Florida Roofing Magazine- March 2022

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for gypsum with a properly sized pre-drilled hole. For base-sheet attachment, there are a few options – all drive style – installed with a weighted “pogo” driver. The extendable barb-style used in cementitious wood fiber or a base-sheet fastener may be used. Again, fastener pullout tests are required for this deck type.

Lightweight Insulating Concrete

When it comes to lightweight insulating concrete, density is all over the place. Depending on density, mix design and the condition of the existing deck, fastener pullout resistance can vary greatly. Curing of new pours also impacts pullout performance as well as the amount of force needed to install fasteners. The practice of using a screw that is normally used in steel decks has become common for these applications, especially in some high-wind zones. Fasteners in these applications are driven through the lightweight insulating concrete and into the steel form-deck, which helps to hold the lightweight insulating concrete in a “sandwich” between the steel deck and insulation. More commonly, a basesheet is attached to this type of deck (Figure 6). Fasteners that are formed from light-gauge steel have been used to attach base sheets for over 30 years and have proven their worth in high-wind areas such as South Florida where lightweight insulating concrete is common. Insulation or additional plies can then be mopped in or adhered with insulation adhesive and an adhered single-ply, Figure 6: Base Sheet Fastener built-up or modified-bitumen system installed on top. Mechanically-attached single ply over lightweight insulating concrete is seldom used unless there is a steel form-deck under the lightweight insulating concrete that is capable of providing adequate pullout resistance. The membrane can also be fastened to the purlins using special fasteners with fine threads and heavy drill points designed for heavy gauge steel thicker than 18 gauge. The auger-style fasteners are not acceptable for use with lightweight insulating concrete. This deck type also requires pullout tests.

popular fastener choices for structural concrete is a heavy-duty screw with a symmetrical thread design (Figure 7). Unlike a buttress thread, symmetrical threads have matched angles on the top and bottom of the thread and will cut a clean thread-pattern when properly heat-treated and installed into a properly sized, pre-drilled hole. What has become a popular style for structural concrete is a drive-type fastener that has a deformed or fluted shank (Figure 8) that, when driven into a properly sized, predrilled hole, creates an interference fit, providing very high pullout resistance.

Figure 7: Symmetrical Thread

Figure 8: CD-10 Fastener

Each fastener has its advantages. The drive fasteners are easier to install but cannot be easily removed. Threaded screws require a good-quality, high-torque screw gun and a little more time and finesse, but they can be backed out and removed. As drill bits wear, the drilled holes get tighter and the fasteners are harder to install, resulting in a short life span for drillbits. The hole also must be drilled sufficiently deep to accommodate any debris that falls back into the hole when the fastener is installed. Fastener pullout tests are always a good idea for these decks: to properly size the hole for optimum pullout performance and installation ease. Perhaps a better option for these more difficult deck types (e.g., cementitious wood fiber gypsum, lightweight insulating concrete and structural concrete) is a good quality roofing insulation adhesive. The material costs may be higher but the labor can be significantly reduced. Finally, it’s not only important to use proven code-compliant fasteners, but also fasteners that are resistant to corrosion. Today’s corrosion-resistant coatings have been refined by decades of real-world use and testing. The industry has transitioned from Structural Concrete “sacrificial” coating to “barrier” coating and in some When it comes to structural concrete decks, there cases, both are used to maximize corrosion protection. are many fastener options available. Both insulation Sacrificial coatings react with the corrosive and membrane can be secured to structural concrete elements and, as the name implies, sacrifice themdecks with very high pullout resistance in sound decks. selves to protect the base material. Eventually the Regardless of the fastener used, pre-drilling is resacrificial coating will be depleted exposing the base quired. As a result, installing fasteners into structural material and corrosion will quickly consume the steel. concrete is a labor-intensive process. One of the most Depending on the corrosiveness of the environment,