Raising the (Green) Roof

A 525-square-foot green roof at East Austin's Stanley Studio (Photo courtesy of Marsha Miller, University of Texas)

On Thursday, Oct. 28, the Green Roof Advisory Group heads to City Hall – home of Austin's most prominent green roof project – to report on the state of Austin's "living roofs." Also known as "sky gardens," green roofs blanket the tops of buildings with vegetation – often native plants well-adapted to the local climate – and are known to improve both buildings and their surroundings (or, in city parlance, provide both public and private benefits). Created in August 2009, the advisory group was charged with exploring the feasibility of using city policy to promote green roofs around town.

Green roofs are credited with a surprising array of advantages. They can mitigate the urban heat island effect, reduce storm water drainage (helping both to prevent flooding and to protect downstream water quality), provide wildlife habitat, reduce the costs (and associated greenhouse-gas emissions) of cooling a building, and extend the life expectancy of a building's "roof membrane." Stud­ies also show that they can reduce noise pollution and enhance property values. All around the world, cities have been using green roofs to tackle urban problems, says GRAG Chair Eleanor McKinney. They've been popular for decades in Stuttgart, Ger­many, she says, where they are mostly mandatory downtown. Among U.S. cities, Chicago has the most square footage of green roofs – the result, says McKinney, of a municipal program that provides grants, density bonuses, and staff assistance for such projects.

Here in Austin, the trend has been relatively slow to catch on. In fact, one of the advisory group's first projects was simply to take inventory of the existing green roofs in town. The official (so far) grand total, 23, includes residential properties as well as institutional and commercial developments, such as Dell Children's Medical Center, the Escarpment Village (a southwest Austin retail center), and City Hall (which has two green roofs designed by McKinney and landscape architect Carolyn Kelley.

GRAG also combed through city policies to identify those that either already encourage green roofs but aren't typically interpreted with that goal in mind (such as open space requirements for planned unit developments) or could encourage them if the language were tweaked (e.g., green roofs can somewhat function as "pervious cover" – allowing rainfall absorption – but are not currently defined as such). GRAG has documented the results of that analysis, along with recommendations for further changes (such as a suggestion that green roofs be included in the Downtown Density Bonus Program), in a final report to be presented at council.

GRAG also plans to request a yearlong extension to continue its work, primarily to develop locally relevant performance standards for green roofs. As McKinney points out, good performance standards are an integral part of the process: "Say the city does give a density bonus credit for a green roof – they want to make sure that it's viable," says McKinney. "The density bonus obviously is given before they start construction. ... They don't want someone to just say, 'Yes, I'm going to do a green roof,' but then not have ... the plants survive."

As it turns out, however, developing adequate standards is "tougher than anyone ever thought," says sustainable design consultant Dylan Siegler, also a GRAG member. "You need to create performance standards, some design criteria, to say, 'Here's what a green roof is,'" she explains. "If you put three planter boxes on top of your high-rise, you don't get a green roof credit." But, she says: "The definition of green roofs that exists in the rest of the world is not perfectly applicable here. ... It so happens that it's been cold climates that have really grasped the green roof thing and run with it for 30 years." In Austin, the weather is "dry as hell – and then we get a deluge of rain. There are very certain plants that are adapted to work in that, and it's even more exaggerated on a roof," says Siegler. "We're going to be some of the first people to really codify this in a semiarid – or some people call us subtropical – climate."

Luckily, there is still a lot of useful wisdom to be gleaned from colder climates, say Sieg­ler and McKinney. According to a Canadian report on green roof projects around the world, there's a six-step process involved in bringing the trend into the fold of city policy. GRAG's very existence, they say, puts Austin at step three, while a city like Stuttgart – which invested in more than 1.1 million square feet of green roofs between 1983 and 2006 – is a solid six. "If you understand where you are, you understand that it will take a while to get to stage six," says McKinney. "We just are kind of getting council interested, and so you don't have to ask for the moon, right? You can kind of just go from, this is where we are, and we're at a good place."

To see the advisory group's final report, which includes a policy analysis and documents other achievements from the past year – such as initiatives to create an online repository of green roof information, to study rainfall runoff at the Lady Bird Johnson Wildflower Center, and to include green roofs in the city's Urban Heat Island Miti­ga­tion program – see www.cityofaustin.org/council/place1.