print this pageNews & Events

Green Roofs Over Texas - Nov 7, 2005

Roofing consultant Brian Gardiner, in white shirt, explains the techniques being used in the demonstration project at the Lady Bird Johnson Wildflower Center in Austin where 25 simulated green roof models are being monitored.

This article appears in the November/December issue of Texas Architect.

Texas may seem an unlikely locale for green roofs. Dry in some regions, humid in others, but often very hot through most of the year, Texas is a challenging terrain for a technology proven in moderately wet and temperate climates such as the Pacific Northwest. Closer consideration, however, yields a more complex and optimistic outlook on this time-honored and versatile building technology.

Green roofs – perhaps more aptly called “planted roofs” to avoid the connotation of a lushly verdant lawn – are also “green” roofs, that is, significant sustainable-design tools for creating high-performance buildings. Gradually, the architectural and construction communities across the U.S., and even owners in the public and private sectors, are becoming familiar with the methods and reasons for building them. Texas, while still at the beginning of its own green roof learning curve, stands to make contributions to the practice as architects, owners, and builders embrace and develop regionally appropriate systems.

Although deep-soil, intensive rooftop landscapes have been done for years, usually on large civic projects, extensive green roofs are typically implied by the term. With soil depths of four to eight inches, extensive green roofs are a streamlined sandwich-layering of roof membrane, drainage material, filter fabric, soil matrix, and plantings—a combination that does not require excessive structure for support (and sometimes no additional structure at all). Some roof manufacturers offer their own component assemblies, botanical consulting expertise, and proprietary soil mixes. There have been efforts in some areas of the country to expand the conventional green roof user base from large commercial and institutional projects reliant on proprietary systems to smaller commercial and residential buildings for which generic assemblies and off-the-shelf components would make green roofs more economically feasible. Regardless of the system size or type, the advantages of green roofs extend beyond the immediate user to benefit the community at large. As additional green space, planted roofs reduce the urban heat island effect through vegetative transpiration, dampen the surging effects of stormwater events, decrease and filter run-off, improve urban air quality by trapping airborne particulates, including heavy metals, and provide habitat for wildlife. Also, as another layer on top of a building, they augment sound and thermal insulation, dramatically increase a roof’s longevity by reducing rooftop temperature swings and thermal stress on the membrane, reduce a building’s life-cycle cost, modulate ambient temperatures around mechanical equipment, beautify unsightly rooftops, and offer potential outdoor space for additional occupancy. All these features translate directly or indirectly to economic savings and improved quality of life.

So how are green roofs relevant in Texas? What might Texan green roofs look like? In a reg ion k nown for hot temperat ures and extended droughts, an extensive green roof design may differ significantly from one intended for a cooler, episodically rainy climate. Still, we can learn from experience garnered elsewhere that the universal rules of roof construction – water needs to be shed, membranes must remain intact, etc. – hold true regardless of region, and develop local best practices. The success of a site-specific green roof is ultimately contingent on how well the plants do over time, and the design of the whole system works backwards from there. Plant regimes are critical, as is the soil matrix and the ability of a system to retain moisture when it is needed and drain when it’s not.

A firm grasp of a region’s distinct characteristics and traditional practices is essential. For example, rainwater collection systems developed historically out of necessity and have become a well-established practice in Texas, one that could be drawn upon to direct a portion of harvested rainwater for light irrigation to droughttolerant roofscapes. The system of aquifers and rivers constitute one of our state’s most precious resources, hence the safeguarding of our water systems is an abiding and increasing concern. Planted roofs work toward this goal by functioning as a slowing and filtering device, tempering the flows of heavy rainfalls and allowing more water to permeate into the ground rather than flood parking lots, roadways, and storm conduit systems. As such, they might also be integrated into more complex graywater systems, in which roofs are harnessed as available surface area to filter graywater through plantscapes, at some point along the water-processing cycle. As for plantings, native grasses and flowering species that survive on rocky shelves or in the cracks of hot asphalt paving are perfect candidates for rooftops, and native wildflowers are ideal from the standpoint of showing local color and pride of place. And because the Texas sun is particularly powerful, green roofs afford a considerable opportunity to reduce urban heat loads created by acres of paved and impermeable surfaces. Vegetation absorbs and transforms solar radiation through evapo-transpiration rather than reflecting it, and lower ambient temperatures help counteract the heat-generating conditions of urban and suburban areas.

Recent projects in central and eastern Texas are beginning to illustrate the “regional” green roof. The Lady Bird Johnson Wildflower Center in Austin is host to a unique demonstration study which brings together expertise in native landscapes and the roofing industry. Begun in early spring, the study presents the first thorough research into local application of extensive green roofs with local flora. The brainchild of roofing consultant Brian Gardiner, the project received its primary funding from the Roof Consultants Institute Foundation last year and was aided in design and implementation by Steve Windhager Ph.D., project director, Mark Simmons Ph.D., Heather Venhaus, and Jeannine Tinsley, all at the Wildflower Center.

