Green Roofs, A Heads-Up Way of Providing Urban Environmental Benefits

Back to Summer 2009 Newsletter

An increasingly frequent urban sight (although not in Arizona and the West) green roofs demonstrate a new meaning and purpose for roofs. Roofs, a hitherto taken-for-granted, inauspicious urban feature, are being adapted to take advantage of  the natural elements of  water, sun, soil and vegetation, to achieve environmental benefits. In the process a new word is coined: roofscape. 

Green roofs, also called “vegetated roof  covers,” “living roofs,” or “eco roofs,” are conventional roofs of  residential or commercial structures used to grow vegetation. Like a constructed wetland, a properly designed and maintained green roof  is a stable, living ecosystem that replicates many of  the processes occurring in nature. 

Predominantly developed and used in Europe, especially Germany, and increasingly used in some U.S. cites with compatible environmental conditions, green roofs are a rare and uncommon sight in Arizona and the West. Brent Jacobsen, a student in the University of  Arizona’s School of  Architecture and Landscape Architecture, is working to encourage green roof  development and use in the western region. Jacobsen received support for his work from the University of  Arizona’s Technology and Research Initiative Fund and the Water Sustainability Graduate Student Fellowship Program.

He says green roofs in Arizona and the West is a concept still to be tested and demonstrated. “It is still very early [in Arizona] as opposed to a lot of  other municipalities in the US. Arizona as well as the entire West is figuring out whether to use green roofs, and, if  so, how to use them.”

Green roof  benefits     

Green roofs offer varied benefits, water resource advantages among them. Jacobsen describes them as “multifunctional or multi-beneficial systems.” For example, green roof  vegetation captures and holds precipitation, thus reducing potentially ripariandamaging peak runoff  loads. By reducing the amount of  runoff  and extending runoff  time, green roofs lessen flooding risks and allow more water to infiltrate into the ground. In effect, green roofs mimic processes occurring in nature that intercept and delay rainfall runoff. 

Not to be overlooked are water quality benefits: green roofs filter runoff  thus reducing the amount of  pollutants washed into a riparian area. Also, green roofs add an aesthetically pleasing touch to the urban environment. 

The energy benefits also are to be considered: green roofs reflect heat, thus reducing heat gain and thereby lessening heating and cooling costs and reducing the urban heat island effect. Energy savings mean water savings, a fact increasingly realized, since water is consumed to generate power and electricity. 

Green roofs also can increase wildlife habitat. Jacobsen says, “This was an interest in my study, the way Europeans are discovering how to design green roofs to be used by different wildlife species. It is becoming a way of  enhancing urban habitat provisions.”

Rethinking roofs

Jacobsen views green roofs as fitting into the urban space differently than did the traditional roof. He says, “Roofs are generally thought of  as separate objects, a surface separate and distinct from the ground. ... There is an opportunity to consider connecting the ground plane to the roof  and creating a different relationship between the roof  and the ground. The result would be an integrated urban design fostering wildlife habitats.”

Ron Stoltz, Director of  the UA School of  Landscape Architecture, agrees. He says a single greenroof  is an isolated ecosystem. “The future of  green roofs in Arizona is not just a green roof  but in complexes of  green roofs, not one roof  but, in fact, dozens of them working in concert witheach other. That is something we are starting to think about fairly seriously.”

He says, “I think of  it more as the terrestrial equivalent of  the fish ladder where you can allow reptiles and the ground mammals to come up on the roof  and start to interact so you get morepredation and a lot more healthy ecosystems than if  you just isolate the roof  in the air.”

Thinking about green roofs clearly involves reconsidering the traditional roof; it means making do with what is available and at hand to accommodate urban environmental projects in need of  open space. With roofs making up about 32 percent of  horizontal surfaces in urban areas, their use for landscape purposes is tantamount to discovering and opening up new lands for green development. Arizona conditions considered

Green roofs once again raise the issue that what works in Europe and in other parts of  the United States may not work in arid and semi-arid Arizona. Jacobsen says more green roof  development is occurring in the Midwest and the East Coast. “They have been pretty successful transporting systems from Europe, using them the way they were designed. That would not fly here because the soil depths aren’t right and the plant material wouldn’t do well in our extreme heat.” 

The Tempe Transportation Center is the most ambitious green roof  project in Arizona. The planning of  the center as a green building was viewed as compatible with the goal of encouraging various and alterative means of  transportation. The green roof  concept being such a novelty in the state Tempe officials who approved the green roof  design hedged their bets. Tempe principal planner Bonnie Richardson says, “To get approval to do this I had to have a back-up plan for what if  it did not work. ... If  for some reason this was a total mistake we would remove the dirt, clean the roof  and put one more layer of  white, reflective membrane on what exists.”

Richardson says their task was to develop a green roof  appropriate to Arizona conditions and convince officials that it was, in fact, a good idea. To determine different methods of  planting, test were conducted at Arizona State University using boxes containing different soil samples and vegetation watered on different schedules. The test were run over 18 months or two summers. 

Plants were selected that would provide a good life cycle. But, also, Richardson says, “We chose plants that had different color and textures to make the roof  interesting.” The plants found to be most suitable for the Tempe roof  were Rocky Point Ice Plant, Slipper Plant, Red Yucca, Bear Grass and Candelilla. 

In choosing appropriate plants, Richardson says special attention was paid to the particular conditions of  the roof. She said, “A lot of  plants could survive without additional water in the desert, but when you put them on a roof  in an urban area surrounded by lots of  concrete and streets you are creating more severe conditions than you have in the desert.”  Richardson says, “We found that the depth of  soil was very important in the Southwest because of  the heat. Standard green roofs might get by with 4 inches of  soil; our test showed we needed a deeper soil base to retain moisture and cool the roof.” They settled on a 12-inch soil depth. 

Jacobsen says, however, a roof  thickly layered with soil limits its placement and use because of  the weight load on the structure. He says, “The Tempe Center has a very deep soil basin, about 12 inches. ... Roof  systems in Europe can be as low as about 4 inches of  soil.  If  our roofs end up having to be 8 to12 inches its going to affect cost. You are not going to be able to install those on houses as easily as you might in other areas.”

Stoltz is conducting research at Biosphere 2 looking at issues affecting green roof  installation in the Southwest. (His research team includes Jacobsen, project manager, and students Daniel Bradshaw and Kristin Van Fleet.) Topics being explored include the amount of  water needed to maintain healthy green roof  vegetation and the effect the weight of  the water has on structural support requirements. The test sight includes 38 plots, four by four feet by about ten inches, that enable researchers to compare two soil types (heavy vs. light); two irrigation regimes (rainfall only vs. supplemented) and three plants species (grass vs. succulent vs. small shrub).

Demonstration and use of  green roofs

Are green roofs a realistic strategy for private homeowners? It would likely mean a dwelling would need to be retrofitted. This is an expensive proposition, to structurally reinforce a building to support the weight of  a green roof. The expense of  retrofitting is better borne when done large-scale, on a structure with commercial or institutional use. Still a fledgling effort in the West, green roofs would be more efficiently installed during construction of  a house or building. 

A green roof  demonstration project is planned for the roof  of  the UA Architecture and Landscape Architecture Building, applying research conducted at Biosphere 2. The design of  the green roof  demonstration project is expected to be completed by Christmas with work beginning in the spring.

Meanwhile the Tempe Transportation Center is serving as a green roof  demonstration project. Richardson says it has attracted many visitors interested in the concept. She says, “We are trying to learn as much as we can. I would like to see graduate students from UA or ASU come up with some interesting thesis to test. If  we learn a better way to do things that would be great.”