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An Interview with Stephan Brenneisen



To kick off Ecoroof Month, the City of Portland welcomes Dr. Stephan Brenneisen, green roof and biodiversity expert and head of  the Green Roof Competence Center at Zurich University. His presentation, titled "Green roofs from pilot to mainstream. Pushing the market with wild life visions" will take place at 5:30 PM on Thursday, March 3rd at the Portland Building in downtown Portland.  The presentation is free and open to the public. (Get all of the details here.) 

In advance of his visit, Dr. Brenneisen was kind enough to answer some questions, and we've posted the interview below. 


Portland Ecoroofs (PR): What are the direct (e.g. cooling cost savings) and indirect (e.g. species preservation) benefits of green roofs? Why are they better than a conventional roof?

Dr. Stephan Brenneisen (SB): In Switzerland we usually don’t cool down our buildings, we have a building code that says you have to find other solutions to prevent your building from heating up in summer (insulation etc.). We have calculated the energy savings if you have a green roof instead of an ordinary roof.

Indirectly we are supporting many rare wildlife and insect species (red data book listed), because of the undisturbed conditions on the green roofs (if correctly planned). These species include birds, butterflies, wild bees, beetles, grasshoppers etc. Conventional roofs obviously don’t provide similar habitat benefits.

PR: Why do our cities, in particular, need more green roofs?

SB: The most important reasons are adaptation to general climate change, mitigation of the urban heat island effect, and bringing more wildlife into the surroundings of human beings.

PR: How important is the use of local materials in constructing an effective green roof?

SB: It’s important because it shows an understanding of the approach “giving nature something back”, but adapted to the local conditions. It is not the really important point, though. Of course green roofs can be installed with many different materials, but the sustainable approach requires us to think about where the materials come from.

PR: Are professional design services needed to build a green roof? Or can they be a Do-It-Yourself project?

SB: If people ask the right person and study the right books it can be done by “do it yourselfers”. Important information may be needed from roofing companies (membrane, is it root “safe”) and a structural engineer (load capacity).

PR: Are thinner, non-varied substrate green roofs worth the investment? Or do you need the extra thickness of soil and variation for them to be effective?

SB: To answer this question one would need to calculate the amount of water capacity that is held in the thinner substrate layer compared to a shingle (gravel) roof and a “better” green roof with more biomass and water capacity. So one could compare the effects correctly.

PR: Do we need to design differently for stormwater management than for species preservation? Or can both be accomplished within the same design?

SB: The design criteria can be combined, meaning they are not “against each other” and one can achieve both approaches on the same roof.

PR: Can a few green roofs make a difference at the community benefit level? Or is scale necessary to achieve those broader environmental impacts?

SB: Scale is surely something to achieve, most problems are “quantitative” (heat, storm water). In terms of biodiversity one could argue that some specific roofs can contribute a lot, I would say it is less a need to green us much as possible. But the more is always the better…

PR: How do you think we can achieve a greater adoption rate of green roofs in our cities and suburbs?

SB: Planners, roofing industry and contractors need to have an interest to find functional and cost-effective solutions to reduce the prices per m². Actually the prices in USA are a factor higher than in Switzerland, where we have implemented green roofs in the building code. 


We hope you can make the presentation on March 3rd to hear about Dr. Brenneisen's fascinating work. In the meantime, here's an excellent video of him touring a green roof in Basel Switzerland. 



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