Studies On Green Roofs

People facing illness must rely on their doctors for advice or treatment. Depending on the issue, doctors often base their decisions on patient outcome studies, preferably over a 5- to 10-year period. However, in some cases, especially with new treatments, doctors and patients select the treatment even though there is little information to back it up.

Surprisingly, many building owners have been in the same predicament when it comes to installing green roofs. Since 2001, there have been many Green roof installations, and in certain cities, such as Chicago and Toronto, Green roofs are quite common. By now, many North American developers, building owners, and roofing contractors have either installed a green roof or know something about them.

However, because there are still few published studies available, we still do not always know whether all the claimed green roof benefits – among them, storm-water management, energy savings, extended existing roof life, sound insulation, and reduction of the urban heat island effect – will be obtained and, if so, whether they will be significant enough to warrant the green roof’s added cost. These issues are important to building owners, roofing contractors, green roof manufacturers, landscapers, and others involved with green roof installations. Contractors do not want to advise their clients to install or reject a green roof unless they have facts to back up their recommendations.

New Studies
Although Green roofs have been installed in Europe for more than 40 years, until recently there were few studies on the benefits; those studies that did exist tended to be difficult to understand and not involve a North American city. However, new studies have been underway all over North America.

The Lady Bird Johnson Wildflower Center, an Organized Research Unit of the University of Texas at Austin, that started a research project in 2005 to study the benefits of green roofs and to install a pilot project to determine what native plants work best in a green roof application. Their research began with the installation of an 8,000-square-foot modular green roof on top of a new Starbucks building. This green roof, planted exclusively with native vegetation, was one of the first commercial green roofs in Austin, Texas.

More recently, the Lady Bird Johnson Center completed a comparative study pitting six different extensive green roof designs vegetated with native species against roofs with both non-reflective (black) and reflective (white) roofing materials. Although many items could be studied and may be analysed in the future, the primary focus was on stormwater retention and thermal performance.

Twenty-four test platforms were created using the two conventional reflective and non-reflective roofing materials and the six types of green roof assemblies, each replicated three times. The six green roof types included five monolithic (built-in-place) systems and one modular system. Each test roof features a five-foot by six-foot roof platform atop an insulated metal box. The study involved thermocouples placed throughout the roof strata on all 24 platforms, collecting temperature data from the roofs and inside the platforms, connected to data loggers. Stormwater run-off was collected by a scupper and directed through a flow meter, which sent the results to the data loggers.

Among The Findings:

  •  All of the green roofs performed similarly with regard to thermal performance. On one 91-degree Fahrenheit (ºF) day, one of the black-topped boxes without air conditioning reached an indoor temperature of 129ºF while the green roof boxes recorded indoor temperatures of 97 to 100 ºF.
  •  An even greater temperature difference was found on roof surfaces. There, black-top roofs reached 154ºF on the 91ºF day. By comparison, the soil temperature of the green roofs was between 88 and 100ºF.
  •  The green roofs showed maximum stormwater run-off retention between 88% and 44% for medium and large rain events, respectively; however, some green roof types showed very limited retention capabilities. Variations of design and soil medium can explain the varying performance.
  •  The presence of native plants likely helped all the green roofs capture water. In comparison to sedums, a type of succulent traditionally used on most green roofs, native plants can take in more water and release more of it to the atmosphere. The center will study these factors in future green roof research. It is important to note that, for green roofs, plant selection is a key to long term success. Native species may not be the best choice for all areas.

More Studies Needed
Current studies on green roof performance are just ‘scratching the surface’ of what can be learned. While there is significant evidence of green roof benefits, more research is needed to discover the effect green roofs will have on our future urban landscapes. Continued research will also feed the development of improved green roofs technologies. Universities and agencies such as the U.S. Environmental Protection Agency are looking to green roofs and other new technologies to answer global concerns like energy conservation. We are moving into an age of environmental consciousness and concern, and are feverously looking for technologies that will allow us to lead environmentally sustainable lives. It is clear that green roofs can have a significant role in getting us there.

To learn more about the research at the Lady Bird Johnson Wildflower Center go to www.wildflower.org