In my quest to learn which green roof system (to use pre-fab trays or not) performs better, I went to see one of the earlier green roofs in Chicago. The head engineer at the Notebaert Nature Museum, Chris Dunn, was kind enough to give me a tour and indulge all my questions. He is very pleased with the installation. Since the green roof was installed in 2003 by Intrinsic Landscaping, Inc. (so cutting-edge that LEED wasn't even a program yet), it has been an interesting lab for observation. Plant materials, originally mostly sedum, have morphed to include a bunch of aster and moss. Chris and Kurt Horvath (President of Intrinsic Landscaping) were on-site investigating what might have caused these changes (soil pH, less maintenance) and one of the reasons I appreciate them is that they approach the situtation as curious scientists, not demanding prima donnas. It is a public building after all and therefore we should all be able to learn from the process. Unlike architecture which is a bit more exact, plant culture is constantly in flux. Of course, green roofs incorporate both, so the intersections most fascinate.
This was one of Intrinsic's earliest installation using a non-tray system. They now install all over the US and are quite in demand from some of the more progressive institutions. The discussion of pros & cons (to tray or not to tray) is too complicated to investigate here. For the moment, let's say that if a leak occurs, you don't know where it originated in either system, so as far as I can tell, that's not a good argument for using the trays. Although apparently there is now some electronic device which can pinpoint the leak exactly. Also, once you remove the tray, that system is compromised and has no filter. This green roof has three components over the building roof membrane: a pvc permeable layer, a root barrier and a filter cloth. The soil medium is contained by grills on a slope with about a 4% grade. About 3/4s of the way down the green roof sit three drains. Intrinsic set up dams before installing the layers in order to test drainage.
In general, even after a large storm, the roof garden seems to retain water at a rate of about an hour before the excess is released back into the drainage system...in this case a retention pond (or the lagoon as it was known in my childhood wanderings). Who wouldn't applaud that?
Inside the museum, you can watch the process of the green roof installation from a worm's eyes view. Most people know the Notebaert as the butterfly museum, but of course it offers more ways to spread our wings and rise above the crowd.
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