Stephen Childs, Technical Services Manager, OMG Roofing Products
Commercial, industrial and institutional buildings can be constructed with several different roof deck types, including steel, wood, concrete, gypsum and others. Each has a unique set of characteristics that require different mechanical fasteners to meet the various needs of the roof assembly. Regardless of the specific deck type, contractors should always ensure that the fasteners used to secure any roofing components are proven for the application and are code compliant (e.g., Factory Mutual, Miami-Dade County).
Since first used to attach insulation in the early 1980s, roofing fasteners have evolved a great deal. Prior to 1980, fasteners had not been specifically designed or engineered for commercial roofing applications. Fastener pullout, back out and corrosion resistance requirements were unknown and, as a result, fastener performance was not always up to the task. As fastener manufacturers started working closely with roof system manufacturers, more sophisticated and task-specific roofing fasteners were developed.
At first, wood and steel were the only roof decks suitable for mechanical fastening. Cementitious wood fiber (Tectum), gypsum, lightweight insulating concrete and even structural concrete decks possessed unique properties that did not allow insulation or membrane to be secured with traditional screws. Fastener manufacturers began to recognize the roofing world as a new opportunity and dedicated resources to design and engineer products to address market demands. New fastener companies solely dedicated to the roofing industry emerged, bringing with them new fastener designs and better performance. Below is a general description of the most common decks and fastener options.
The most common commercial roof deck is steel and it is available in several different configurations, thicknesses (gauge) and tensile strengths. Thickness and tensile strength play a major role in fastener performance. It’s important to understand that when a screw pulls out of a steel deck, it’s the deck that fails not the screw but there are some fastener designs that perform better than others. The change in test protocols at Factory Mutual in 1992 – after Hurricane
Andrew severely damaged South Florida – from a 5’ x 9’ to the current 12’ x 24’ test sample size for all mechanically-attached systems over 4-feet wide and for fully adhered, built-up and modified bitumen systems requiring wind uplift ratings over 90 psf, not only exposed the limitations of many of the screws used in steel roof decks at the time but also helped spark new designs for specific applications. Today, there are a wide variety of roofing screws for steel deck
applications.
The specific screw used depends on the component being installed. Membrane and insulation fasteners have different performance characteristics and are not generally interchangeable. “Standard”
screws, often referred to as #12, are typically used to secure insulation (Figure 1 Click here .), while larger diameter screws (#14, #15 and #21) are typically used to secure membrane. These larger diameter screws also have buttress style thread designs (Figure 2.) as well as point configurations that maximize pullout and backout resistance for improved wind-uplift resistance. Insulation attachment is less dynamic and insulation fracture is more likely to occur than fastener pullout, so insulation screws are less sophisticated in design.
Regardless of the screw type, all steel deck fasteners are designed to be used with a stress plate that has also been designed for the specific application. Most roof cover manufacturers have fastener programs and generally require the use of their fasteners to comply with warranties. Approval listings, especially Factory Mutual approved systems, also have minimum deck requirements. The roof cover manufacturer should always be consulted for specific deck and fastener
requirements.
Dimensional lumber, plywood and oriented strand board (OSB) can all be used in wood roof deck construction. Currently there are not any commercial roofing fasteners on the market specifically designed for use in wood decks. Instead, steel deck screws – especially the “standard” screws – are commonly used in wood substrates due to a coarser thread profile that provides the best pullout resistance for wood.
In addition to the “standard” fasteners, #14 “heavy duty” or all purpose fasteners (Figure 3) can also enhance pullout resistance in wood decks. The pullout resistance values in dimensional lumber are usually high due to the thickness and density of the wood. However, in plywood and OSB, especially in boards less than 3/4-inch thick, obtaining consistent and adequate pullout values can be very challenging due to limited thread engagement and voids in the laminations. This is of concern with mechanically attached single-ply, where the wind uplift forces are more dynamic. Having a qualified technician perform a fastener pull test is recommended for plywood and OSB construction or where there is question of the roof deck’s condition.
Some roof decks, like cementitious wood fiber, are not capable of holding a “traditional” screw. The wood fiber and cement construction are simply not dense or stable enough to hold small-diameter, shallow-thread screws. Instead, large-diameter coarse thread auger- type fasteners made from glass-filled nylon (Figure 4) were developed to accommodate the varied density of these decks. Some can be installed without pre-drilling; however denser decks typically require pre-drilling.
There are other fasteners with this general design that are made from steel and look like monster screws. It is also common to attach base sheets to this kind of deck. Drive-type fasteners with barbs that extend into the deck (Figure 5) after the fastener is seated, provide an excellent attachment method. A fastener pull-test is a must with this type of deck.
As with cementitious wood fiber decks, gypsum roof decks do not effectively hold traditional roofing fasteners. While many have tried putting concrete screws into this type of deck and have even achieved relatively high pullout resistance, over time the deck connection with the fastener fails. This is because movement of the insulation or roof cover moves the screw back and forth and the deck is not resilient enough to take this movement. The deck becomes damaged,
greatly diminishing the pullout resistance. The fastener performance can become inadequate to support the roof assembly. The same auger-type fasteners that are used for cementitious wood fiber are used 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.
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 base sheet
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, 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.
When it comes to structural concrete decks, there are many fastener options available. Both insulation and membrane can be secured to structural concrete decks with very high pullout resistance in sound decks. Regardless of the fastener used, pre-drilling is required. As a result, installing fasteners into structural concrete is a labor-intensive process. One of the most 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.
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 drill bits. 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 “sacrificial” coating to “barrier” coating and in some cases, both are used to maximize corrosion protection.
Sacrificial coatings react with the corrosive elements and, as the name implies, sacrifice themselves to protect the base material. Eventually the sacrificial coating will be depleted exposing the base material and corrosion will quickly consume the steel. Depending on the corrosiveness of the environment depletion of the sacrificial layer will occur at different rates.
In contrast, barrier coatings form a protective layer between the potential corrosive elements and the steel. Since these coatings are not consumed, they typically provide longer corrosion resistance and longer fastener life.
All corrosion needs a catalyst. For roofing that catalyst is water. Leaks, condensation and reroofing over wet insulation and decks can introduce and trap moisture and lead to the potential risk of accelerated corrosion. While the fasteners may be protected, the deck may be vulnerable. The potential for problems varies with deck type. Bottom line: existing materials that are wet must always be removed.
High performance roofing fasteners are a critical component of nearly every roofing system on the market. Proper fastener selection is imperative if the system is to perform to its full potential. It is important to work closely with the roof cover manufacturer to make sure the right fastener and stress plate is being used for the assembly. It is also important to follow all the manufacturer’s specific fastening patterns and installation procedures.
There are several constructions that have very high-wind ratings if installed in accordance with strict guidelines, including deck attachment and preparation. Don’t be intimidated, work with the cover manufacturer to help with proper selection and field support.
As the Technical Services Manager for OMG Roofing Products, Stephen Childs leads the team that manages all technical services, including codes and approvals, pull tests, application reviews, warranty support and technical training. In addition, he serves as OMG’s technical representative to various industry associations and groups. Stephen is a member of the National Roofing Contractors Association (NRCA), the Single-Ply Roofing Industry (SPRI) and the International Institute of Building Enclosure Consultants (IIBEC) and holds a Bachelor’s degree in Mechanical Engineering from the University of Massachusetts, Amherst, Mass.
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