Mike Silvers, CPRC, Owner, Silvers Systems Inc. and FRSA Technical Director
Anyone doing a fair analysis of roof-related construction failures after high wind events knows that materials pulling through or over fasteners (pullthroughs), are often seen. From my observations, pull-through is far more often the failure mode than fastener pull out. There are many different types of roofing fasteners that are installed differently depending on the material being fastened. We see this pull-through condition with asphalt shingles, nailed underlayment and even sheathing.
I want to concentrate primarily on those materials that are fastened with nails and, most often, by pneumatic (air powered) nail guns. Shingle failures observed often demonstrate that the shingle has pulled over the fastener, leaving holes in the shingles found in debris piles and by the nails still present on the roof. When these are observed, the question comes to mind, did the uplift forces created by high winds pull the material over the nail head or is it possible that the fastener was driven at least partially through the shingle during its application? The answer is hard to determine after the fact but we do know that overdriven fasteners are regularly observed during shingle applications. Why is this seen so often? This condition is often blamed on improper air pressure being supplied to the nail gun. This does have merit but there may be more to consider.
Proper pneumatic tool performance depends not only on consistent pressure measured as pounds per square foot (psf) but also sufficient volume delivered to the tool measured as cubic feet per minute (cfm). In theory, the best way to provide both is with a compressor dedicated to a single tool or gun with a relatively short hose. In practical application, that arrangement is seldomly provided or used. Instead, one compressor will supply several applicators. The job set-up may include multiple fittings, connectors, manifolds or tees and several long hoses. Each of these will limit the cfm being delivered to the tool. When the tool is not in use a constant pressure is achieved throughout the set-up but when air is used, the pressure begins to drop. If sufficient volume (cfm) cannot be maintained, the pressure will continue to drop. One way to compensate for this is by starting with more pressure. However, this leads to overdriving the nail and will eventually lead to lower pressure. The problem of maintaining consistent volume and pressure also arises when multiple users stop and start using their tools, which occurs often when installing roof coverings.
Since the observed pull-through failures may often be caused by these air pressure and volume installation issues, it may be unfair to blame the product.
Why are overdriven fasteners observed more often during inspections than underdriven fasteners? An underdriven fastener will affect the application by causing an easily observed bump in the shingles. It will eventually create a hole in the shingle causing a leak and a call back. To avoid this, an applicator must stop and use a hammer to drive the nail flush with the material. So, if your job performance is judged by the quantity of material that you install which of these might you choose? Overdriven nails that go unseen and allow you to proceed unfettered or underdriven nails that interrupt the pace of your work every time they occur? Near perfection can be hard to obtain but, in this instance, one has to strive for proper roof covering performance.
Are there possibly other reasons nails applied with nail guns are so often overdriven? Overdriven nails were not a problem when nails were hand driven with a hammer. I’m not suggesting that we abandon nail guns but why weren’t overdriven fasteners a problem when hammers were used? One could think that hand driving would cause greater care in driving the nail but I can tell you that someone proficient in hand nailing shingles doesn’t tap them in until they are flush. Instead, they give the nail a tap to set it in place and to allow them to move their fingers from under the head and then drive the nail flush to the shingle in one hard hit. The nail can’t be overdriven, not because of strength (pressure) being applied but because the hammer head contacts the surrounding material and the fastener is then properly set.
A pneumatic nail gun has a driver or striker that drives the nail like a hammer head but, unlike a hammer head, it is smaller in diameter than the head of the nail it is driving. This allows the nail to be driven deep into the material, acting like a hole punch and eliminating or greatly reducing the ability of the nail head to prevent the shingle or other material from pulling through (or over) the nail. This reduces the ability of the applied material to perform properly.
This phenomenon is not limited only to roofing material. While presenting a report to the Florida Building Commission on the University of Florida’s mitigation assessment team findings from Hurricanes Helene and Milton, Dr. David Prevatt noted that roof sheathing pulling over the nails was observed by the team. This obviously causes serious water intrusion and sometimes catastrophic structural failure. The data wasn’t available to determine if there was a difference in performance because of oriented strand board (OSB) or plywood use. But one must wonder if fasteners being driven partially or totally through the sheathing is related to the failures.
Clearly, I’m not in the pneumatic nailer business. Having used, purchased and repaired dozens of them, I know that much hasn’t changed in their basic design in over 30 years. However, expectations for buildings, building components and claddings (roof coverings) certainly have changed. The wind resistance of roof coverings has never been more important and more scrutinized than they are now. Yes, I am aware that most things are easier said than done. I do, however, have great faith in our ability to solve problems, especially one that persists and is important to so many. Is a nail gun with a larger head or a depth sensor that eliminates over driven fasteners too much to ask for? Maybe so but it could go a long way towards increasing the wind resistance performance of many critical building components.
Mike Silvers, CPRC, is owner of Silvers 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 50 years of industry knowledge and experience to FRSA’s team.
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