Fasteners for Metal Roofing: Screws, Clips and Where Each Is Used
Exposed screws, hidden clips, stitch screws and deck fasteners all do different jobs. Getting the wrong one — or driving the right one badly — is the most common cause of leaks.
More metal roofs leak at fasteners than anywhere else, and it is rarely because the fastener was defective. It is because it was the wrong type for the application, or it was driven at the wrong angle, or to the wrong depth, or into something that could not hold it. Fasteners are the part of a metal roof that requires the most judgment per unit of material, and the part most often treated as though it requires none.
Exposed Fastener Screws
On exposed-fastener panels — agricultural profile, corrugated, ribbed panel — the screw goes through the face of the panel and is visible. It is a hex-head screw with a bonded washer, usually EPDM rubber vulcanized to a metal cap, and that washer is the entire waterproofing strategy for the hole the screw just made.
Driving depth is everything. The washer needs to be compressed just enough to spread slightly beyond the edge of the metal cap. Under-drive it and the washer never seals, leaving a hole with a rubber disc loosely covering it. Over-drive it and the washer squeezes out from under the cap, the rubber extrudes and tears, and the panel dimples into a small crater that holds water around the fastener. Both leak, and over-driving is by far the more common of the two because it happens in a fraction of a second with a high-torque driver.
- ✓Use a screw gun with an adjustable depth-sensitive nose or a well-set clutch. An impact driver has no depth control and will overdrive every screw it touches.
- ✓Drive perpendicular to the panel. A screw driven at an angle compresses one side of the washer and leaves the other side open.
- ✓In North American practice, exposed screws on ribbed panels are generally driven in the flat of the panel so the fastener pulls the panel down against its support. Follow the panel manufacturer's stated pattern.
- ✓Land every screw on solid structure. A screw into open sheathing between purlins holds nothing and will back out.
- ✓Match the fastener metallurgy to the panel. Coated carbon steel and stainless-capped screws are the common choices; putting bare galvanized fasteners against an aluminum panel invites galvanic corrosion.
Why Exposed Fasteners Eventually Need Attention
Exposed-fastener roofs are legitimate systems and they have their place, particularly on agricultural and utility buildings. But the fasteners are consumable in a way that hidden-clip systems are not. Every day the panel expands and contracts around a rigidly fixed screw, working the hole slightly larger. Sunlight and ozone slowly harden the rubber washer. Over years, holes elongate, washers go brittle, and screws back out. This is why an exposed-fastener roof gets inspected and re-screwed periodically, and it is the single biggest functional difference between that system and standing seam.
Standing Seam Clips
Standing seam moves the fastener out of the water. A clip sits on the deck at the panel seam, is screwed down to the sheathing, and its upstand is captured inside the seam when the adjoining panel is engaged and the seam is closed. The panel face is never penetrated in the field of the roof, so there is no hole to leak and no washer to age.
Fixed Clips
A fixed clip is a single piece of metal. Once the seam is closed over it, the panel is held rigidly at that point. Fixed clips are appropriate on short panel runs, where the total thermal movement over the length of the panel is small enough that nothing is stressed. Manufacturers publish the run length above which fixed clips are no longer acceptable, and that threshold is not a suggestion.
Floating Clips
A floating clip is two pieces: a base that screws to the deck, and a tab that engages the seam and slides within the base. The panel can lengthen and shorten while the base stays put. On longer runs this is mandatory. It is also why a standing seam panel is fixed at one point only — usually at the ridge or at the eave — so the movement has a defined direction and the panel is not fighting itself from both ends.
Clip Spacing and Uplift Zones
Clip spacing in the field of the roof comes from the manufacturer's tested assembly for the design wind load. What catches people out is that the spacing is not uniform across the roof. Wind uplift is far higher along the perimeter — the eaves and the rakes — and higher again at the corners, where flow separates over two edges at once. Those zones need tighter clip spacing, and a roof laid out with one spacing everywhere is under-fastened exactly where it will be tested first.
Deck Fasteners, Stitch Screws and Everything Else
The clip screw itself is usually a pancake-head or wafer-head wood screw chosen so that it seats flush and does not interfere with the panel. Nails are not an equivalent — the withdrawal resistance of a threaded fastener in sheathing is what the uplift calculation depends on. Stitch screws are short, small-diameter self-drilling screws with sealing washers used to join metal to metal: panel end laps, trim laps, closures, and ridge caps to panel ribs. They fasten sheet to sheet, not sheet to structure, and they are almost always paired with butyl in the joint they are closing.
Never mix incompatible metals in a wet assembly. Copper fasteners against a Galvalume panel, bare steel against aluminum, or a mismatched trim material against a panel will set up a galvanic cell that eats the less noble metal wherever water sits. On a roof, water sits at fasteners. Keep the fastener, the panel and the trim in the same metallurgical family.
Where Fasteners Should Not Be
There is an equally important list of places a fastener does not belong. Not through the face of a standing seam panel in the field of the roof. Not through the pan of a valley. Not through the visible face of a trim where a cleat could do the job. Not through a panel into a chimney curb or a wall, pinning a panel that needs to move. A useful rule for reviewing a detail: if a fastener is the only thing keeping the water out, redesign the detail so that geometry keeps the water out and the fastener only holds things in place.
Frequently Asked Questions
How often do exposed screws need to be replaced?
There is no fixed schedule — it depends on the fastener quality, the panel movement, the exposure, and how well they were driven originally. The practical approach is to inspect them periodically, look for backed-out screws, cracked or flattened washers, and rust around the heads, and replace as needed with a slightly larger diameter fastener where a hole has elongated.
Why are my screws backing out?
Usually one of three things: they were driven into sheathing rather than solid structure, they were overdriven so the threads stripped the wood, or the panel is long enough that thermal cycling has worked the holes oversized. All three are addressed by relocating or upsizing the fastener into sound material.
Can I use an impact driver to install a metal roof?
Not for gasketed fasteners. An impact driver delivers uncontrolled torque and will overdrive and destroy sealing washers. Use a screw gun with a depth-setting nose so every washer is compressed identically.
Do standing seam clips ever need maintenance?
No. They are inside the seam, protected from water and sunlight, and there is nothing to service. That inaccessibility is exactly why the deck they are screwed into, and the fasteners themselves, need to be right the first time.
Should the screws go in the flat or on the rib?
Both approaches exist in the industry. North American ribbed-panel practice generally puts the fastener in the flat so the panel is pulled tight to its support; crest fastening is used in some markets and requires a profiled spacer. The panel manufacturer's installation instructions decide it for their product, and mixing approaches on one roof is a mistake.
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