complicated than it seems.

In most places, plants aren’t green all the time, for instance. Should green roofs stay green when the plants growing on them aren’t naturally in season? The living roof at the California Academy of Sciences could go dormant for San Francisco’s warmest months, but the museum has chosen to irrigate the roof to keep it a lovely green. And in some climates, plants that will thrive are less adaptable to a rooftop. In Texas, Dr. Astrid Volder, a horticulturist at Texas Agrilife Research, found that native plants that can live in heat depend on deep, extensive root systems to keep hydrated, so they’re not great candidates for shallow roof tiles.

Similarly, white roofs aren't as simple as they’re sometimes made out to be. They do cool down cities, absorbing more heat than green areas and releasing it more slowly. But not all climate scientists agree that they can effectively combat climate change: A new study at Stanford, for instance, found that if roofs worldwide went white, they’d actually heat the planet by reducing cloudiness and increasing the amount of light black carbon the atmosphere absorbs. The study offered an alternative, however: Solar panels, researchers found, offered all the benefits of white roofs, with none of the negatives.

Multi-hued roofs are going to be part of greener buildings and cities, but it won’t be enough simply to build them. We still have a lot to learn about how to make them work as hard as they can to keep people and the planet cool.

Photo by Sarah Laskow