Metal Roof Fasteners: Exposed, Concealed, Where They Fail

A metal roof is a few hundred square feet of steel held to a wooden deck by small pieces of hardware. Whether those fasteners sit in the weather or under the panel is the single biggest design difference between systems, and how they were driven is the single biggest install difference. Here is how to think about both.

Exposed fasteners

On an exposed-fastener roof, screws go through the face of the panel into the deck or purlin. Each one carries a rubber washer under its head, and that washer is the seal. The panel is not sealing the hole. The washer is.

Here's the tradeoff. It is the cheapest and fastest way to attach a metal panel, and it has been done for a century. But the washers sit in direct sun, and the panel expands and contracts under the screw head every day. The rubber hardens, cracks, and stops sealing. Somewhere around year ten to twelve the fasteners need inspecting, tightening, and in many cases replacing with a larger diameter screw, because the original hole has worked loose.

Claims about coated, capped, or "lifetime" fasteners do not change the mechanism. A stainless-capped head resists rust longer, which is worth having. It does not stop the washer from aging or the hole from wearing. I have yet to find an exposed fastener design that is impervious to the elements.

Concealed fasteners

On standing seam, the fasteners go through a clip that is hidden inside the seam, and the seam is folded over it. On an interlocking metal shingle, the fastener sits under the lap of the next shingle. Either way, no screw head is in the weather and no washer is sealing anything. The geometry of the joint does that.

The clip does one more job. It holds the panel to the deck while letting it slide as it heats and cools. A 30-foot panel moves roughly a quarter inch across a Midwest temperature swing. A fixed screw through the face fights that movement and the hole elongates. A clip allows it.

The two ways screws get driven wrong

This applies to exposed fasteners and to the screws through clips alike.

Overdriven. The washer is crushed and squeezed out past the head. It looks tight. It is already failing, because the rubber has been deformed past what it can recover from.

Underdriven. The washer never compresses and the seal never forms. Water gets under the head from day one.

The right drive compresses the washer until it just spreads slightly past the head, and a crew that does this all day sets the clutch on the driver for it. A crew that does asphalt on Monday and metal on Thursday often does not. This is one of the reasons certification matters: it is training on exactly this kind of detail.

Material and compatibility

Fasteners should be matched to the panel metal. Galvanized or zinc-aluminum-coated screws with a steel panel, stainless where the manufacturer calls for it, and never plain steel into aluminum. Dissimilar metals in contact with moisture corrode each other. The same rule applies to copper flashing against galvanized steel and to treated lumber against any panel.

Fasteners also have to be long enough to reach solid wood. A screw that bites only OSB at a seam between sheets holds until the first wind event.

What "long-life fastener" should mean

If a quote advertises long-life fasteners, ask what that means. A good answer names the coating, the head material, and the washer compound, and says what the manufacturer expects of them. A vague answer is a sticker.

Who should think about this

If you are choosing an exposed-fastener system because the budget is the binding constraint, go in knowing the fasteners are the maintenance. Plan to have them looked at every year or two and expect a fastener service around year ten to twelve. That is not a flaw in the system. It is the system.

If you want a roof you do not think about, the concealed-fastener systems exist precisely so that nothing sealing the roof is in the weather. They cost more, and this is a large part of what you are paying for.

Now you have the context to read a fastener spec for what it is.