A New Technique for Green Roof Evaluation

Aug. 24, 2017

Earthquake engineering inspires vegetated roof performance analysis.

A technique normally used in earthquake engineering is helping researchers determine how well green infrastructure works and communicate those findings with urban planners, policymakers and developers.

It’s tough to predict how the buildings underneath green roofs will respond to the variable weight between wet and dry conditions, notes University of Illinois at Urbana-Champaign graduate student Reshmina William, who partnered with Civil and Environmental Engineering Professor Ashlynn Stilwell to develop new green roof analysis methods.

Another challenge is determining how well green roofs retain and process stormwater when storms themselves vary greatly in intensity, duration and frequency. Williams noticed a parallel between the uncertainties of green roofs and the concept of fragility curves, a mathematical calculation that helps earthquake engineers predict what earthquakes will do to buildings.

“Green infrastructure has a lot more variability, but that’s what makes fragility curves ideal for capturing and defining the dynamics involved,” Williams explains.

Williams and Stillwell opted for green roofs specifically over other forms of green infrastructure because a green roof on campus was already outfitted with instrumentation to measure soil moisture, rainfall, temperature, humidity and other variables that can be entered into the fragility curve model. Most green roofs don’t have monitoring equipment installed, Stillwell adds.

“One of the biggest barriers to the acceptance of green infrastructures is the perception of financial risk,” William says. “People want to know if the benefit of the green roof is going to justify the cost, but that risk is mitigated by knowing when an installation will be most effective. That’s where our model comes in.”

The duo published the results of their model and risk analysis in the Journal of Sustainable Water in the Built Environment. The results from this one specific roof don’t offer a one-size-fits-all approach to all green infrastructure, but the model is adaptable across multiple technologies and environments, according to William and Stillwell.

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