Choosing the right screw for a metal roof sounds pretty straightforward until you start looking at the options. Several screws may look like they’ll do the job, but choosing the right one depends on the roof you’re working on and what you’re attaching the panel to.
Then there’s the gasket or washer under the screw head. On an exposed fastened roof, the washer helps seal the penetration through the panel, but not all washer designs perform the same way. The fastener and washer both need to be appropriate for the roof you’re working on. What you do need to know is which fastener assembly is right for the job and why.
Fasteners can look pretty similar when they’re sitting next to each other, but that doesn’t mean they’re interchangeable. The point, threads, diameter, length, and other features are designed for particular applications. The right fastener depends on the panel, the substrate, and the attachment requirements of the roof system, which is why choosing a screw based on appearance or what you used on the last job can get you into trouble.
You also need to know whether you’re dealing with an exposed or concealed fastened roof system. On an exposed fastened roof, the screw penetrates the metal panel and attaches it to the deck or substructure below. The head of the screw and its sealing washer remain exposed to the weather, making that washer an important part of keeping water out around the penetration.
Most standing seam systems work differently. The panels are typically attached with clips and fasteners that are concealed beneath the finished roof, so you aren’t making the same type of exposed penetration through the panel. That difference matters when you’re talking about which fastener you need and whether a sealing washer is part of the assembly.
Start with where the screw is being used. A screw used to attach an exposed fastened panel isn’t the same screw you’d use to attach a clip in a standing seam roof system or for trim that will eventually be covered. If the screw is going through the exposed face of the panel, the washer becomes an important part of keeping water out around that hole.
Next, look at what you’re fastening into. Wood and metal require different fasteners, and when you’re fastening into metal, the thickness of the substrate matters too. That’s why simply looking for a “metal roofing screw” doesn’t tell you enough to choose the right one.
When you’re attaching an exposed fastened panel to wood, you need a metal-to-wood roofing fastener with a washer. SFS offers several fasteners for this type of application, including its Woodgrip™ screws.
The point of the screw doesn’t tell you everything you need to know. SFS also offers sharp-point screws without washers that can be used to attach clips in a standing seam roof system. They may be designed to fasten into wood, but that doesn’t make them the right screw to use through the exposed face of a roof panel.
When you’re attaching the panel to steel, you’re generally looking at a self-drilling fastener. The thickness of the steel becomes an important part of that selection because different drill points are designed to work with different substrate thicknesses.
This is one of those places where two screws can look very similar but still be designed for different jobs. The drill point, thread design, and length need to work with the steel you’re fastening into, so moving from one substrate thickness to another may also mean changing the fastener.
A longer or larger screw isn’t automatically a better screw. You need the fastener to provide the required attachment into the substrate, and that requirement depends on the connection you’re making. The panel installation details and fastener specifications should give you the information you need, rather than relying on a general rule about how long a metal roofing screw should be.
If you’re making a substitution, that’s also the time to stop and verify the specifications. A screw that looks similar or happens to be the same length may not have the same point, thread design, or performance characteristics as the fastener it is replacing.
On an exposed roofing fastener, the gasket, more commonly called the sealing washer, is part of the fastener assembly and helps seal the penetration through the metal panel. McElroy Metal fasteners are primarily manufactured by SFS and come with the appropriate sealing washer assembled with the screw, so you aren’t choosing those two components separately.
That washer matters because the fastener passes directly through the metal panel. Once you’ve made that penetration, the screw has to provide the attachment while the washer helps maintain the seal around it. Different fastener assemblies may use different washer designs depending on the fastener and application, which is another reason to choose the complete fastener assembly based on the connection you’re making.
Fastener material matters, especially when the screw head will remain exposed to the weather. One option you may come across is a ZAC® fastener from SFS. ZAC® fasteners have a carbon-steel core with a zinc-aluminum alloy cap over the head to provide additional corrosion resistance.
That cap is one of the biggest differences between a ZAC® fastener and a standard painted fastener head. SFS says the zinc-alloy cap will not develop red rust and is designed to resist UV degradation without chalking or fading like a painted head. That can make ZAC® fasteners worth looking at when long-term appearance and corrosion resistance are important to the project.
ZAC® isn’t one specific screw, though. SFS offers different ZAC® fasteners for different applications, including options for attaching metal panels to wood and different thicknesses of metal. The substrate and connection still determine which fastener you need; the ZAC® cap adds another level of protection to the fastener head.
You can have the right fastener in your hand and still create a problem if you don’t install it correctly. With exposed fasteners, the screw needs to go in straight so the sealing washer seats evenly against the panel. Driving the screw at an angle can prevent the washer from making even contact around the penetration.
You also have to watch how far you’re driving it. An underdriven fastener may not compress the washer enough, while an overdriven fastener can damage the washer or dimple the panel around the screw. You’re looking for proper compression of the sealing washer, not the tightest screw you can possibly install.
The tool you’re using can help with that consistency. SFS recommends a screw gun with a depth-sensing nose piece for many of the roofing fasteners McElroy Metal provides and does not recommend impact guns or hammer drills for those applications. A depth-sensing screw gun helps control how far the fastener is driven, making it easier to maintain consistent washer compression across the roof.
The exposed fastener installation FAQs show the difference between a properly installed fastener and one that’s underdriven or overdriven. Seeing the three side by side makes it much easier to recognize what you’re looking for during installation.
There isn’t one screw location or fastening pattern that applies to every exposed fastened metal roof. You’ve probably heard the rib-versus-flat debate, but giving one answer for every metal roofing panel ignores how individual systems are designed and tested.
Fastener location and spacing are part of the installation requirements for the specific panel. A fastening pattern you’ve used on one panel shouldn’t automatically carry over to another, even when the panels look similar. Use the product and installation information for the exposed fastened panel you’re installing rather than relying on a general rule.
Because exposed fasteners and their sealing washers remain outside the panel, they should be included when the roof is inspected. Look for fasteners that have backed out, damaged or deteriorated washers, corrosion, missing fasteners, or visible changes around the penetration that could affect the attachment or seal.
Metal panels expand and contract as temperatures change, and that movement can affect exposed fasteners over time. If something doesn’t look right, look at the entire connection before assuming the solution is simply a new screw. The condition of the panel, fastener, and substrate can all affect the right repair.
Start with the connection you’re making. What panel are you installing? Are you fastening into wood or metal? If it’s metal, how thick is it? What fastener does the panel specify, and are there project conditions that could affect your choice?
Once you have those answers, it’s much easier to narrow down the screw length, diameter, point type, material, and sealing washer you need. You’re not really choosing a screw and gasket as two separate components. You’re choosing a fastener assembly designed for that particular connection.
If you’re looking at two screws that seem almost identical, don’t assume they are interchangeable. Check the specifications before making the substitution. That’s a much easier question to answer before installation than after there’s a problem on the roof.