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| The best thing you can say about the roofs of most city buildings is that you don't have to look at them much. That's very good, since an urban landscape viewed from above can be an unlovely thing-blockafterblockof tarred(铺有沥青的 ) black rooftops , stickyin summer. windswept in winter and ugly year-round.Or at least that's the way it used to be. But urban roofs are going green. Fnvironmental designers have begun to realize that the tops of buildings don't have to be wastelands. Indeed, they can be gardens, planted with grasses,flowers and bushes. A planted roof usually comes in one of two varieties : extensive or intensive. The extensive type is wide and shallow, with a soil depth of less than 8 inches, able to support smaller plants. The intensive type may be smaller,but it's relatively deeper and home to larger plants. Whatever the design,green roofs are not so simple as ordinary gardens. They have multiple layers beneath the soil, including a drainagelayer , waterproofing , structuralsupport , andso on. But this system can do a great deal of good. A recent paper in the joumal BioScience tells that green roofs can control temperature , contain water and clean the air. And most impressively , they can cut heat loss from a building by 50% ,lower air-conditioning costs by 25% , and reduce the urban-heated-island effect by 2℃. Of course,apart from the square feet greened and heat reduced. green roofs are even more valued since people can gain some psychological comfort simply by having a quiet place to go. As so often happens,what's good for the planet can also be good for the spirit.  1.________2._________3.__________4.___________5.___________ 6.________7._________8.__________9.___________10.__________ |