Putting a roof over your head is one thing. Putting a garden or any sort of greenery on top of that roof is a far more complicated matter, usually requiring the combined talents of a landscape architect and a structural engineer.
That hasn’t discouraged a number of local homeowners from fulfilling their dream of having a green garden way above ground, whether for functional or decorative purposes. In some circles, the “green” — as in environmentally friendly — roof is a trend even when the roof is pitched as much as 45 degrees.
When retired AOL executive David Cole and his wife, Maggie, wanted to turn the space above their Watergate penthouse into an outdoor room, they turned to the internationally known landscape design firm of Oehme, van Sweden & Associates at 800 G St. SE, knowing that building codes would influence results.
Their ideal, Mrs. Cole says, was “a gracious space to get ‘above’ the mad scramble of the city so we could enjoy our wonderful views in a garden setting. We especially wanted a venue to barbecue our farm’s — Sunnyside Farms in Little Washington, Va. — organic meats.”
Water features they hoped to include were shot down because of weight issues involved in the final design.
“Direct planting into a layer of soil wasn’t possible either,” she says.
The solution was using three types of relatively light false-bottomed planters made of wood, zinc and fiberglass. The soil contains a lot of lightweight vermiculite to aid aeration and hydration. Zinc linings in the planters keep water away from the wood exterior. Plants were chosen to ensure flowering and greenery each season. Automatic sprinklers are built into each planter for irrigation. Soft river stones from Japan and flagstone pavers mark pathways.
The Coles have used the site for Fourth of July parties, fund-raisers, birthdays, graduations, wedding anniversaries and “no event, kick back and enjoy yourself soirees,” Mrs. Cole says.
The building’s age — Watergate was constructed in 1971 — also put some constraints on the design. Planters had to be distributed with weight considerations in mind. “We did it right because I didn’t want anything to collapse,” says James van Sweden, whose firm recently completed plantings on the roof of a Pepco electric power substation in Georgetown along the Whitehurst Freeway.
He describes the Pepco roof as a “tapestry like a Helen Frankenthaler painting. It’s not for living but for looking, so residents in the adjoining Flour Mill can look down on it.” Likewise, the Outback chain, based in Tampa, Fla., hired an architect to create a roof garden modeled after an Italian hillside for its Carrabba’s Italian Grill in Bowie, strictly as a visual motif. Customers can’t walk or dine amid the tree-filled planters.
Even decorative designs can have a beneficial environmental impact, however, as more and more building developers and homeowners are opting for the best of both worlds. Though costs may be greater upfront when a green roof is chosen for its environmental features, there can be savings later in energy bills and, in some cases, even tax relief from local authorities wanting to encourage such plans.
In addition to helping cool a building’s interior, such roofs help manage storm-water drainage in cities by reducing and slowing down the amount of runoff, says Ed Snodgrass, owner of Emory Knoll Farms in Street, Md., a nationally known supplier of plants and plant expertise for extensive green roof systems (www.greenroof plants.com).
“The plant list [for such buildings] is limited, but nice-looking,” he says. He has sold plants for homes in Silver Spring; Takoma Park; Pasadena, Md.; and Annapolis and also the new Human Rights Campaign building at 1640 Rhode Island Ave. NW. “For some people, it is part of their citizenship.”
Mr. Snodgrass distinguishes between what he calls intensive and extensive green roofs. By his definition, an extensive roof, often planted in soil placed directly on the roof, has low-lying, rapid-growing plants with high drought tolerance and tubular rather than tap roots to help to protect roofing membranes. Extensive green roofs are not meant to be used for people to walk or sit upon, he says.
By contrast, an intensive one is meant to resemble natural landscape using plants with foliage up to 15 feet high that may require several feet of soil and professional service to maintain it. Roofs with planters are considered intensive because they have soil depth and foliage more than 6 inches high.
Functional and aesthetic considerations both were important when horticulturalist Diane Cina at the National Gallery of Art picked 15 flowering Japonica Snowbell trees in planters for one seventh-floor roof terrace and Natchez crepe myrtle trees for another. (Neither space is open to the public.) Both species withstand reasonably well the heat reflected off the building’s marble walls, but every other day, staff members hand-water the 150-pound wood planters, which have fiberglass insulation and 2 to 3 inches of gravel at the bottom for drainage.
“Would you rather see a green roof or a blacktop?” architect Frank Mobilio asked rhetorically before several members of the American Society of Landscape Architects during their recent tour of the Human Rights Campaign building.
“That is an ugly roof,” he said, looking down on the black tar top of a nearby building. “No heat reflects off this one,” he noted, pointing to the 2,000-square-foot brown-green carpet on the Human Rights Campaign building’s second floor. That green roof can be seen from seven floors above the second-floor level where he was standing.
Sedum, a succulent that cost $8 a square foot and weighs 45 pounds per square foot, was the plant of choice for the roof. The soil is 23/8 inches thick above membrane and barrier layers that keep the sedum roots from growing down too far.
Mr. Mobilio, a member of the architectural firm of Hellmuth, Obata and Kassabaum, which helped renovate the structure, is sold on the concept of rooftop gardens for both environmental and decorative effects.
“They really are becoming quite a necessity, to help with urban heat-island effect that the TV weather channel talks about,” he says. “And they help mitigate storm runoff in this area now that the cost of building sewer pipes is getting to be astronomical.”