Lessons Learned from 30 Years of Designing Green Roofs

At the 2010 ASLA Annual Meeting, OLIN Studio’s David Rubin, ASLA and Susan Weiler, FASLA highlighted the lessons learned from their three decades of experience with building green roofs. “Many clients and landscape architects have a very narrow understanding of green roofs,” argued Weiler. “The key to understanding their role and purpose is to think of them as a floor rather than a ceiling.” Especially in urban environments, green roofs offer tremendous opportunities to create outdoor gathering spaces in densely built environments. In addition to their social value, green roofs provide opportunities for environmental stewardship and economic revitalization.

One of OLIN’s most successful and highly-touted projects is Canary Wharf in central London, which is now London’s main financial district. “The green roof space was critical to the success of the Canary Wharf development,” said Weiler. “The landscape became the selling point for the office and retail space.”  One of the unique aspects of this project was that it incorporated trees as large as 18 to 20 inches in trunk diameter. Trees this large generally have very large root balls, which become a major obstacle when building a landscape over structure. “In Germany, they use advanced root pruning techniques that minimize the size of the root ball, while keeping the tree viable.” This project was one of the first to take advantage of this process to incorporate trees of such girth. “The space looks like terra firma, yet it built entirety over structure,” said Rubin.

Many of OLIN’s major green roof development projects have been affiliated with transporation centers, including Patsaoures Transit Plaza in Los Angeles, Columbus Circle and Bryant Park in New York City (see a case study), and several projects in Philadelphia. “If we can get people around cities without using cars, then we are making progress toward sustainability,” said Weiler. Because major transit centers are often in densely built areas, green roofs create opportunities for public green space that can encourage more people to use public transportation. Many of these projects become very complex due to the amount of gas, electric, water, and sewage and other utilities buried beneath the ground.

To deal with the complexity, “coordination is key. Civil engineers, plumbing engineers, electrical engineers, architects, and landscape architects must communicate clearly and get involved at the very beginning of the planning process.” One way that OLIN does this is by asking engineers to do ‘surface expression drawings’ in which they draw the pipes, electrical lines, and other infrastructure to scale. “Rather than drawing a 12-inch pipe as a simple line, it is drawn to look like a 12-inch pipe. A backflow preventer looks like a backflow preventer,” said Rubin. “This way we are able to anticipate and avoid many problems and alterations that would inevitably occur during construction.”

Many landscape architects are concerned with the liability involved with taking on green roof projects. The number one concern is with waterproofing of green roofs. If the waterproofing fails, it can lead to tremendous damage in the building below. To help avoid these problems, OLIN typically hires a waterproofing consultant to oversee the process. There are many different strategies for waterproofing a space based on soil types, slope, and vegetation. “It is extremely important to choose the right strategy for the right situation, said Weiler. “It’s either you pay for it now or you pay for it later.” In addition, Rubin recommended that you have someone on site overseeing the waterproofing process very carefully. The problem is that most green roof construction crews are not to dealing with such a delicate process. Many times, problems can occur when workers put spades through the waterproofing creating a leaky under layer that is very difficult to find once it is covered over with soil. “The only way to avoid these problems is through close monitoring and good communication with the contractor.”

Check out Susan Weiler’s well-regarded book on green roofs: “Green Roof Systems: A Guide to the Planning, Design, and Construction of Building over Structure

This post is by Matt Busa, ASLA 2010 advocacy and communications intern.

Image credit: Canary Wharf / Dennis Gilbert

One thought on “Lessons Learned from 30 Years of Designing Green Roofs

  1. Jorg Breuning 09/16/2010 / 4:33 am

    Structures covered by streets, walk ways, parks, plazas or soil always require special attention in regards of the waterproofing and drainage. These structures are also not new in construction and projects go back much more than 100 years. Green roofs or modern green roof technology was introduced approximately 30 years ago in Europe and 12 years ago to the US by different architects.

    The big difference to traditional techniques implemented on the projects mentioned in the article is simply the difference in the performance of the systems and engineered layers. The high performance of well engineered modern green roof technology allows thinner (soil) profiles, higher water retention, better nutrient exchange capacity and the use of advanced irrigations systems. Modern green roof technology made the installation of live green more affordable and easier integrable for literally any projects. I could read about the fear of having leaks during the construction. However any experienced green roof installing crew knows exactly what to do – that’s why they do it. In most of our cases the waterproofing itself and the waterproofing details are not designed correctly and lead to problems. Experience showed that water proofing consultants typically are not familiar with the different types of green roofs, green roof research or international guidelines (i.e. FLL). Root resistance, leak detection are another topics and have to be part of the architectural drawings and specification long before the project will be executed. jbi

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