WASHINGTON — Just a few weeks ago, I was standing in a garden beautifully landscaped with shrubs, flowers, grasses and a small forest of 20-foot trees. Part of it was formally ornamental and part was planted as a natural prairie of grasses and wildflowers.

But my admiration was more than aesthetic.

This gorgeous garden was on a roof — the roof of the conference center of the Church of Jesus Christ of Latter-day Saints in Salt Lake City. Colleagues at the landscape-design conference I was attending had been telling me I had to see this "green roof," but I was skeptical. Most green roofs are simply a collection of turf or sedum. This one was a complete landscape design on all planes, ground, vertical and overhead.

When I got home and started thinking about it, I realized that what's truly amazing about the whole roof-garden concept is that more people don't have them.

The benefits of gardening on the roof are so extensive that the Environmental Protection Agency is encouraging cities to start roof-garden programs. The EPA's target is the heat-island effect found in cities where nearly every flat surface is paved or built on and nearly every roof is made of dark materials. The result is that temperatures in urban and suburban areas are raised by several degrees. EPA estimates that increasing an urban area's acreage of planted space by just a few percentage points can lower temperatures several degrees, significantly reducing smog and saving millions in energy costs.

Roof gardens have great advantages for anyone with a suitable structure. They cut water runoff by as much as 50 percent and put to good use an otherwise unused space, providing habitat for birds.

Obviously, not everyone with a roof can have a roof garden. You need to figure out whether the roof and the structure are strong enough to support extra weight. Get an opinion from a structural engineer or architect. You can also ask what, if anything, you should do to the roof or to the flashing to make sure you're not going to encourage leaks.

There are many ways to protect a roof — there's membrane sheeting just for that purpose. If you're going to be using turf, you may need a system of under layers, including plastic foam or gravel, as a foundation.

The weight you put on the roof will include sheathing or underlayers, some kind of flooring, structures (fences or railings, screens, trellises or gazebos), plants, growing medium, water, planters, boxes or containers. Decorative objects, such as fountains, might be considerations.

Once you have determined that your roof will support a garden, you can begin thinking about plants and water. Roofs are not always the most hospitable places, and plants will have to contend with heat, cold, wind and air pollution.

How will you water your garden? Sophisticated roof gardens might have a system of drip or spray irrigation, but home gardeners are more likely to have hoses and buckets.

Establishing a roof garden can be expensive and labor-intensive or simple and affordable, with just plants in containers, but either way, the rewards are enormous. You are helping to improve the local environment, as well as your own living conditions.