On a recent summer morning, Nancy Striniste was perched on the gently sloping roof of her front porch, pulling a few weeds. Unlike the other houses in Ms. Striniste's Arlington neighborhood, hers has a porch roof covered in vegetation, one example of a small but growing trend among environmentally conscious homeowners.
In recent years, vegetated green roofs have become increasingly common in large commercial or governmental applications, such as the green roofs found on several government buildings in downtown Washington, nonprofit headquarters like that of the World Wildlife Fund, and at Nationals Park. Although green roofs are less common on private homes, this may be changing slowly, especially as more municipalities enact regulations to control storm water on site rather than allowing it to drain into local sewer systems.
Green roofs provide numerous benefits to the environment, supporters say. They absorb and filter rainwater, reduce pollutants in runoff and mitigate the urban "heat island" effect. In urban areas like the Washington region, roofs with vegetation may offer outdoor green space and garden options to people who otherwise may not have access to them, said landscape designer Julie Udani, principal of Arlington-based Greener Gardens LLC.
"For homeowners who are less inspired by the ecological benefits of green roofs, they can also increase the life of the roof by protecting it from exposure to the elements," Ms. Udani said. "And they can increase the insulation on the roof, preventing temperature fluctuations in the home."
Generally, green roofs are constructed with a layered system that may include a waterproofing membrane, a root barrier, a drainage system, a growing medium -- usually a combination of soil and gravel -- and plantings. Green roofs can range from shallow mats of lower-maintenance plants such as succulents to deeper systems with larger, diverse plants.
The former style is what Ms. Striniste, a landscape designer and principal of Early Space LLC, chose for her green roof, which features a range of succulent and flowering plants including talinum, allium and grape hyacinth. The roof required regular watering after it was first built, Ms. Striniste said, much as any new garden would, but once the plants were established, she no longer had to water it. She said people routinely slow down as they drive by to look at the roof.
"Now it's just interesting to see what the plants are doing," said Ms. Striniste, who, in addition to her own roof, has designed green roofs for a Clarendon child care center and a Montessori school. "It's really fun to look out the bedroom window at it."
Green roofs require some planning, however. Homeowners are encouraged to have a structural engineer make careful calculations to determine whether the roof structure can withstand the weight of the plants, soil and trays, especially when saturated with rain or covered in snow. Landscape designers, structural engineers and specialty firms such as Capitol Greenroofs in the District can help. Ms. Striniste also recommends the book "Small Green Roofs: Low-Tech Options for Greener Living," published in 2011 by Timber Press.
"The most important thing is to make sure that your roof is structurally strong enough to support this," Ms. Udani said. "Most of the time it is, and it depends on the system you use. If the soil is only about 31/2 inches thick, for example, it's not crazy heavy for a regular roof. But I would have an engineer come and see how much load your roof would be able to carry."
Vegetated systems also work best on flat roofs and ones with a relatively shallow pitch, Ms. Udani added, because steep roofs would require additional infrastructure to keep plants in place.
Another consideration is the effect a green roof might have on a home's property value.
"As far as whether they affect a house sale positively or negatively, I would respond with an emphatic 'It depends,'" said Carolyn Connell, an associate broker with Keller Williams Realty in McLean. "Many people are very interested in conservation in general and in practice in reducing their energy bills and keeping their homes cooler. For those people, a green roof would be a plus and would add value."
Others might see a green roof as simply a source of maintenance, Ms. Connell said.
"I liken it to selling a home with a pool," she said. "Some people love it and will buy a home because it has a pool. On the other hand, some people will never buy a home with a pool because of the maintenance."
Gayle Fleming, a real estate agent with Keller Williams in Arlington who has both an EcoBroker designation and the "Green" designation from the National Association of Realtors, said the percentage of homeowners who might attempt a green roof remains pretty small. But she said she thinks more and more homeowners will be enticed to try them.
"Most people who buy houses do so because they want a green footprint," she said. "They have an urge to plant something. So the idea of a vegetated roof and why it's good for the environment would appeal to most people, if it was properly marketed."
One local house tested that theory this summer. Called the Passive House, this recently completed and sustainably designed house in South Arlington, developed by Eric and Roger Lin of Southern Exposure Homes in Burke, features a green roof over the garage. Before being rented to a family of four this month, the house and its green roof garnered positive comments from visitors during open houses, Roger Lin reported. The roof features a modular scheme that combines shallow square trays of plants with same-size decking tiles, so the design can be shifted around as the residents desire (or the plants warrant).
"When we researched green roofs, we decided to go with the modular tray system because if there is a leak or some other issue, you can lift up the tray and solve the problem very easily," Mr. Lin said.
For now, most residential roofs are still covered in shingles, not plants. But that may change as time goes on.
"As with the native-plants movement, green roofs will become more popular as people become more familiar with them," Ms. Udani said. "We have to retrain our eyes to find beauty beyond clipped hedges and aggressively green lawns."