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Revisiting Green Roofs, Firm Finds Surprises (and Daisy Fleabane)

Green roofs have gotten a lot of publicity in recent years. They’re meant to help insulate buildings, to capture rainwater that would otherwise run into storm sewers, and to moderate the heat-island effects of clustered structures. To do all that, they’re designed to use special soil blends and carefully planned mixes of plants—most often hardy varieties of succulents in the sedum family.

Among green roofs revisited for the survey was the one on Middlebury College's Atwater Commons. (Middlebury College photo)

Among green roofs revisited for the survey was the one on Middlebury College’s Atwater Commons. (Middlebury College photo)

But nature, it seems, is not particularly respectful of architects’ intentions. KieranTimberlake, a Philadelphia firm that has designed buildings for a number of colleges, just won an Architect magazine R&D award for revisiting green roofs on six buildings dating as far back as 2003 to see how they’ve fared and what lessons they offer for future designs.

The survey found, among other things, that dozens of additional species were sprouting alongside the original plantings. On the roof of a 2005 Cornell University residence hall called Carl L. Becker House, for instance, the survey located 54 new species growing with the planned three kinds of sedum and one grass, prairie dropseed. The volunteers included trees, shrubs, vines, grasses, and wildflowers with names like spotted sandmat, mouse-ear chickweed, October-skies aster, bull thistle, and daisy fleabane.

The survey also discovered that while vegetation on some roofs was lush and healthy, on others it was sparse—which would compromise the roof’s ability to perform as designed. On top of Middlebury College’s Atwater Commons, for instance, plants appeared to be doing well around skylight cones, which provided some shade, but were feeble elsewhere. A report on the survey noted that the building’s unusual design had created microclimates on the roof, which clearly was not the architects’ plan.

The magazine’s account of KieranTimberlake’s research notes that the firm is now raising money to construct a green-roof test facility on a new building in Philadelphia’s Navy Yard, the Penn State Center for Building Energy Education.

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