Need to Weed Your Roof?

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--block after block of tarred black rooftops, sticky in the summer, windswept in the winter, ugly year-round. Or at least that's the way it used to be.

But urban roofs are going green. Environmental designers looking for new ways to soften cities 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 shrubs that control temperature, conserve water and clean the air. A newly published paper in the journal BioScience reveals just how much good green roofs can do.

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 in. (20 cm), able to support smaller plants. The intensive type may be smaller, but it's deeper and home to larger plants. Whatever the design, green roofs are a lot more complicated than ordinary gardens. They have multiple layers beneath the soil, including a filter membrane, a drainage layer, waterproofing, insulation and structural support.

more from Time