What happens when high velocity winds meet your roofing system? Well, there’s no simple answer and no way to predict precisely which roofs will have trouble; Mother Nature is just unpredictable. Sometimes, on buildings with the same type of roof that are located side by side, one roof blows off or suffers other catastrophic damage while the other roof has very little damage. Let’s compare the two most common styles of roofing in Florida.
Built-Up Roofing Systems (BUR)
The tried and true performers of roofs that weather storms are conventional built-up or modified bitumen membranes fully adhered, especially when they are adhered to a structural concrete deck. Historically, after a major storm, many have puncture damage to flashings and cuts as a result of flying debris, but the roofs stay intact and in place. Gravel-surfaced, built-up roofs fared slightly better than the modified bitumen systems because the gravel that was well embedded in the flood coat provided protection from flying debris. The weight of the gravel also helped keep the roof down. Unfortunately, most of the built-up roofs lost a good portion of the gravel surfacing, causing other damage to surrounding buildings, but the roofs that the gravel came from suffered the least amount of punctures.
Less effective, but still very good, were the modified bitumen and built-up systems mechanically attached to steel decks. The asphalt-based roof systems that were attached to lightweight concrete or gypsum deck tended to have more wind-related damage than others. Generally, the membrane stayed intact, but sections pulled up from the deck, leaving the membrane loose on the roof but looking as if nothing had happened to it. With lightweight concrete and gypsum decks, whether the roof stayed in place mostly depended on the condition of the lightweight concrete or gypsum and its ability to resist fastener withdrawal.
Single-Ply Roofing Systems (TPO/PVC/EPDM)
Fully adhered single-ply membranes resisted blow off about as well as asphalt systems but were more susceptible to damage from flying debris. Almost all types of flexible membrane systems suffered cuts and punctures during the storms. There was one exception: Roofs that had no mechanical units or other penetrations were almost always free of puncture damage.
Overall, fully adhered single-ply membranes were more hurricane-resistant than mechanically attached systems. The mechanically attached systems weathered the storms well as long as they remained intact. Once the membrane was cut, the incidence of blown-off membranes increased. Wind is able to get into the cuts, so the roof was subject not only to negative pressure above the membrane — which pulled the membrane up — but also to positive pressure from underneath, which pushed the membrane up.
Because roofs are mostly designed for uplift pressures only, the wind was able to loosen poorly fastened screws. This causes a ripple effect: one screw pulls out, putting extra stress on those around it, which in turn causes them to pull out. This cycle repeats itself until the membrane comes all the way off.
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