Coruscation

Putting living vegetation on the roof is not a new idea. For thousands of years people have made sod roofs to protect and insulate their houses, keeping them cooler in summer and warmer in winter. The modern movement for green roofs began in the last 50 years in Europe. Germany, where about 10 percent of roofs are green, is the leader; some parts of Germany require green roofs on all new buildings.

Greening a roof is not simple or cheap. Over a black roof -- flat is easiest but sloped can work -- goes insulation, then a waterproof membrane, then a barrier to keep roots from poking holes in the membrane. On top of that there is a drainage layer, such as gravel or clay, then a mat to prevent erosion. Next is a lightweight soil (Chicago City Hall uses a blend of mulch, compost and spongy stuff) and finally, plants.

An extensive roof -- less than 6 inches of soil planted with hardy cover such as sedum -- can cost $15 per square foot. An intensive roof -- essentially a garden, with deeper soil and plants that require watering and weeding -- can double that. But because the vegetation is thicker, it will do a better job of cooling a building and collecting rainwater. Plants reduce sewer discharge in two ways. They retain rainfall, and what does run off is delayed until after the waters have peaked.