Atop the National Aquarium in Baltimore, away from the crowds, is a hidden oasis where the turtles and lizards sometimes come to bask in the sun. Their caretaker says it's also a nice spot for wild birds - and him, too.

The 4,000-square-foot "green roof," a 4-inch deep collection of soil and succulent plants over a rubber liner, has been serving many purposes since it was built in 2004. Besides acting as an urban refuge, it's helping cut utility bills and control rainwater into the harbor.

Each situation is different because, for example, some cities have storm water fees or incentives, and roofing systems vary in size and costs. The group anticipates more cities will encourage the roofs and has launched a professional accreditation program so those hiring designers and contractors can know they have specific training.

"Ideally, we'd like to see buildings providing more resources than they take, but we're very far away from that," he said. "Green roofs are a start. And I wouldn't call it a trend. We are seeing the beginning of a transformation in the building industry."

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