Those who think that living beneath a roof alive with plants is medieval haven’t seen the world through the eyes of Michael Perry.
Perry runs a company that makes roof systems for people who want a verdant layer between themselves and the sky. Variously called green roofs or living roofs, these wafer-thin gardens are increasingly common in Europe and parts of the United States, particularly in Chicago.
With a capacity to cool the building below by as much as seven degrees, green roofs are seen as a way to reduce the heat island effects of large cities. If Paris had far more, this month’s lethal heat wave there would have been tempered, say green-roof experts. . According to Perry, of Building Logics in Virginia Beach, in parts of Europe, 30 percent of new roofs are living roofs.
Green roofs offer a second key environmental benefit: Like lawns around homes, they hold and filter rainwater, reducing pollution and erosion by rivers and streams.
And there’s another aspect. Building tops clad in succulent plants become living kaleidoscopes with ground covers that grow together and change color, influenced by varying growing conditions and seasons. The sedum, a mainstay plant for such difficult places, gyrate through hues of green, pink, yellow and white, depending on species.
“When you’re up there, it’s a pleasant experience,” said Jeanette Stewart, a resident of a Yorktowne Square condominium in Falls Church that has such a roof. “The birds seem to gravitate to it, and the squirrels don’t hurry across it.”
Stewart is president of the condo association and works for a nonprofit environmental group called EcoStewards Alliance, which she convinced to seek grant money to make the project happen. Earlier this month, she hosted a site visit by Perry and local officials.
The observers clambered up a stepladder through a hatch to the four-story roof, which covers 14 of the 296 units at the condominium. Yorktowne Square was built 35 years ago as a garden apartment community on 15 acres, nestled between the Capital Beltway and Route 50.
The new roof was designed and fabricated by Perry’s company. Two months after completion, hundreds of little sedum plants are beginning to grow together. By next spring, the flat roof should be a tapestry of three succulents, Sedum album, S. reflexum and S. sexangulare.
The choice of suitable plants is narrow — sedums are among a relatively small palette of hardy plants suited to roof duty, varieties usually coveted by wall builders and rock garden fans because of their ability to grow with little rainfall and in poor soil. An aerial lawn wouldn’t work; grass needs to set down deep roots, and besides, mowing would be a nightmare. Certain dry-loving perennials, grasses and shrubs would work if you were willing to install a system that contained, and could support, at least eight inches of soil.
Flat roofs such as Yorktowne Square’s are obvious candidates for the green-roof treatment, and they can be employed three ways: as a place to be (with adequate railings); an area merely to look at from a higher place; or simply as an unseen covering you live under in primal comfort. “Like a hobbit,” said Marc Aveni, whose agency, the Virginia Department of Conservation and Recreation, contributed $29,000 to the $95,000 project. The National Fish and Wildlife Foundation granted an additional $50,000.
At Yorktowne Square the growing layer is a scant two inches deep and so poor that weeds, by design, grow and then sizzle to death in the sun. It is all part of the science of building a roof in which desirable plants grow with little fuss, weeds die, and the roof remains durable and waterproof for years.
The roof is built up like a lasagna, layered, from top to bottom, and includes two waterproof membranes, the upper containing a blanket of copper and gel that absorbs great quantities of rainwater. The gel holds rainwater and provides prolonged moisture for the sedums; the copper forms a barrier to roots. “The key word in green roof is roof,” said Perry, “It if doesn’t keep water out, it’s not a roof.”
Green roofs hold up to 70 percent of the rainfall that otherwise would gush into storm sewers, but they must also be properly drained to handle deluges. Ponding water would kill plants and create other problems.
In the United States, most green roofs are installed on industrial, commercial or governmental buildings, or in multi-family housing such as Yorktowne Square, not single-family houses. “Most houses have yards [to absorb rainfall] and are not under the same regulatory pressures commercial buildings have,” said Ed Snodgrass, a farmer and horticulturist in Street, Md., who began growing roof-garden plants five years ago. And yet, he says, there is potentially a huge demand among residents of various housing types for this sort of roof treatment.
Perry said the technology can be installed on roofs with up to a 45-degree pitch, though the hardware is more elaborate and expensive and plants must be pre-grown. At a commercial-scale property such as Yorktowne Square, a green roof costs about 30 percent more than a conventional bitumen roof, but lasts much longer because the roofing material is shielded from the sun’s highly destructive ultraviolet rays.
Single-family houses are proportionately more expensive because of the scale of the jobs, said Perry. He said a roof on a 10-by-15-foot flat porch roof would be about $20 a square foot, or $3,000. A pitched roof increases the cost “by five or six dollars per square foot.”
A desire for sturdier, more vertical plants, and the heavier soil medium they require, could increase costs exponentially because the superstructure must be engineered to hold it. This is not as much an issue in Europe, Perry said, because buildings, particularly old ones, are made of materials designed to stand for centuries and can take heavy roofs.
But don’t despair; the range of plants that will grow in a couple of inches of aggregate, while still limited in the whole repertory of hardy garden plants, is relatively rich. There are more than 50 species of sedums alone that are hardy in Washington. Roof gardeners could also pick among species and varieties of sempervivum and jovibarba, said Snodgrass. Funny names for funny plants, perhaps, but together they could return some nature to places that need it the most.
“I think this is an example of how we can bring our dwellings back into the realm of the natural world,” said Stewart. “Psychologically, that would be a wonderful step for all of us.”
By Adrian Higgins
Washington Post Staff Writer