On an exposed-fastener metal roof, the screws are the difference between a 40-year roof and a leaky one. They're cheap, but choosing the wrong type, length or spacing — or driving them badly — causes most metal-roof leaks. Here's what you need to know.
What makes a metal roofing screw different
A proper metal roofing screw isn't a generic screw. It has three critical features:
- A bonded sealing washer — an EPDM rubber washer under a metal cap that compresses to create a watertight seal around the hole.
- A self-drilling or self-tapping point sized for the metal (and the wood or purlin beneath).
- A corrosion-resistant coating so the screw doesn't rust and stain the panel.
That rubber washer is the whole game: it's what keeps water out of the fastener hole. When washers dry out and crack with age, the roof starts to leak — which is why re-screwing is a normal part of an older metal roof's life.
Screw types
| Type | Use |
|---|---|
| Wood screws (Type A / AB) | Panels fastened into a wood deck or wood purlins. Most residential roofs. |
| Metal/self-drilling (Type 3) | Panels fastened into steel purlins or framing. Common on metal buildings. |
| Stitch screws | Short screws joining panel-to-panel at the overlap seams. |
What size and length
The most common diameter is #9 or #10, with #12 and #14 for heavier applications. Length is chosen so the screw fully penetrates the substrate with thread to spare — a common rule of thumb is that the screw should penetrate the wood by at least about an inch (and pass through steel purlins with a few threads showing). When in doubt, follow the panel manufacturer's spec.
How many screws and where
Spacing follows the manufacturer's pattern, but a typical exposed-fastener layout runs screws roughly every 12–24 inches along each support, with closer spacing at the eaves, ridges, edges and overlaps where wind uplift is highest. As a rough planning number, many installs use on the order of 80 screws per "square" (100 sq ft) — but always defer to the panel's installation guide and local code.
In the rib or the flat?
This is a long-running debate. Driving screws in the flat of the panel (against the deck) gives the most solid, leak-resistant seat and is what most manufacturers specify. Screwing in the rib can leave the washer unsupported and more prone to leaks. Follow the manufacturer's instructions for your specific panel.
Over- or under-driving screws. Too tight crushes and deforms the rubber washer; too loose leaves a gap. The washer should be compressed just enough to mushroom slightly and seal flat — not squeezed out the sides.
When to replace metal roofing screws
Exposed-fastener roofs typically need a re-screw at some point in their life — often around the 15–20 year mark — as the rubber washers age, or sooner if you see backed-out screws, rust streaks, or leaks at fastener points. Re-screwing with slightly larger-diameter screws restores the seal and can add many years. (Hidden-fastener standing seam roofs avoid this maintenance entirely — one reason they cost more.)
Use real metal roofing screws with intact sealing washers, the right length for your substrate, manufacturer spacing, driven snug (not crushed) in the flat of the panel. Get those right and an exposed-fastener metal roof performs for decades.
Seeing rust streaks or backed-out screws on a metal roof? Have it inspected before a fastener leak reaches the decking.
Frequently asked questions
What screws are used for metal roofing?
Metal roofing uses special screws with a bonded EPDM rubber sealing washer, a self-drilling or self-tapping point, and a corrosion-resistant coating. Wood-type screws go into wood decks or purlins; self-drilling metal screws go into steel framing; stitch screws join panel overlaps.
What size screws for a metal roof?
The most common diameters are #9 and #10, with #12 and #14 for heavier jobs. Length should fully penetrate the substrate with thread to spare — roughly an inch into wood. Always follow the panel manufacturer's specification.
Do metal roofing screws go in the flat or the rib?
Most manufacturers specify driving screws in the flat of the panel, where the washer seats firmly against the deck for the best seal. Screwing in the rib can leave the washer unsupported and more prone to leaks. Follow your panel's instructions.
How often do metal roof screws need replacing?
Exposed-fastener roofs often need a re-screw around 15–20 years as the rubber washers age, or sooner if you see backed-out screws, rust streaks or leaks. Hidden-fastener standing seam roofs avoid this maintenance.
How many screws per square for a metal roof?
It varies by panel and wind zone, but many installations use on the order of 80 screws per 100 sq ft, with tighter spacing at eaves, ridges, edges and overlaps. Always follow the manufacturer's pattern and local code.
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