Mark T. Simmons

Green roofs are not created equal: the hydrologic and thermal performance of six different extensive green roofs and reflective and non-reflective roofs in a sub-tropical climate

Green roofs have the potential to retain stormwater on the roof surface and lower the thermal loading on buildings. Because of this, the greatest environmental benefits from green roofs might be achieved in subtropical climates characterized by high temperatures and intense rain events. There is, however, little research to support this. In a replicated study in Texas, we compared the performance of six different extensive green roof designs vegetated with native species, to non-reflective (black) roofs, and reflective (white) roofs. Preliminary hydrologic and thermal profile data indicated not only differences between green and non-vegetated roofs, but also among green roof designs. Maximum green roof temperatures were cooler than conventional roofs by 38°C at the roof membrane and 18°C inside air temperature, with little variation among green roofs. Maximum run-off retention was 88% and 44% for medium and large rain events but some green roof types showed very limited retention characteristics. These data demonstrate indicate that: 1. Green roofs can greatly affect the roof temperature profile—cooling surface layers and internal space on warm days. 2. Green roofs can retain significant amounts of rainfall, this is dependent on the size of the rain event and design and can fail if not designed correctly. We suggest that as green roofs vary so much in their design and performance, they must be designed according to specific goals rather than relying on assumed intrinsic attributes.

The effect of roofing material on the quality of harvested rainwater

Due to decreases in the availability and quality of traditional water resources, harvested rainwater is increasingly used for potable and non-potable purposes. In this study, we examined the effect of conventional roofing materials (i.e., asphalt fiberglass shingle, Galvalume(®) metal, and concrete tile) and alternative roofing materials (i.e., cool and green) on the quality of harvested rainwater. Results from pilot-scale and full-scale roofs demonstrated that rainwater harvested from any of these roofing materials would require treatment if the consumer wanted to meet United States Environmental Protection Agency primary and secondary drinking water standards or non-potable water reuse guidelines; at a minimum, first-flush diversion, filtration, and disinfection are recommended. Metal roofs are commonly recommended for rainwater harvesting applications, and this study showed that rainwater harvested from metal roofs tends to have lower concentrations of fecal indicator bacteria as compared to other roofing materials. However, concrete tile and cool roofs produced harvested rainwater quality similar to that from the metal roofs, indicating that these roofing materials also are suitable for rainwater harvesting applications. Although the shingle and green roofs produced water quality comparable in many respects to that from the other roofing materials, their dissolved organic carbon concentrations were very high (approximately one order of magnitude higher than what is typical for a finished drinking water in the United States), which might lead to high concentrations of disinfection byproducts after chlorination. Furthermore the concentrations of some metals (e.g., arsenic) in rainwater harvested from the green roof suggest that the quality of commercial growing media should be carefully examined if the harvested rainwater is being considered for domestic use. Hence, roofing material is an important consideration when designing a rainwater catchment.

The ecological imperative for environmental design and planning

Environmental design and planning are important tools for human adaptation. Designers and planners depend on experience, craft, and environmental knowledge to shape preferred futures. Ecological literacy would enhance the design and planning of built environments. The concepts of “resilience” and “ecosystem” offer opportunities for collaboration between ecologists and practitioners in the design and planning disciplines. Urban resilience to natural disasters and coastal “green” infrastructure represent two areas where design and planning based on ecological principles should be applied. The Sustainable Sites Initiative is a practical example of interdisciplinary collaboration.

Toward Ecosystem Services as a Basis for Design

Environmental design has a long history of concern for ecosystems but has often lacked explicit assessments of, or goals associated with, site performance. Ecosystem services provide an organizing concept around which to make a wide array of environmental and, to some extent, social design goals explicit. Additionally, they allow assessment and evaluation of site-design decisions through both pre-construction modeling and / or post-occupancy evaluation. The U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) rating system and the Sustainable Sites Initiative (SITES) are used as examples of how performance-based site design can be incorporated into the design process. We suggest that the maintenance of ecosystem services become a standard and increasingly monitored goal for the practice of environmental design. This move toward performance goals linked to ecosystem services for which success or failure can be determined is essential if environmental design is to offer a substantive contribution to the achievement of a more sustainable culture.

Timing of Prescribed Burns Affects Abundance and Composition of Arthropods in the Texas Hill Country

Abstract Prior research has demonstrated that fire can be an important structuring force for plant communities in prairies and grasslands. However, investigations of land-management techniques, such as prescribed fire, often overlook responses of local fauna, particularly the arthropods. In this study, we examined a previously unappreciated, although potentially important, component of fire ecology by asking, does the timing of prescribed burns alter community structure of arthropods? At a site in the Texas Hill Country, we used sweep-net sampling to collected arthropods from experimental plots that had been treated with a summer-burn or winter-burn regime. Summer-burn plots supported >170% more individual arthropods than winter burns. Although overall diversity of arthropods did not significantly differ between treatments, there were significantly more carnivorous arthropods and marginally more herbivorous arthropods after fire in summer relative to fire in winter. Effects of timing were particularly strong for Cicadellidae (leaf hoppers) and Tetragnathidae (long-jawed spiders). Our results demonstrate that timing of prescribed fire can substantially alter composition of resident communities of arthropods. Furthermore, these data highlight the importance of examining composition of the community, in addition to diversity indices, when assessing response of arthropods to land-management techniques.

Climates and Microclimates: Challenges for Extensive Green Roof Design in Hot Climates

Green roof systems have been developed and adopted in the temperate and cool-temperate climates of Europe and North America. Although these regions can get extreme weather, they generally do not experience climatic extremes of high temperatures, prolonged drought, and intense rainfall events of tropical and subtropical regions. This presents challenges for green roof design to not only provide adequate growing conditions for plants, but also to improve roof performance with respect to intrinsic (e.g. cooling building, extension of roof membrane lifetime) and extrinsic (e.g. flash flood mitigation, building cooling, reduction of heat island effect) benefits. Therefore, the components of conventional green roof including plant palette, growing media composition and the other synthetic layers need to be modified. The characteristics of green roof water retention, plant water availability, plant selection, and thermal properties are all critical factors which need to be adapted to help address the harsher environmental conditions and performance demands of hot climates. If these problems can be overcome, the combined environmental, ecological and sociological benefits suggest green roofs could be an imperative technology for towns and cities in tropical and subtropical regions of the world.