Roofs can face serious problems when fasteners are installed improperly, especially in high winds. Improper fastener use with exposed fastener roofing can compromise the roof's performance. Proper fastener use goes a long way in maintaining structural integrity and weathertightness.
In this article, you'll learn about the roles of fastener spacing, fastener patterns, purlin bearing (PB) legs, and sidelap sealant in proper fastener use.
Fastener spacing is one of the most critical factors to consider when installing exposed fastener metal roofing. It directly affects the roof's load-bearing capacity and resistance to environmental stresses, such as wind and snow loads.
Tighter spacing typically provides greater strength, but simply reducing fastener spacing may not be the most economical or efficient choice. Always follow the fastening requirements for the specific panel and project conditions.
Many different variables, from building shape to geographic location, 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, which can affect fastening requirements.
Different roof attachment materials also have different physical properties that may affect fastener pullout strength and spacing. You may encounter structural steel purlins, wood framing, or even concrete as a supporting surface for metal roofing. Each material provides different fastening characteristics, so the appropriate spacing depends on the panel, attachment material, fastener type, and project requirements.
Roof elements such as slope and shape affect wind uplift and snow loads, which directly impact recommended fastener spacing. It's important to know whether your roof is gable, hip, single-sloped, or curved. Roof slope, also known as pitch, impacts fastener spacing due to differences in how environmental forces, such as wind, react to low- versus high-sloped roofing systems.
This is especially true for areas that experience greater wind uplift, such as roof edges and corners.
As eave height increases, wind pressures may also increase. Depending on the building's design and location, higher eaves may require closer fastener spacing.
Geographic location affects prevailing weather conditions. Mountainous locations may experience higher wind speeds and snow loads, while coastal locations may be susceptible to hurricanes and strong winds. Therefore, you should account for the location of your project before installing exposed fastener panels.
Depending on the project, you may need to consult a licensed structural engineer to accurately calculate fastener spacing and ensure the roofing system meets applicable structural requirements.
Your manufacturer can provide section properties and load tables that show how much pressure a panel can withstand under certain load conditions. As a contractor, you should use these resources, along with your engineer when necessary, to determine whether to increase or decrease fastener spacing.
Fastener patterns refer to how fasteners are placed on metal roofing and siding panels to meet required capacity to resist pullout forces, provide diaphragm strength where applicable, and maintain weathertightness. The correct fastener pattern plays an important role in how a roofing system performs under wind, snow, and rain loads.
It's common to use combinations of fastener patterns because different roof areas are exposed to varying forces. Combining patterns helps ensure panels perform as expected while securing panel edges and compressing sealants and closures to reduce the potential for water infiltration.
Framing fasteners at panel ends provide the primary attachment of roof panels to the roof substructure. The location, number, and spacing of framing fasteners vary based on the specific panel style and manufacturer's recommendations.
Many manufacturers offer what they call a purlin bearing leg on their R-Panel products. A purlin bearing (PB) leg is an extended leg or lip on one side of the panel that rests on the roof purlin to provide additional support and improve the panel's overlap.
During fastener installation, the PB leg helps prevent the leading edge rib of a metal panel from bending or shifting downward when installing a fastener, sometimes called "ducking out," by keeping it aligned and steady.
Unlike framing fasteners, stitch fasteners primarily attach two metal pieces without penetrating the substructure. Panel sidelaps are common locations for stitch fasteners.
Due to capillary action, it's customary to use sealant at exposed fastener panel sidelaps for slopes below 3:12 to reduce leak potential. However, specific sealant requirements depend on the panel system and manufacturer's installation instructions.
If you've ever held two microscope slides close together with a bit of moisture between them, you may have noticed that water tends to rise between the two surfaces. This is due to capillary action. Similarly, this phenomenon can cause water to infiltrate the sidelap between metal roofing panels.
The diagram below for our R-Panel and Multi-Rib panels is an example of the manufacturer's recommended fastener patterns. Request this type of information from your panel manufacturer before beginning installation.
When selecting a fastener pattern, it's essential to follow manufacturer specifications to maintain weathertightness, reduce the risk of installation failures, and comply with applicable warranty requirements.
Proper fastener spacing, patterns, and additional components such as a PB leg and sidelap sealant all contribute to the performance of your roof installation. Following the appropriate installation requirements helps reduce the risk of structural problems, water infiltration, and panel attachment failures.
Ultimately, you should refer to manufacturer specifications and consult qualified professionals for the best results.