Green Roofs for Healthy Cities (GRHC), an association that promotes the development of sod roofs and walls, reports that the green roofing industry grew by a whopping 115 percent in 2011. Steven W. Peck, president of the Toronto, Ontario–based group, says the industry has seen increases every year since 2004, when it began keeping score.

What accounts for such robust growth? “We’re beginning to see results from policy support that has increased over the past few years,” GRHC chair Jeffrey L. Bruce said when announcing the figures. “The industry is also benefiting from the [550] accredited green roof professionals in the market, who are committed to driving future industry growth.”

Wide acceptance by the eco cognoscenti doesn’t hurt. Green roofs have been lauded by consultants, embraced by cities, and are even eligible for LEED credits, and the reasons are many. Roof vegetation, advocates say, will manage stormwater, reduce the heat island effect, decrease air conditioning usage, clean the water, improve the air, and contribute to the overall health of the environment. But residential architects and their clients are motivated by other benefits.

“It’s a functional amenity that creates usable space,” Peck says. “[Residential architects] and consumers are not doing it for the energy benefits or the stormwater retention. That’s more for commercial projects.”

Patrick Farley, founding principal of Watershed Architects in Richmond, Va., confirms Peck’s assessment. “Because of the scale of commercial projects, you can measure the benefits of a green roof,” he says. “But in residential it’s about qualitative more than quantitative benefits.” As an example, Farley points to a green roof he did recently to re-establish the habitat that the firm removed to build the house and to establish a new habitat for plants and insects. He also included the roof so that the clients would have a better view from the interior and exterior living spaces on the second level.

Green roofs fall into two categories—an “extensive” system with 6 inches or less of growing medium and an “intensive” system with more than 6 inches. An extensive roof is lightweight and requires less maintenance, but it also supports less plant diversity, Peck explains. An intensive roof, on the other hand, is “durable, supports a wider range of plants and trees, and acts as an outdoor space.” A popular and growing option for green roofs is a modular system in which 2-foot crates are pre-planted with growing medium and plants. Farley, who favors modular systems by Spring Lake, Mich.–based LiveRoof, says that the company’s systems are easy to install and remove if a membrane leaks and needs repair.

For many years, Farley tried to include a green roof on one of his homes, but the cost proved prohibitive. The firm got its first roof approved in 2006, and subsequent projects proved easier. “One of the simple things that surprised us on the first project is that a green roof is an ecosystem, and it comes with some issues,” Farley says. “The birds flew over the roof or they would visit the space. Their droppings contained seeds, and grass and weeds started growing in the sedums. As a result, the roof required a fair amount of maintenance.” Today, Farley emphasizes to his clients that a sod roof is not a set-it-and-forget-it deal; it will require upkeep. “There is no such thing as a maintenance-free green roof,” he says.

Maintenance is not the only issue architects must consider. There are design limitations on when and where a green roof is appropriate. Sod, for example, is ideal on a flat roof, but it gets more challenging to install on a sloped deck. According to the voluntary German FLL Green Roofing Guideline from 1982, a green roof should not be installed on a deck that slopes more than 40 degrees. “The issue with the slope is that you get less water retention and subject the plants to more wind and solar radiation,” Peck says. “The moisture content in the growing medium gets compromised.”

There are numerous commercial installations that disregard this slope standard, however; Farley has done a modular green roof on a 12/12 deck. “It can be done, but you have to do an elaborate anchoring detail to hold the modules in place.”

Perhaps the most important consideration of a green roof is the structural requirement, and its significance can’t be stressed enough. “The structure has to be designed to carry the load, obviously, but the calculation has to be done early in the process and with additional considerations,” Peck explains.

Farley says the issue goes beyond the basics of what architects naturally do. For example, the roof design has to take into consideration the weight of the growing medium and plants, but it also has to calculate rain, snow, people, furniture, and more. “It can get extremely heavy,” Farley says. “If there is ever a time to overbuild on a house, this is the time to do it.”