Roofs flying off under normal conditions isn’t unheard of. More often than not, the underlying error is credited to improper fastener use with exposed fastener roofing. Proper fastener use goes a long way in maintaining structural integrity, weather tightness, and preventing collapse.
In this article, you will understand the roles of fastener spacing, fastener patterns, PB leg, and side lap sealant in proper fastener use.
Fastener spacing is arguably the most critical factor to consider when installing. Fastener spacing directly affects the roof's load-bearing capacity and determines the roof's weather resistance to environmental stress, such as wind and snow loads.
Tighter spacing configurations typically confer more strength, but in real-time applications, simply having smaller spacing distances might not be the most economical or efficient choice.
It’s important to realize that many different variables, from building shape to location, all impact the appropriate fastener spacing for your project.
The thickness, or gauge, of the roofing panel directly impacts fastener spacing. For example, lighter gauge panels have less available strength to resist bending and, therefore, require more fasteners to prevent buckling.
Different substrates also provide different physical properties that may ultimately affect the fastener pullout strength and spacing. You may encounter structural purlins, wood, or even concrete as a substrate for your metal roofing attachment. As expected, steel purlins tend to have a firmer grip than other materials and provide a firm anchor point, allowing for wider fastener spacing.
Roof elements such as roof slope and shape affect wind uplift and snow loads. In turn, wind uplift and snow loads directly impact the recommended fastener spacing. It is important to know if your roof is a gable, hip, single-sloped, or curved shape. Roof slope, also known as pitch, also directly impacts fastener spacings due to differences in the way environmental forces, like wind, react to low versus high-sloped roofing systems. This is especially true for areas that experience greater uplift, such as eaves and ridges.
Higher positions suffer from increased wind interactions. Consequently, you should adjust accordingly when carrying out fastener installation on higher eaves, as they will require more frequent fastener spacing.
The geographical location of an area affects the prevailing weather conditions. Mountainous locations may be affected by higher wind speeds and snow loads, while coastal locations may be susceptible to hurricanes and sea breezes. Therefore, you should account for the location of your project before installing exposed fastened panels.
Often, you will need to consult a licensed structural engineer to accurately calculate spacing to avoid legal liability for structural failure or panel blow-off.
Your manufacturer will provide section properties and load tables with information on how much pressure a panel can withstand under certain load conditions. As a contractor, you should use these resources, along with your engineer, to determine whether to increase or decrease fastener spacing.
Fastener patterns refer to the unique placement of fasteners on the metal roofing and metal siding to achieve the required capacity to withstand pull-out forces and confer diaphragm strength and weather tightness. The correct fastener pattern could be the make-or-break factor under wind, snow, and rain loads.
It is common practice to use combinations of fastener patterns at different roof areas are exposed to various forces. Combining patterns aids in ensuring that panels perform as expected, ensuring the edge conditions are sealed up and compressing the sealants and closures so that the system retains its integrity against water infiltration.
Framing Fastener at Panel Ends: The framing fastener is responsible for the primary attachment of the roof panels to the roof substructure. The location, number, and spacing of framing fasteners vary based on the specific panel style and manufacturer’s recommendation.
Many manufacturers offer what is referred to as a purlin bearing leg on their R-panel type products. A purlin- bearing Leg (PB) is an extended leg or lip on one side of the panel that rests on the roof purlin to provide additional support and structural integrity and improve the panel’s overlap. During fastener installation, the PB leg evenly distributes weight across the purlins, reducing deflection and allowing for a secure connection.
Additionally, during fastener installation, the PB leg prevents the leading edge rib of a metal panel from bending or shifting downward when installing a fastener (ducking out) by keeping it aligned and steady.
Stitch Fasteners: In contrast to the framing fasteners, the primary purpose of stitch fasteners is to attach two pieces of metal to themselves without penetrating the substructure. Panel sidelaps and conditions are both common locations for stitch fasteners.
Due to capillary action, It is customary to use a sealant at exposed fastened panel sidelaps for slopes below 3:12 to reduce leak potential.
If you’ve ever held two microscope slides closely with a bit of moisture between them, you notice that water tends to rise between the two surfaces. This is due to capillary action. Similarly, this phenomenon causes water to infiltrate the side lap between panels in construction.
The diagram below for our R-Panel and Multi-Rib panels is an example of the manufacturer’s recommended fastener spacing. It’s critical to request this type of information from your panel manufacturer before beginning installation.
When selecting a fastener pattern, it is essential to follow manufacturer specifications to avoid compromising weather tightness or installation failures. It also helps to remain within warranty specifications.
Proper fastener spacing, patterns, and additional components such as a PBR leg and side lap sealant all contribute to ensuring your roof installation project proceeds without the threat of structural failure or panel blow-off. Ultimately, you should refer to manufacturer specifications and consult certified professionals for the best results.