Fly over any town or city and you'll see a lot of hard surfaces -- paving and roofs -- that, cumulatively, create serious environmental problems.

Hard surfaces shed rain water, rushing it to storm sewers and frequently flooding them. In many cities the storm and sanitary sewers are combined, so the flooding can cause serious health hazards and pollute natural waterways.

When the sun shines, hard surfaces heat up and can create an "urban heat island" -- a hot, dome-shaped air pocket that covers the entire city, trapping pollutants and increasing air-conditioning demands. Heat islands can make cities 10-20 degrees warmer than their surrounding regions.

Forests, by contrast, remain cool on hot days because leaves turn the sun's energy into plant tissue, not excess heat. Trees also transpire water through their leaves to cool themselves. When it rains, forests absorb water like a sponge -- they typically "dump" 10 to15 percent of a heavy rain as runoff, while cities typically dump 60 to 80 percent of their rainwater.

The solution to hard surfaces, it seems, would be to make cities more like forests.

Enter "eco roofs" -- roofs covered with a thin layer (four to six inches) of soil and vegetation that provides many of the environmental services of natural ecosystems. Eco roofs reduce stormwater runoff by holding rainwater and gradually releasing it back into the atmosphere. Their soils filter pollutants and heavy metals out of rainwater, while their plants filter pollutants out of the air and absorb carbon dioxide, a greenhouse gas. The living surface reduces the heat island effect in much the same way that trees do, and simultaneously provides habitat for a variety of flowers, insects and even small birds.

Eco roofs also benefit the buildings they cover, reducing heating and cooling costs by increasing insulation. They increase the life span of roofs by protecting their waterproofing membranes from the sun.

While eco roofs have been popular in Europe for some time, they are just catching on in America, where most of them are on public or commercial buildings because of their initial cost. Residential eco roofs are becoming more practical, however, since some cities such as Portland, Ore., and Chicago, Illinois, are encouraging their installation with stormwater-fee rebates and other incentives.

Although many eco roofs are "flat" (actually, low-sloped), soil-and-root systems can be stable on slopes up to 45 degrees if a support grid is provided. Eco roofs require a heavy-duty waterproofing membrane that is also resistant to root penetration, and they are usually equipped with a permeable drainage layer to release excess moisture and a filter layer to prevent soil from clogging drainpipes.

Eco roofs are also available as pre-fabricated "kits" -- trays filled with soil and plants that can be placed on ordinary roofs. The advantages and disadvantages of the tray system versus an installed eco roof are debatable, and buyers should investigate carefully before deciding between them.

An eco roof should take care of itself, requiring little or no watering and just a little weeding or trimming. The choice of soil and plants is critical. Most eco roofs employ special soil mixes that hold up well under local moisture conditions and nurture selected plants. Soil from your back yard might dry up, wash off your roof or weaken your plants.

Native plants should be used wherever possible, as they are adapted to local climate conditions and support native fauna. The plants should have a low profile and strong roots to avert wind damage, but the roots should be shallow so they don't disturb the roof membrane.

Members of the Sedum genus of succulents make good candidates, as many Sedum are hardy, drought-resistant but water-tolerant, low-profile and attractive spreading plants. Some Sedum provide food for butterfly larvae. Sedum Spathifolium, for example, is the host plant to the endangered San Bruno elfin butterfly of the San Francisco peninsula. (Similar symbionts can be found in most regions.)

Building codes are still being written for eco roofs. Weight, the potential fire hazard of dry plants and other issues must be resolved before eco roofs become standard building practice. But their environmental advantages are spurring many communities to find ways to help you put an eco roof on your ecological house.

Philip S. (Skip) Wenz is a freelance writer specializing in ecological design issues.