There’s No One-size-fits-all Green Roof, Studies Show

by |April 24, 2012

Image: Inhabitat.com

By Lorena Galliot

Green roofs – rooftops covered with a layer of vegetation–are getting a lot of credit for providing environmental benefits. They have been found to reduce storm water runoff from buildings, conserve energy by moderating rooftop temperatures, restore fragile ecosystems and beautify urban spaces. From Toronto to New York, cities are investing billions in green infrastructure programs that rely on this kind of technology and ongoing research is helping refine its application.

Researchers are now looking at how the types of vegetation used in green roof affect their functioning. To date, many designers have used sedum–a genus of low-growing succulent plants that hold water in their leaves–to cover rooftops. Sedum, also known as stonecrop, is found throughout the Northern Hemisphere, and has been favored for its hardiness, shallow root systems and low maintenance requirements.

However, two recent studies suggest that grasses or taller shrubs may actually be more effective than sedums at reducing storm water runoff, often a major selling point for green roofs. These studies suggest there is no one-size-fits-all green roof.

Stuart Gaffin, a climate scientist at Columbia University’s Earth Institute who specializes in green roofs, says that this kind of study can help contractors and urban planners optimize green roof performance. “More research is always a good idea,” he wrote in an email.

One of the studies compared different soil depths and plant types on an experimental green roof in northern Italy. Its authors found that “the use of succulent plants like sedum […] might not always represent the best choice to improve green roof technical functions.”

A second study, conducted in the UK, also concluded that grasses were most effective at reducing water runoff, followed by forbs (herbaceous flowering plants like sunflowers) and, in last place, sedums.

According to Brad Rowe, a professor of horticulture at Michigan State University who did not participate in either study, these findings come as no surprise.

The more a plant transpires, releasing the water it absorbed back into the atmosphere, the more it mitigates water runoff, he explained. Sedums close their stomata (the microscopic pores on its leaves) to retain water and thus transpire very little. Even if they  keep water in for a while, it is eventually released and adds to runoff.

The  small size of sedums also puts them at a disadvantage compared to taller or wider plants. “It makes sense. A plant whose biomass is up three feet rather than three inches will capture and retain more water even before it reaches the ground,” said Rowe, adding that he thought the findings in the UK studies were “spot on.”

According to Nigel Dunnett, an author of the UK study, previous research on the efficiency of green roofs had focused on their design and structural components, not on the type of plants used. Understanding the impact of vegetation type on the functioning of the system is important step for the field.

“There is a tendency in the green roof world to imply that all green roofs are the same, and deliver benefits in the same way,” he wrote in an email. By contrast, he said, his study showed that “vegetation choices can, to some extent, also influence green roof performance.”

Sergio Andri, co-author of the Italian study, also stressed that rooftop vegetation should reflect local growing conditions.

“If our aim is to build low-maintenance green roofs, we must imitate natural ecosystems,” he wrote, pointing out that sedum-type plants do not naturally grow in most parts of Italy. He believes that in temperate and Mediterranean climates, like the ones where his experiment was carried out, “the natural evolution will be to use more herbaceous plants and shrubs.”

Gaffin agrees that using local grasses can be an effective and low-maintenance way of covering a green roof. He and Columbia plant ecologist Matt Palmer have begun experimenting with such plants on New York City rooftops.

“We planted seeds from a native grassland ecosystem, the Hempstead plains [on Long Island], on the roof of a school in the Bronx, and they’re thriving beautifully,” he said. Using native plants for green roofs could have the additional benefit of restoring patches of fragile or disappearing ecosystems, he added.

But Gaffin cautioned against grass as a general trend, explaining that efficiency of green roofs depends on many things. “The type of plant used plays a role, but also the depth of the substrate used, and the structural design of the green roof,” he said, pointing to a recent study by his team which showed that the use of pre-fabricated modules containing plants – favored because they are easy to install on any roof – retain storm water much less well than green roofs built from the floor up.

Neither would Gaffin write off sedum. “I like sedum. It’s a very tough plant, usually a low-risk choice” for use on green roofs, he said.

As more and more cities are integrating green roofs into their planning, scientists are hard at work looking for ways to streamline the use of this technology while accounting for differing conditions and aims.

“There’s water retention. There’s energy. There’s [the] urban heat island [effect]. There’s aesthetics. When green roofs are designed to take all of these factors into account, that’s when they’re the most valuable,” said Rowe. The problem, he said, is that it’s “not always easy.”


5 thoughts on “There’s No One-size-fits-all Green Roof, Studies Show

  1. For those of us in the green roofing industry, whether design or research, these recent findings come as absolutely no surprise. I learned this in Hort. 101. In 2007, Berghage et al published a paper reporting plants contribute up to 40% of a green roof’s water management capability. http://www.decentralizedwater.org/documents/04-DEC-10SG/04-DEC-10SG.pdf

    The plants have always been the star of the show. The whole purpose of the green roof system is to support the plants and provide them with the environment they need to thrive.

    Roof tops are an extreme environment; do not be fooled into believing they will ever be able to mimic the surrounding environment. Extensive green roofs should always be designed without irrigation and plants must be able to tolerate drought conditions. Irrigation systems that use potable water are a disgrace. This is why succulents will always be the first choice green roof plant. Extensive green roofs just do not have soil depths deeps enough to support larger-leaf plants that transpire more water. A good green roof designer with years of experience will be able to recommend a larger pallet of plants that include herbaceous wildflowers, but nothing is ever guaranteed when dealing with micro-climates.

    As a designer, I always strive to explain to clients the thicker the profile of the green roof system, the more stormwater we will retain and the great the diversity of plants we can establish. This is also why we always specify a granular drainage layer over plastic drainage boards. But 9 times out of 10 we are dealing with structural restrictions and feather weight systems are necessary.

    In order to install more green roofs with greater diversity of plants, architects need to add additional structural capacity into their initial roof designs and specify built in place green roof systems based on performance characteristics, not product names. When the average profile of a green roof increases from 3-4 inches to 7-10 inches we will see a proliferation in the types of plants we find on roof tops.

  2. I also wish to say I am disappointed each of the studies cited were to websites that require you to purchase their papers. So much for the free dissemination of ideas.

  3. Polyroof says:

    We agree with the majority of this article and especially the comment that one size does not fit all. This area is still very much in its infancy and while storm water run off is a major issue in some areas there are many more widely accepted reasons for choosing a green roof. For most residential property owners their decision making process is far more geared to the environmentally friendly benefits of the green roof. However, most are put off by the cost, especially if they have a particularly forceful designer or architect.

  4. Ashton says:

    This seems to be a trending industry although it does not seem to have taken off so far in the residential areas of London. Prohibitive costs I would imagine.

    I certainly hope that it will take off in the future.

  5. Dennis says:

    I own a roofing, siding and gutter company in Ohio. I’m seeing more and more commercial Client interest in Green roofs but the cost is definitely the factor that sways them against installation. I’ve done alot of research on Green roofs and have been interested in possibly installing them in the future. I’ve also worked as a Staff Reporter for the city newspaper nearby and covered a story on this subject. It certainly would be beneficial to the environment and I’m all for that but it would be great if we could find a way to lower cost.

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