In commercial construction, the right tool can make the difference between success and failure. The same is true in low-slope commercial roofing applications; the correct fastener can make the difference between a long-lasting installation or one that is plagued with constant problems and poor performance.
There are several commonly used deck types in commercial construction. Each has its own unique set of structural characteristics, and each requires specific fastener types, some of which are unique, in order to meet all of the needs of the roofing system.
Roofing fasteners have only been on the market since the mid 1970s when they were first used to attach insulation. However, there were not any fasteners specifically engineered for roofing applications until Factory Mutual created the need for them in the late 1970s. In those days, issues such as pullout, backout and corrosion resistance requirements were not understood and, consequently, fastener performance was not always up to the task. As fastener manufacturers began to work closely with roofing systems manufacturers, applying what they experienced in the real world and learned through testing at Factory Mutual, roofing fasteners became significantly more sophisticated, as well as deck and task specific.
Originally, steel and wood were the only deck types that were acceptable for mechanically attaching insulation or membrane. Structural concrete, lightweight insulating concrete, gypsum and cementitious wood fiber decks such as Tectum® had unique properties that prevented roofing components from being attached with traditional screws.
As fastener manufacturers started to see these opportunities in the roofing industry, they dedicated significant resources, designing specific fasteners to meet commercial roofing demands as they were understood at the time. At the same time, new fastener companies emerged that were solely dedicated to the commercial roofing industry, and several new fasteners were developed with improved performance on conventional roof decks as well as on decks that would not accept traditional screws.
Many innovative ideas and designs were developed as a result of this situation. Some became very successful and resulted in patented designs that are still in use; others, while interesting, never made it past the prototype stage.
Today, there is a wide range of fastener options available in the market for every application. Outlined below is a generic description of the most common decks and fastener options as they currently exist.
Without question, steel decks are the most commonly specified roofing substrate in the market. It’s available in several different configurations, gauges, and tensile strengths, all of which plays a major role in fastener performance. In fact, when a screw pulls out of a steel deck it’s the deck that fails, not the screw.
When Factory Mutual changed the wind uplift test frame from 5 feet by 9 feet to 12 feet by 24 feet for all mechanically attached systems over 4 feet wide and for fully adhered, built-up and modified bitumen systems that required wind ratings of over 90 psf, the limitations of many of the screws used in steel became clear. As a result, there are several types of screws specifically designed and engineered for steel deck applications today.
The insulation or membrane being attached determines which specific fastener is used in steel deck applications. “Standard” screws, often referred to as No. 12s, are typically used to attach insulation, while larger diameter screws (No. 15 and No. 21) are used to attach membrane. Larger diameter fasteners used for membrane attachment generally have a buttress tread design where the angles of the tread are not the same on the top and bottom plane and point configurations that maximize pullout and back-out resistance. (See Figure 1.)
Insulation attachment is less dynamic than membrane attachment, and insulation fracture is more likely to occur than fastener pullout so insulation screws are less sophisticated. However, in both membrane and insulation applications, all of the fasteners are used in conjunction with a stress plate designed for the specific application.
Most roofing system manufacturers have fastener programs and generally require their fasteners to be used to meet warranty requirements. Approvals, especially Factory Mutual approved systems, have minimum deck requirements, therefore the roof cover manufacturer must be consulted for specific deck and fastener requirements.
Dimensional lumber, plywood and OSB are all typical wood decks. Currently there are no roofing screws designed specifically for these materials. Instead, the common practice is to use steel deck screws, especially “standard” screws.
Fasteners with coarse thread designs generally provide the best pullout resistance in wood. Heavy-duty No. 14 or “all-purpose” fasteners are also available that can enhance pullout in wood decks. The pullout resistance of dimensional lumber is generally fairly good due to the thickness and density of the material. However, achieving consistent or adequate pullout values can be difficult in plywood and OSB, especially when it is less than three-quarters of an inch thick. This is due to the limited thread engagement as well as voids in the laminations. Where wind loads are dynamic, especially in mechanically attached single-ply applications, this can be a great concern. Fastener manufacturers continue to search for an affordable fastener design that will accommodate plywood and OSB decks.
Cementitious wood fiber (CWF) decks are not capable of holding a “traditional” screw. The wood fiber and cement construction are not dense or stable enough to hold small diameter shallow thread screws.
Many years ago, the common practice was to use quarter-inch diameter toggle bolts in long lengths with 4 inches of thread to attach insulation and membrane to these types of decks. While this was a good solution, it was highly labor intensive and not very economical.
As an alternative, fastener manufacturers developed large diameter, coarse thread, auger-type fasteners made from materials such as glass-filled nylon. These engineered fasteners were designed to accommodate the varied density of CWF decks and could often be installed without pre-drilling; however, the more dense CWF decks do require pre-drilling.
Some of these fasteners are made with anti-backout mechanisms for mechanically attaching membrane. In addition to glass-filled nylon fasteners, there are others on the market made of steel that look like monster screws.
