What the corrosion classes are
AS 3566 is the Australian Standard covering self-drilling screws for building construction. It rates the corrosion resistance of a screw's protective coating in classes — Class 1 through Class 4 — with each step up rated for a harsher environment. The class tells you how well the coating stands up to moisture, salt and airborne contaminants over time. It is not a strength rating and it says nothing about the screw's structural capacity — it is purely about how long the fastener resists corrosion in a given exposure.
| Class | Environment | Typical application |
|---|---|---|
| Class 1 | Mild interior | Interior lining, cabinetry — zinc plated is acceptable |
| Class 2 | Mild exterior | Inland residential, more than 50km from the coast |
| Class 3 | Moderate exterior | Coastal residential, roughly 1–10km from the surf |
| Class 4 | Severe exterior | Direct marine exposure, within about 1km of the surf, industrial areas |
| Stainless | High to extreme | Where exposure goes beyond Class 4 territory — 304 for high-corrosion coastal work, 316 for direct marine and pool surrounds |
Why it matters on the Sunshine Coast
Almost every job on the Coast lands in Class 3 territory or worse. Beachfront suburbs — Mooloolaba, Kings Beach, Caloundra, Noosa Heads, Coolum Beach, Mudjimba, Marcoola, Twin Waters — sit in direct marine exposure, within a kilometre of the surf. Most of Maroochydore, Buderim, Sippy Downs, Caloundra West and Pelican Waters sit in the moderate coastal band. Only once you're properly inland — Nambour, Maleny, Montville, Palmwoods, Landsborough — does the exposure genuinely ease off.
The National Construction Code carries durability requirements for coastal zones through its roof and wall cladding provisions, and fastener selection flows directly from them. Underspec the fastener and the coating breaks down years before the cladding it's holding — red rust streaks, seized heads, and rework nobody budgeted for.
How to choose
- Within 1km of the surf: Class 4 minimum. Stainless is the stronger call, especially on exposed fixings.
- Within 10km of the surf: Class 3 minimum.
- Sunshine Coast general rule: Class 3 is the safe default for most exterior applications. If in doubt, don't go below it.
- Pool fencing and barriers: 316 stainless is the spec we work to — chlorinated water is one of the harshest environments a fastener sees.
- Mixed metals: the class rating only covers the coating. If you're fixing stainless into galvanised steel, or aluminium into anything, check galvanic compatibility as a separate step — see our galvanic corrosion guide.
Common mistakes
- Zinc plated outside. Zinc plated screws are an interior product. Used externally on the Coast they can show red rust within months.
- Judging by distance alone. Microclimates matter — estuaries, canal frontage and tidal reaches can be far more corrosive than the straight-line distance to the surf suggests.
- One class for the whole job. A sheltered soffit and an exposed ridge fixing on the same house can justify different specs. Rate each fixing for its own exposure.
- Fastener below the sheeting spec. Roofing and cladding manufacturers publish minimum fastener requirements for their products. Fix with less and the screw becomes the weak point of a system that was otherwise specced correctly.
- No paper trail. When the class you fixed with isn't documented, you can't demonstrate what went in. Keep the spec with your job records.
Related guides
For the practical Class 3 versus Class 4 decision on a specific job, read Class 3 vs Class 4 Screws. If the job has stepped beyond coated fasteners into stainless, 304 vs 316 Stainless Steel covers the grade call.
How we handle it
Spec it once, spec it right. Tell us the suburb and the application and we'll match the corrosion class at the counter, check galvanic compatibility, and put the spec in writing on request so you can file it with your QA records. Final selection for a specific job always rests with the builder, engineer or certifier — our job is to give you accurate information and the correct product options to choose from.