--Speculating on why the green roof collapsed in St. Charles, Illinois and who is at fault.

A little over a month ago, the largest sloped green roof in North America collapsed.  There has been speculation as to why this roof collapsed, but no firm conclusions have been made as the investigation continues.  The president of Aquascape, commented on the incident in a press release saying that the snow had melted on the roof but could not drain because of ice blocking the drainage system.  It has been further explained that the sequence of unordinary weather conditions involving almost two feet of snow, freezing temps at night and 40 degree weather the next day, had allowed the circumstance to occur.

While this seems like a likely explanation reinforced by weather data on the corresponding days, I cannot come to agreement with this theory.  As an Architectural Engineering student, I find it hard to believe that not enough safety factors had been built into the structural design of this roof that even if the loads involved were applied, the structure would not fail.  And while green roofs are a newly implemented technology in commercial construction in the United States, I do not think that the engineers handled the calculations incorrectly.  Designing with green roof loads is not very difficult as dead loads (green roof system’s components, thickness of soil, and vegetation per square foot) are fairly uniform.  In addition, the live loads (snow, ice, and maximum water retention of the soil are also uniform and would be designed for peak load simultaneously using even safety factors over 50%.  For this reason the behavior of a green roof and loads applied are known and will be, in essence, over-engineered by code to avoid this type of failure.

Keeping all of this in mind, what else could have caused this collapse if the beam sizes and connections were designed appropriately?

This is the type of question that has construction managers and contractors shaking in their work boots.  As new sustainable technology is implemented into large construction projects there is a lot of room for error.  Rarely, are these failures a result of poor technology, but a lack of compliance to specified installation instructions.  Contractors unfamiliar with this new construction tend to make mistakes that get covered up until forensic investigations uncover them after tragic events.

Incidents such as the Aquascape green roof collapse should not discourage people from using sustainable products and technologies in new buildings but should emphasize the need for construction managers and contractors that are familiar with the new technology.  It will be interesting to see what the forensic investigation concludes.