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.

A garden has to be pretty spectacular to dazzle me and this was, with its carefully designed feeling of privacy and its wide variety of plants. 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 Salt Lake City roof garden is huge. Grasses, trees of all sizes and extensive water features grace the four-acre rooftop. However, a garden can be adapted to fit any roof or terrace that will allow it.

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.

The 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 of dollars in energy costs. It may also save lives, as excessive heat can be deadly, especially to people who are already frail because of youth, age or illness.

Roof gardens have great advantages for anyone with a suitable structure. They reduce heat loss in winter and heat gain in summer, cut water runoff by as much as 50 percent and put to good use an otherwise unused space, providing habitat for birds and butterflies. Using the rooftop expands living space for activities such as dining or recreation, takes advantage of city views and might increase property value.

Besides the practical value, roof gardens can be as varied and as stunning as in-ground gardens. In their 1999 book "Gardens in the City: New York in Bloom," author Mary Jane Pool and photographer Betsy Pinover Schiff capture some spectacular roof and terrace gardens. They found a Japanese garden with bamboo plants and screening; a turfed play yard with a swing set, slide and sandbox; and a penthouse terrace with trellised vines, a fountain, half a dozen types of trees and a screened lattice gazebo. In several cases, the plants not only provide beauty but also screen mechanical systems or ugly views.

Obviously, not everyone with a roof can have a roof garden. Not all roofs are flat enough or can stand the extra weight. They can't all be easily reached. If your roof or terrace belongs to a landlord or a condo-owners association, use may be restricted or forbidden. (In such a situation, check with the owner to see whether you would be allowed to build.)

But if you have the right slope, a sturdy enough structure, a good way to get up there and a little imagination, you can have the crowning glory of 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. You might need to consult a landscape professional with some experience in green roofs.

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 or statues, and people or pets might all be considerations.

You can, to some extent, control how much weight you add to the roof. Cedar planking is lighter than paving stone, fiberglass or plastic planter boxes are lighter than wood or stone, flowers and grasses are lighter than trees, and some planting mediums are lighter than soil. The planting medium used on the roof garden of the Mormon Conference Center is a stone that has been superheated to become porous and lighter in weight. In pots, plastic peanuts weigh almost nothing and can be substituted for gravel in the bottom of a container to aid drainage.

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. In "Gardens in the City," Pool compares roof gardening to gardening on a mountainside, noting, "When the garden is up high, maintenance must be kept low."

How will you water your garden? Even if you are just stepping out of a window, watering can be a real nuisance. Sophisticated roof gardens might have a system of drip or spray irrigation, but home gardeners are more likely to have hoses and buckets. Watering is maintenance, and plants in containers might need to be watered every day.

You can uses fences, screens and trellises to reduce the effects of sun and wind, but tough plants that withstand local conditions are good choices. The book "Planting Green Roofs and Living Walls," by Nigel Dunnett and Noel Kingsbury offers a thorough overview of practices for planting green roofs. This reference also offers lists of plants that have performed well on roofs and walls.

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.

There's something almost magical about being in a garden far from the hustle and bustle of the streets. Even on the roof of the Casino Montreal in Quebec, the chef of their much-lauded restaurant, Nuances, has an herb garden that is his piece of quiet and greenery and provides many of the seasonings for his kitchen. This sense of enclosed privacy is what I noticed most on the rooftop garden in Salt Lake City.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. E-mail or contact him through his Web site, www.gardenlerner.com.

Pinyon pine stands tall among 21 native grasses on the roof garden at the Mormon Conference Center.

Trees and shrubs are terraced in planters along the side of the Mormon Conference Center in Salt Lake City. A roof garden crowns the building.