Turning a Roof Into a Garden in the Sky
Green roofs are increasingly fashionable, but they're nothing new -- elaborate roof plantings were used in Rome and Mesopotamia 2,500 to 3,000 years ago. The best known were probably the Hanging Gardens of Babylon.
Simpler green roofs have also been used for millennia by Scandinavians and Kurds. Homes were made of mud; weeds grew on the mud and created sod that helped keep the home warm or cool.
Going green on your roof will improve visual and aesthetic impact, and can prolong the roof's life. It reduces heat loss in winter and heat gain in summer, and can reduce water runoff by as much as 50 percent while putting to good use an otherwise unused space, providing habitat for birds and butterflies. Using the rooftop expands living space for activities such as dining, recreation and enjoying city views.
The benefits of gardening on the roof are so many that the Environmental Protection Agency is encouraging cities to start roof garden programs. The EPA's goal is decreasing the heat-island effect found in cities, which raises temperatures in urban and suburban areas by several degrees. The agency 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.
In the 19th century, flat roofs with gardens became popular in Europe and North America. In "Planting Green Roofs and Living Walls" (Timber Press, 2004), Nigel Dunnett and Noel Kingsbury refer to extensive green roofs as being naturalistic in appearance, like a meadow, and able to be planted over a thinner growth medium.
Dunnett and Kingsbury say that the 1868 World Exhibition in Paris included a planted concrete "nature roof," the first of several in Western Europe. In 1914, architect Frank Lloyd Wright used a roof garden for a Chicago restaurant.
The first green roof I saw was on the National Mall, installed atop the underground complex that houses the National Museum of African Art and the Arthur M. Sackler Gallery of Asian art. These are actually ground-level rooftop gardens and must meet specifications for vehicular traffic.
The roof of the conference center of the Church of Jesus Christ of Latter-day Saints in Salt Lake City is green on a monumental scale, with fountains, prairie, alpine meadow and woodland, including conifers and aspens. It is a landscape design on ground, vertical and overhead planes.
Some gardens at the J. Paul Getty Museum of Art in Los Angeles are green roofs, too. There's a lush garden on the roof of Chicago's City Hall. Green roofs dot the tops of offices and residences in New York, Philadelphia, Boston and San Francisco. In Washington, there are several buildings with such roofs, including the national headquarters of the American Society of Landscape Architects.
In Christian Werthmann's "Green Roof: A Case Study" (Princeton Architectural Press, 2007), readers are taken through each step of installing the green roof at the landscape architects' building. Every step of the installation for the rooftop and terrace gardens is clearly explained. This book has excellent graphics and color photographs. It is an up-to-date how-to volume that shows that it is possible anywhere, with the proper planning, to install a rooftop garden.
Greening your roof requires study and planning. Structural integrity, grading, waterproofing, drainage, growth medium and plants are all important.
In addition to considerations that help in the development of the foundation of the garden (hardscape), the planted environment (softscape) is the ultimate goal of a green roof.
Not everyone with a roof can have a roof garden. Not all roofs are flat enough or can handle the weight. Some aren't easy to reach. But if you have the right slope, a sturdy enough structure and way to get there, then with a little imagination you can have a roof garden.
If you don't own the space, find out whether you are allowed to build on it. Whether you own it or not, you also need to know whether the structure is sturdy enough to support the weight. Get an opinion from a structural engineer or architect. Ask what, if anything, you should do to the roof to make sure you're not going to encourage any leaks.
There are many ways to protect a roof -- there's membrane sheeting just for this purpose. If you're going to use 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 under layers, some type of flooring, structures (fences, railings, screens, trellises or gazebos), plants, growing medium, water, planters, boxes or containers. Decorative objects, such as fountains or statues, and the weight of people or pets might all be rooftop 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 containers 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 super-heated to become porous and lighter. For drainage, plastic peanuts are lightweight and can be substituted for gravel in the bottom of containers.
Once you've determined that your roof will support a garden, begin thinking about plants and water. Roofs are not the most hospitable sites, and plants will have to contend with heat, cold, wind and air pollution. "Green Roof Plants: A Resource and Planting Guide," by Edmund C. Snodgrass and Lucie L. Snodgrass (Timber Press, 2006) offers a thorough explanation of plants, including color photographs; that have performed well on roofs. Use fences, screens, and trellises to mitigate the effects of sun and wind. Tough plants that withstand local conditions are good choices.
How will you water your rooftop garden? Even if you are just stepping out of a window, watering is a 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.
To learn more about how to build a green roof, go to http:/
To find out more about the roof at the Washington headquarters of American Society of Landscape Architects, and to arrange for a tour, see http:/
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.