Getting the Garden off the Ground

BJ Louws, president of Louws Truss Inc. in Burlington, Washington, said he won’t be starting a rooftop garden on his own home anytime soon. “I have a 1913 house,” he explained, “so it’s probably not going to get a green roof. It doesn’t even have trusses,” he sheepishly admitted.

The data suggests that Louws’ house, traditional in framing method and rooftop design, might be outmoded. Green roof installations dipped slightly in 2014, according to the most recent survey data reported by industry association Green Roofs for Healthy Cities. But Louws said the last few years have brought the greater Seattle area on board with a trend that has been building across the U.S. over the past decade.

Green roofs, Louws said, are making up a bigger and bigger share of his flat-roof business. The vast majority of the new green roofs are for multi-family housing, but the trend is spreading. “More often than not,” Louws observed, “if it’s a flat roof, it has a green roof on it.”

The increase in the past three years, he says, has been “fairly exponential,” both in terms of square footage and number of projects. By his estimate, green roofs are now on well over 80 percent of Louws Truss’ flat roofs.

A Growing Market

The data collected by Green Roofs for Healthy Cities, already a couple of years old, put Seattle ninth on the list of top ten metro areas seeing green roof growth. Washington, DC  topped the list of most square footage of green roof space in a metro area, a leading position it held as far back as 2011.

Second on the list was Toronto, Ontario, followed by many U.S. cities currently experiencing housing market growth: Philadelphia, Chicago, New York and Denver. Louws said that’s exactly as he’d expect. “The greater Seattle area is a pretty environmentally conscious place,” he admitted, but he was surprised to find it in the vanguard of landscape architecture design. “The thing about Seattle,” Louws said, reviewing the list of green roof leaders, “is it’s so much smaller than any of those places.”

He explained that along with popular opinion, a variety of incentives are driving or at least dovetailing with the change he’s seen in Seattle: both a continued interest in LEED certification and local efforts by the city of Seattle to curb the urban heat effect “create some major incentives for our customers to put in green roofs.”

Many of the top ten metro regions, the Green Roofs for Healthy Cities report points out, have incentive programs in place to encourage green roofs in new construction. A June 2015 post on the U.S. Green Building Council (USGBC) website cites Washington, DC’s financial incentives as the force behind the city’s hundreds of green roofs, covering over 50 acres of space.

According to the District’s website, the financial incentive program began in 2007 and has increased since in an effort primarily aimed at managing stormwater. The site points out that green roofs hold rain water, reducing runoff and preventing stormwater system overflows, a key asset for urban areas that dedicate so much square footage to rooftop space.

What makes a roof green?
Plants, of course; but simply setting a potted plant on a flat roof generally doesn’t qualify. BJ Louws, president of Louws Truss Inc. points out that there are a wide variety of roof types and uses that fall under the label “green.”
 
There are those, he says, that “are green just to be green.” They don’t include much space for activity. Think of the flat roof of a commercial building—it might hold an air conditioning unit or even solar panels nestled into low profile, low-maintenance ground-cover plants like small grasses. These are extensive green roofs.
 
The other option is some variety of intensive green roof. This style has more varied vegetation, a deeper or more complex soil and drainage system, and generally requires a little more maintenance. When you think of a lush rooftop garden, with park-like landscaping and planter boxes sprouting full-size trees, it’s this style of green roof.
 
Even those two descriptions probably don’t bring to mind the full range of possibilities. Living walls, urban farms or elevated parks are all potential green spaces that structural component manufacturers might be asked to support.
 
Louws says the active use spaces are what’s popular in Seattle. Almost every flat roof they design, he says, has some element of deck, landscaping or play space.

New Garden, Same Plant

The green roof trend, Louws said, is not a topic of great concern in most component manufacturing circles. “I know there are projects in other major cities,” Louws said, “but I haven’t heard about it from the industry.” Part of the reason for the silence might be that, in many ways, green roofs aren’t a stretch for truss manufacturers.

“We pretty much just work [green roof design] into the regular process,” Louws said. “The loads are generally heavier,” he explained, but otherwise they’re just roofs. “Some of the planter boxes can get really heavy,” he points out, especially in a damp climate like Seattle’s, where the growing medium and irrigation system supporting trees or grass can retain an astounding amount of water weight.

In the end, though, Louws said a roof truss destined to support a garden isn’t really any different from a roof truss destined to support HVAC equipment—which is actually a key selling point. “It’s really intensive on the design side,” Louws explained. Extra time with architects and engineers is necessary to account for the large and varied loads involved in a green roof, but that can be a great opportunity for educating the market. “We sell them on the roof truss aspect,” Louws said, “because we’re able to do a lot more” with trusses than with joists.

Doing more with structural elements is crucial, because builders and residents are continually doing more with their roofs. “There’s a lot more live loads,” Louws observed. For example, he says, “a typical roof is a 50-450 pound dead load and a 100 pound live load.” With a green roof, on the other hand, “in some of the areas a planter alone is a 20 pound dead load and 30 pound live load.” The green roof for a commercial building currently in the design stage at Louws Truss goes well beyond those figures. It’s designed to support not only grass, pavers, planter boxes and gravel, but also a bocce ball court.

The large loads, though, Louws said, aren’t the real issue. Increasing the amount of live or dead load a truss needs to support is a simple adjustment with truss design software. What complicates projects, Louws explained, is how those huge loads are distributed across the trusses. The bocce ball building, for example, has 39 different load cases—but Louws says that’s not what jumped out to his designer. “He said the nice thing about this project is that all the load ran either parallel or perpendicular to the trusses,” Louws recalled. The diagonal loads are what make a project challenging and time-intensive.

Putting Down Roots

The spread of vegetative rooftops isn’t all green pastures. For starters, it does raise some tricky legal questions. “The potential liability with a roof deck is that people could be doing more up there than we know about,” Louws said. The trusses are built to design, but a design might not anticipate the swing set added to an apartment garden or a hot tub installed on a shrub-ringed roof. The urban infill projects Louws has been involved in open the door to an array of creative possibilities that could prompt wise truss manufacturers to carefully review their customer contract language.

There’s also the possibility, Louws suggested, that the heavy loads of green roofs could lead some builders to metal rather than wood. Louws has asked his staff for their analysis of the market. “They thought we were gaining market share,” thanks to the green roof trend, he said.

With a host of projects last year and more already in line, Louws Truss certainly isn’t hurting because of customer demand for green roofs. So even though rooftop green space isn’t top of mind for many component manufacturers, perhaps it should be. As the Green Roofs for Healthy Cities 2014 industry survey point out, “There is still enormous potential for new green roofs to be installed on tens of billions of square feet of roofs across North America.”