Mike Silvers, CPRC, Silvers Systems Inc. & FRSA Director of Technical Services
Anyone who has witnessed the aftermath of a major hurricane knows that one thing they all have in common is an abundance of debris. It is everywhere, and the contents of this debris is just about everything imaginable. The type of debris that I want to concentrate on is debris that becomes wind-born.
One thing that’s clear: in a 150-mph wind, almost anything can fly. In Panama City after Hurricane Michael, on a fairly isolated roof with the bay on one side and large parking lots on the other three sides and a roof height of approximately 35 feet, I witnessed a piece of cast iron about the size of a candy bar penetrated the roof surface. Considering the ratio of weight to surface area one would think that nothing short of being used as cannon fodder could cause it to become air-borne. But there it was, stuck in the roof. This
was obviously very unusual even by hurricane standards. Typically, objects like limbs, furniture and pieces of surrounding buildings, including lumber, glass, mechanical equipment or components and sheet metal are thrown around everywhere. Very few unpowered things have the ability to take flight like a piece of corrugated metal does when exposed to high wind speeds. The panels are not only from roofs, but also from sheds, siding, soffit, fences, equipment screens, etc. These items typically have sharp points and edges that can cause damage to buildings, and low slope roof systems in particular.
We have added impact resistance requirements to the building code for windows, doors and skylights in windborne debris regions, but the impact resistance of roofs (other than for hail) is seldom considered. I am not suggesting code requirements to address these concerns, but some understanding of the wind-borne debris resistance of different roof systems would be helpful. A building's use and its contents may make a more impact-resistant roof covering a smart choice. We have incorporated secondary water barriers in steep slope roofing, but very little consideration is given to how well low slope roofs resist breeches from debris impact. There is currently limited data available that would help a designer evaluate this type of impact resistance and, just as importantly, what happens after a breech in the membrane occurs. Does the breech expand when exposed to additional high wind pressures? How much water enters through different roof types when a given size cut or slice is present?
These are questions that we should want answers to. I am not aware of existing test methods that deal with these concerns. If they don’t currently exist, maybe it is time to consider developing them. We are all familiar with the old expression “when the stuff (s#*t) hits the fan” – it’s time to begin to better understand “when the stuff hits the roof.”
Mike Silvers, CPRC, is owner of Silver Systems Inc., and is consulting with FRSA as Director of Technical Services. Mike is an FRSA Past President, Life Member, and Campanella Award recipient and brings over 40 years of industry knowledge
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