What's Happening from Environmental Building News
October 1, 2008
Green Roof Study Finds Big Variation in Stormwater Retention
A recent study of several green roof types monitored their temperature and stormwater retention performance and found that while all were capable of significant thermal moderation, some held water well while others were barely better than conventional roofs at preventing or slowing runoff. Mark Simmons, Ph.D., a researcher at the University of Texas at Austin and the lead author of a paper on the study in
Urban Ecosystems, told
EBN that these benefits had until now not been tested experimentally for tropical or subtropical climates, where high temperatures and large rainfall events make those advantages particularly valuable. A green roof effective in such a climate would presumably also perform well in more temperate zones.
Simmons and his colleagues compared white and black conventional roofs and six extensive vegetated roof designs planted with native perennials and erected on test platforms in the subhumid, subtropical climate of Austin, Texas. They monitored temperature at several points in and under the roof structures and water runoff after storms.
Each type of green roof lowered roof temperatures during warm weather by 30°C–35°C compared to the black roof, with no meaningful difference among the green roofs in the temperature beneath the roofs. The white roof, as expected, was cooler than the black roof but by a much smaller margin. The green roofs also delayed the peak daily temperature by 1–3 hours. During a cold snap, the green roofs were warmer than the conventional roofs. Because the study was conducted within eight months of planting the roofs, the thermal results “largely reflect the insulation and evaporative cooling characteristics of the abiotic component of the green roof,” says the paper, referring to the growing medium.
Simmons said that based on the six roof designs, he had anticipated that they would all retain stormwater effectively. But during three rain events, the most effective green roof captured about four times as much water as the least effective, which did little better than the conventional roofs. Substrates with large planting-medium retention cups, low drainage-hole area in the drainage layer, and a high proportion of perlite in the planting mix correlated with high water retention.
The study’s results indicate that while any common green roof design moderates the effects of hot weather, in regions where heavy rains occur, green roofs should be designed specifically for stormwater retention. Simmons said research is continuing, seeking to identify a “cocktail of specs” that is effective and relatively simple and inexpensive. As vegetation on the test roofs matures, Simmons and his colleagues are studying both irrigated and nonirrigated approaches to green roofs.
– Michael Wilmeth
DISCUSSIONS
Reader-contributed comments related to Green Roof Study Finds Big Variation in Stormwater Retention - EBN: 17:10. Comments are listed with newest at the top.
RELATED CATEGORIES
LEED Credits
SS Credit 6.1
SS Credit 6.2
GREEN TOPICS
IMAGE CREDITS:
1.
Photo: Philip Hawkins
More comments
I am appreciative of this testing and am very interested in the ongoing findings. As I research and develop living roof systems its helpful to have results to measure what I think works to actual results. To my mind most systems are vastly overdesigned for their stated functions, water retention and heat reduction, and it seems, are still not accomplishing their goals.
I am testing recycled, or crushed, concrete as part of my mix. It seems to have many of the qualities that sedum like and is a readily available, recycled/reused material. I have not done water quality testing yet, but initial inquiries real that the greatest concern is residual lime, which gives the soil high alkalinity. As for weight, it does not seem to be any higher than gravel. Because of the rough, porous nature of its surface, the concrete seems to absorb initial rainfall well.