Also common for CWF decks are non-traditional fasteners, particularly for attaching base sheets. These hammer-in or drive-type fasteners have barbs that extend into the deck after the fastener is seated and provide an excellent attachment method. Having a qualified technician perform pullout tests is a must with this type of deck.
As with CWF decks, gypsum decks will not hold a traditional fastener. While many heavy-duty fasteners have achieved relatively high initial pullout resistance values in gypsum decks, those values greatly diminish over time as insulation or roof cover movement works the screw back and forth in the setting. Gypsum decks are not resilient enough to take this movement and the integrity of the pullout resistance values drop dramatically.
Many large diameter, glass-filled nylon auger-type fasteners used for CWF decks can also be used in gypsum decks. Generally speaking, you have to pre-drill gypsum decks before these fasteners can be installed.
In addition, non-threaded drive-style fasteners can also be used to attach base sheets to gypsum decks. These fasteners are typically installed with a weighted “pogo” driver. As with CWF decks, you should always have a qualified technician perform pullout tests with this type of deck.
Depending on mix design and condition of the existing deck, density is all over the scale when it comes to lightweight insulating concrete (LWIC) decks. Material curing also affects both the amount of force needed to install a fastener as well as the pullout performance. In some cases, a drive type fastener typically with barbs can be used to attach insulation.
Using steel deck screws in these types of decks has also become fairly common, especially in high-wind applications. For this application the screws are driven through the LWIC into the steel form deck, which helps to hold the LWIC in a “sandwich” between the steel deck and the insulation board.
However, it is more common to attach a base sheet to this type of deck. Base sheet fasteners formed from light-gauge steel have been used to attach base sheets for over 20 years and have proven their worth in high-wind areas such as South Florida where LWIC is common. Insulation and 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.
Seldom are single-ply systems mechanically attached on LWIC unless there is a steel form deck under the LWIC that is capable of providing adequate pullout resistance or unless the membrane can be fastened directly to purlins. When attaching directly to purlins that are heavier than 18 gauge, special purlin fasteners with drill points and fine threads are required.
The auger-style fastener discussed earlier is not acceptable for use with lightweight insulating concrete decks. Additionally, these decks also require tests to determine the specific pullout values.
When it comes to structural concrete, roofing contractors have many different fastener options, as both insulation and membrane can be installed with strong pullout values.
Regardless of what fastener you select for the job, you must pre-drill the deck, so installation is labor intensive. Heavy-duty screws with a symmetrical thread design are common in this application. (See Figure 2.) Unlike the buttress thread, symmetrical threads have matched angles on the top and bottom of the thread and cut cleanly when properly heat treated and installed into a properly sized pre-drilled hole. Installing screws into structural concrete requires a good-quality, high-torque screw gun and a little more time and finesse but they can be backed out if necessary. As drill bits wear, the hole gets smaller and the fasteners become harder to install, so it’s imperative that the drill bit be replaced frequently on these projects.
However, drive-type fasteners have become a more popular choice for structural concrete decks. Concrete fasteners, as they are known, typically have a deformed shank that, when driven into a properly sized pre-drilled hole, creates an interference fit providing very high pullout resistance. While these drive-type fasteners are relatively easy to install, they are very difficult to remove. Regardless of whether you are installing a screw or hammer-in fastener, it’s important to pre-drill holes a minimum of a half-inch deeper than fastener embedment in order to accommodate the debris that falls back into the hole when the fastener is installed. Testing the pullout values on these decks is also a good idea in order to determine the proper hole size for maximum pullout performance and installation ease.
Adhesive fastening systems have also become common for installing insulation on structural concrete decks, as well as on some gypsum, lightweight insulating concrete and cementitious wood fiber decks. Material costs are higher but labor is significantly reduced.
Lastly, when it comes to re-roofing, it is important that the deck and insulation is dry. Any moisture trapped under a new roof will work to accelerate the corrosion process. Corrosion-resistant fastener coatings are highly engineered today. The industry has transitioned from “sacrificial” to “barrier” coatings. Sacrificial coatings react with corrosive elements and, as their name implies, sacrifice themselves to protect the base material. Eventually, however, the coating is depleted and the base material (i.e., the steel) is exposed to the moisture, which quickly consumes the steel. Depending on the corrosive, the sacrificial layer depletes at different rates. By contrast, barrier coatings form a protective layer between the potential corrosive and the steel. Since these coatings are not consumed, they provide longer corrosion resistance against more potential corrosives as well.
Fasteners are a key component of nearly every type of roofing system. Proper selection is imperative if the system is to perform to its full potential. It is very important to work closely with the roof cover manufacturer to make sure that you not only have the right fastener but that you also have the correct stress plate and that you are following their specific fastening patterns and installation procedures. Doing so will help to ensure that the roof performs as intended for years to come.