Metal Roofing Screws: Types and Placement

On an exposed-fastener metal roof, every screw is a deliberate hole through your weather barrier — a typical house takes 1,500 to 3,000 of them. That makes metal roofing screws the most consequential cheap component on the entire roof: the panels routinely last 50 years, while bad screws or bad driving technique produce leaks in five. This guide covers the anatomy of a washered roofing screw, the size and length codes, the flat-versus-rib placement debate, how many you need per square, and the over-driving mistake that causes most callbacks.

Part of our Roofing guide — explore the full guide →

Anatomy of a Metal Roofing Screw

A proper metal-to-wood roofing screw has four working parts:

• Sealing washer: a galvanized or zinc-aluminum steel cap bonded to an EPDM rubber gasket. The steel cap controls compression so the rubber spreads into a gasket ring instead of squirting out. Cheap screws with bare neoprene washers and no steel cap are the ones that crack at year eight.
• Head: ¼-inch hex drive is the industry standard (a 5/16-inch hex head exists on #14s). Hex resists cam-out over thousands of drives; painted heads match panel colors.
• Thread and shank: coarse, high-thread designs for wood; fine self-drilling threads for steel purlins.
• Point: Type A sharp point for wood and thin sheet (up to ~26 gauge); Type 17 cutting point for wood-fast driving through panels; self-drilling “TEK” points (#1-#5) for steel framing — a TEK-2 handles up to about 1/8-inch steel, TEK-4/5 for heavier purlins.

Coatings matter for lifespan parity with the panel: standard is mechanically galvanized or proprietary multi-layer coatings (e.g., ZAC zinc-aluminum caps) rated 1,000+ hours salt spray. Stainless-capped screws are the spec for coastal work and for aluminum panels, where plain steel screws create galvanic corrosion spots.

Sizes and Lengths

Length rule: the screw must penetrate solid wood by at least 1 inch (three exposed threads past the underside is the shop rule). Practical picks:

• 1½-inch #10: through-panel flats into plywood deck or 1x purlins — the default residential screw.
• 2-inch #10: when fastening through the rib (adds ¾-inch rib height) or through panel + underlayment + battens.
• 2½-3-inch: ridge caps over closures at ribs, trim stacks, high ribs on R-panel.
• Stitch/lap screws (¼” × ⅞” with washer): panel-to-panel at sidelaps and trim, no framing penetration.

Flat or Rib? The Placement Debate

The oldest argument in metal roofing. Both camps are partly right:

Case for the flat (the majority and manufacturer position)

Screwing through the flat pan, adjacent to the rib, compresses the gasket against solid backing — panel tight to deck, washer seated evenly, no flex. Yes, the flat carries flowing water, but a properly compressed EPDM gasket seals a supported hole reliably for decades. Nearly all US panel manufacturers of exposed-fastener profiles (classic rib, R/PBR-panel) specify flat placement, on the deck side of each rib per their pattern. Follow the panel maker’s diagram — deviating can void the weather warranty.

Case for the rib

Rib-top screws sit above the waterline, so a marginal gasket drips less. But the screw spans a hollow crown: torque control is guesswork, the rib can crush or crack, the longer lever arm works the hole in wind uplift, and a slightly angled screw seals badly on the curved crown. Rib fastening survives mostly in agricultural through-fastened steel over open purlins — and even there, foam closures and correct length are critical.

Practical answer: put screws in the flat per the manufacturer pattern, drive them straight, and control torque. The screws that leak are almost never leaking because of flat placement — they’re leaking because of driving technique.

How Many Screws Per Square?

A “square” is 100 square feet. For typical 36-inch-coverage exposed-fastener panels:

• Field pattern: a screw line at every purlin/deck row, rows 18-24 inches apart, 4-5 screws per row per panel width. That’s roughly 80 screws per square as a planning number.
• Eave, ridge, and end laps: tighter spacing — every rib flat at the panel ends.
• High-wind zones: patterns tighten to 12-16-inch rows and edge zones get extra fasteners; coastal and engineered jobs follow the uplift design, not the brochure.
• Stitch screws: every 12-24 inches along sidelaps and trim.

Buying math: a 20-square roof needs ~1,600 field screws plus laps and trim — order 2,000 and keep the extras for the inevitable 15-year re-torque pass. Bags of 250 #10 × 1½-inch painted screws run $25-$45.

Over-Driving: The Leak Factory

The single biggest fastener failure is technique, not product. The EPDM washer has a narrow happy zone:

• Under-driven: gasket not compressed, visible gap under the washer edge — a slow leak and a screw that backs out.
• Correct: rubber just visible as a uniform ring squeezed slightly past the steel cap edge, screw square to the panel.
• Over-driven: washer dished, rubber extruded and split, panel dimpled — the gasket is destroyed and the dimple funnels water to the hole. This is the classic year-five leak.

Use an impact-rated screw gun with an adjustable clutch or a dedicated metal-roofing screwgun (2,000-2,500 RPM with depth control) — not a high-torque impact driver run to the trigger stop. Drive perpendicular; a screw leaning 10° seals on one side only. And never drive screws into wet panels over dusty purlins; grit under the washer cuts the gasket.

Maintenance: The Re-Torque Reality

Wood framing shrinks and panels cycle thermally, so exposed fasteners loosen over time. Walk the roof (or glass it from a ladder) every 3-5 years: snug any backed-out screws a quarter turn, and replace any screw with a cracked or weathered washer using the next diameter up — a #12 or #14 “re-screw” bites fresh wood in the enlarged hole. A 25-year-old through-fastened roof that’s been re-screwed once at year 12-15 is normal and healthy. This maintenance cycle is the real cost difference versus standing-seam’s concealed clips — weigh it when comparing systems in our metal roofing guide.

Mistakes to Avoid

• Plain steel screws on aluminum panels — galvanic corrosion halos within a couple of seasons. Use stainless-capped.
• Reusing holes with same-size screws during repairs — always upsize.
• Skipping foam closures at eave and ridge on ribbed panels; screws can’t seal a corrugation opening.
• Fastening panels tight at both ends of long runs — 40-foot panels move ~¼ inch with temperature; follow the manufacturer’s fixing scheme.
• Guessing at sidewall details — panel-to-wall joints are flashing problems, not screw problems; see our step flashing guide for how wall intersections shed water.
• Using roofing screws to hang gutters or accessories anywhere off-pattern — every hole is forever.

Quick Buying Checklist

• #10 diameter, length = stack thickness + 1 inch of wood bite.
• Steel-capped EPDM washer — reject bare-neoprene bargain bags.
• Coating rated 1,000+ hours salt spray; stainless caps at the coast or on aluminum.
• Painted heads matched to panel color; buy all bags from one dye lot.
• Order field screws + 15%, plus stitch screws for laps and a box of #12 oversize for future repairs.
• A screwgun with depth control — the $100 tool that protects 2,000 gaskets.

Bottom Line

Buy #10 screws with steel-capped EPDM washers and a coating warranty, drive them into the flat per the panel diagram, stop at a uniform rubber ring, and plan a re-torque pass mid-life. Do those four things and the humble screw disappears from your roof’s list of problems for decades.