A coated steel roof or wall is a corrosion system, and the fastener is the one component that deliberately penetrates it. Every fixing is a hole in the barrier, a small piece of a different alloy, and a place where water collects. When a coated building fails early, the evidence is usually a ring of rust around each screw rather than a failure of the sheet itself.

Why the Fastener Is the Weak Point

The sheet lasts because the coating is continuous. A fixing interrupts that continuity twice: it removes coating where it drills, and it introduces a second metal. Two failure modes follow: the washer may not seal, or the screw may pull over the sheet under wind load. Electrochemically, the fastener and the coating form a galvanic couple wherever an electrolyte — rain, condensation, chloride — bridges them. Neither mode is visible at handover.

The Galvanic Rule: Area Ratio Decides Who Is Eaten

Two metals in contact in an electrolyte form a battery. The more active metal, the anode, corrodes, and the rate depends heavily on the exposed area ratio: a small anode against a large cathode is the worst case, because the whole current is generated by a small piece of metal. Apply that to a roof. A stainless screw in a Z275 galvanized sheet or an AZ150 aluminium-zinc sheet is a small cathode against a large anode, so the coating around the screw — not the screw — is consumed. The result is a de-zinced ring around every fixing, first grey staining, then red rust, and it appears fastest in chloride. Reverse the assembly and the problem disappears: galvanized sheet with hot-dip galvanized or zinc-flake coated carbon steel screws puts both parts on the same side of the series, so there is no driving voltage, and a scratch in the coating still protects itself. The same logic explains why whole structures are galvanized: the hot dip galvanized H-beam and ASTM A53 galvanized pipe are specified for their sacrificial behaviour at cut ends and damaged areas.

Where Stainless Belongs, and What It Changes

Stainless earns its place where the sheet is stainless too, or where the environment exceeds what any metallic coating survives. Type 316 and 316L carry 2-3% molybdenum against none in 304, and the same page records that 316L is preferred in corrosive environments, at 530 MPa tensile and 200 MPa yield against 600 and 210 for 304. Type 304 lists fasteners among its own applications, and 304 and 304L are described as screws, nuts and bolts material with the 1.4301 and 1.4307 designations. 304/304L coil and strip covers 0.3-22 mm in 2B, BA, No.1, No.4 and HL finishes, and 316/316L coil runs 3-10 t per coil at ±1% with a 1 ton minimum. Where chlorides are severe, the duplex family takes over: 2205 duplex pipe is rated 550 MPa yield and 800 MPa tensile with 15% elongation, super duplex 2507 reaches 116 ksi tensile and 80 ksi minimum yield but is limited to below 316 °C, and duplex coil and strip covers 2205 and 2507 in the same thickness range as the austenitic grades. If a stainless fixing is unavoidable on coated sheet, isolate it — an EPDM-bonded washer under the head and a sealing washer over the hole, or a nylon or neoprene cup — so the electrolyte path between the two metals is broken. Prefer A4 (316) to A2 (304) in chloride, and keep the exposed coated area around each fixing as large as practical.

Fixings That Should Not Sit on Coated Carbon Steel

Copper, brass and bronze sit at the cathodic end of the series for construction metals, so a copper fixing in galvanized or aluminium-zinc sheet makes the sheet the anode and the corrosion is both fast and local. Copper carried in run-off does the same thing from a distance: water that has crossed a copper roof attacks zinc downstream. Monel 400 (UNS N04400, 63% nickel minimum with 28-34% copper, 480 MPa tensile, 170 MPa yield) is an excellent seawater material — its own application list names marine fixtures and fasteners — and it is also strongly cathodic to any zinc-based coating, which makes it the right fixing for a Monel or nickel-alloy assembly and the wrong one for a galvanized panel. Grade 2 titanium is more extreme: 99.2% titanium minimum, 345 MPa tensile, 4.51 g/cm³ and a seawater corrosion rate below 0.005 mm per year, one twentieth of 316L — which is exactly why it should not be bolted through coated carbon steel sheet. Stainless wire rope in grades 304 and 316, from 6x19 to 6x37 in 0.8-21.4 mm, belongs to stainless or non-ferrous fittings.

Details That Matter More Than the Alloy

Washer compression decides whether the joint seals, and it has a narrow window. An impact driver without a torque clutch over-drives the screw, dishes the washer and splits it; a driver set too low leaves the washer proud and the joint loose. Use depth-sensing or torque-limited tools and check a sample. Screw point and thread must suit the whole stack — sheet plus purlin — because a self-drilling screw that only just pierces the sheet will strip under wind load. Fix into the crest or the valley, but the same one everywhere on a given profile and in line with the drawing for corrugated aluzinc sheet, aluzinc roofing or prepainted galvalume corrugated roof; alternating positions in the field is a leak path. Long panels move thermally: round holes at end zones and slotted holes in the field are a design decision, not a site improvisation. Clear the swarf: filings left on the coating hold water and rust within days, causing more "the sheet rusted in a month" complaints than any material choice. Touch up every cut and drilled edge. On a steel structure frame, Q235B or Q355B welded H sections are joined by welds, bolts or rivets, then galvanized or painted. The coating itself sets the environment budget the fixings must match: galvalume reflects up to 80% of sunlight and is usable to 315 °C against 230 °C for galvanized, and its service-life order runs rural, industrial, general marine, severe marine, the same ladder a fastener supplier works from on galvalume roofing sheet and galvalume against galvanized.

A Fixing Schedule a Supplier Can Quote Against

• Sheet substrate and coating designation — Z80 or Z275, AZ30 or AZ150, or an anti-fingerprint finish.
• Corrosivity band, so the coating and the fastener are chosen on the same basis.
• Fastener base material and coating class, matched to the sheet where possible.
• Head, washer and sealing compound, with the compression or torque setting.
• Screw point and thread for the sheet-plus-purlin stack, not the sheet alone.
• Fixing pattern and spacing, including end zones and any slotted holes.
• Isolation detail where stainless or aluminium-zinc sheet meets an incompatible alloy, and a touch-up specification for cut edges.

None of this requires exotic material. It requires the coating, the fastener and the drainage path to be chosen as one assembly, and the fixing detail to be drawn before the first screw is driven.