Green roofs are designed to save energy and capture rainwater. Rooftops covered with plants help keep buildings cool. They can extend the life of a roof. And they can reduce water running off into streets and storming water systems.

　　Experts say green roofs usually do not get much above twenty­seven degrees Celsius. So temperatures might not be a problem for growing. But rooftop gardeners have to be careful not to put too much weight on the roof. This is true whether plants are grown in soil or water.

　　Earlier in 2008, researchers in Texas reported that green roofs can reduce a building's air conditioning costs by about one­fifth compared to black­top roofs. But they also reported that not all green roofs perform equally well. They found that designs differ in their ability to keep buildings cool and to capture rainwater. The researchers put experimental roofs on top of metal insulated boxes, to recreate green roof conditions. The study compared products from six companies to help businesses understand how to improve their designs.Each rooftop had sixteen different kinds of plants native to Texas. The researchers say the presence of native plants likely helped all the green roofs capture water better compared to sedums. Sedums are plants that need little water and often are used on green roofs.The native plants could take in more water and release more of it to the atmosphere.

　　【语篇解读】 本文主要介绍了一种新的环保模式——绿色屋顶。

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