"Green" Roofs Could Cool Urban Heat Islands

Green roofs can cool near-surface air temperatures by an average of 16.4 degrees Celsius per unit area—slightly behind street trees in terms of heat island mitigating potential, according to a 2006 report from the New York City Regional Heat Island Initiative. And the obvious advantage to green rooftops is that they don’t take up valuable ground-level space.
Although cooling heat islands is an important aspect of urban climate change mitigation, an even bigger impact could come from large-scale energy conservation. That’s where green roofs hold gallons of potential. Most of the rainwater dripping off black roofs mixes with pollutants in the streets and wastewater in the sewer system as it flows to 14 water treatment facilities across the city—where it costs money and energy to clean and store. During big storms, that tainted water bypasses treatment plants altogether, pouring contaminants directly into rivers.

A four-inch-thick green roof can hold a gallon of water per square foot. If all the rooftops in the city were green, they would represent a billion gallons of storage potential.

Although many like the idea of green roofs, the installation cost ($10 to $35 per square foot, depending on roof size, accessibility for installation, and roofing and drainage components as well as the types of plants) makes them impractical for some buildings.

Scientific American