Over a period of several years, the researchers will monitor a series of 25 waist-high, 5 x 6-foot, flat-top structures to measure and document various aspects of a functioning green roof. To date, control and trial mock-ups for one roof manufacturer have been built on the Wildflower Center’s grounds, including a conventional (dark-colored) membrane roof, a “cool” (reflective) roof, and a planted roof with a proprietary soil mix layered four to five inches deep. Once installation of the full range of planned roofs is completed, the team will evaluate the performance of several green roofing manufacturers and compare data on surface temperatures, moisture retention and drainage, supplemental irrigation, water filtration and quality, and plant regime. Although additional mock-ups will isolate and experiment with different plant types, plant selection is held constant across the majority of the mock-ups, highlighting native species of sages, daisies, primroses, yuccas, and grasses. (Sedum, the green roof “poster plant” elsewhere, is used only sparingly.)

The energy and enthusiasm of the research team and the volunteers, as well as the financial support from several private companies, have made the project a focal point for regionally specific green roofs, and a springboard for more research and activity. For those skeptics who raise red flags at the prospect of leaky roofs, it is worth noting that the effort has been spearheaded by someone with professional roofing expertise. This fact in and of itself helps to push green roof technology into mainstream consideration as a viable roofing option.

One of the early lessons learned at the Wildflower Center appears to be one of the initial successes—the plants, chosen carefully for their hardiness and adaptability, are thriving. A similar approach to plant types has been adopted for use in two other local projects, including this year’s University of Texas at Austin’s entry in the U.S. Department of Energy’s Solar Decathlon. The second project is a new commercial property developed by Austin-based Stratus Properties and architect Gary Payne with Dallas-based Enviroplan Architects & Planners. The project includes an 8,000-sf Starbucks that will showcase planted rooftops using a proprietary assembly of 2 x 4-foot, tall-sided trays set in a continuous field. In both cases, the green roof designers opted for a conservative approach in these early applications by using a deeper soil matrix of eight inches to buffer against temperature and moisture swings. The planted areas – swaths of flowering or grassy natives – are intended to be seen and enjoyed from certain perspectives while also offering an educational glimpse of roofing alternatives.

Demonstrated successes in different parts of the state will certainly encourage awareness, education, and incentive programs to match those of other cities around the world. AIA Houston’s Committee on the Environment recently held a two-day workshop-style seminar that centered on local case studies and holistic thinking, inviting participants as diverse as health professionals and soil scientists, in addition to architects and landscape architects. One of the aims of the gathering was to present an indigenous approach to green roofs in which common design solutions for northern climes are replaced by solutions and species that work in Houston. (Sedums were a disappointing flop in one large project). Another focus of the session was the idea of enabling people to design their own green roofs with a good understanding of the essential principles and elements involved in order to better specify appropriate systems, including non-proprietary components

What are the opportunities, challenges, and constraints for green roofs in Texas? How do they vary from region to region? What techniques can we use to appeal to the public, stakeholders, policy-makers, and architects to advance the technology here? Are green roofs for every building, or should we be discriminating, i.e., pick the best battles? Are the best battles small houses, “big box” stores, flagship civic projects, or a combination of all? How do green roofs mesh with the larger aims of the homegrown sustainability movement? These kinds of questions are the focus of the research of Dylan Sieglar, a Wildflower Center intern who is currently studying for a Master of Science in Sustainable Design degree at UT Austin’s School of Architecture. Particularly interested in the nature and obstacles of technology transfer, she is examining the larger context of issues surrounding the implementation of green roofs in central Texas.

Drawing from her experience and contributions to the Wildflower Center study, and looking at promising methodologies such as co-evolving technologies and capitalizing on local opportunities, Sieglar says her research seeks to uncover “an effective process by which green roof ‘best practices’ might reach policymakers, the building industry, and end-users.” The goal of her research, she says, is to give green roofs a place in the sustainability toolbox in central Texas, if not beyond. Because this work will include input from a wide range of stakeholder types, it stands to offer valuable insight and gainful techniques to overcome barriers, whether perceptual, codebased, or other, to the adoption of a technology that can make great strides in the quality of our environment. In the meantime, there is plenty of room for trial and exploration, and for sharing the news along the way.

Architect Lauren Woodward works with Stanley Architects and Artisans in Austin.

More information about research on green roofs is available at www.wildflower.org, www.greenroofresearch.org, and www.greenroofs.com.