Class 3 coating (AS 3566 fasteners)
Class 3 (AS 3566.2) is the corrosion classification for fasteners in moderate-exposure environments. Required for treated timber, wet areas, mild coastal.
Ask Chalkline about this →A Class 3 coating in residential construction refers to the corrosion resistance classification under AS 3566.2:2002 for self-drilling screws and other fasteners. The standard defines five classes (Class 1 through Class 5) of increasing corrosion resistance, each suited to a defined exposure environment. Class 3 is the moderate-exposure tier required for treated-timber fixings, wet-area work, and mild-coastal applications.
The AS 3566.2 corrosion classes:
| Class | Exposure | Typical coating |
|---|---|---|
| Class 1 | Internal dry | Standard zinc-plated (electroplated, 5-8 microns) |
| Class 2 | Internal damp, mild external | Mechanical zinc, HDG light (10-15 microns) |
| Class 3 | Treated timber, wet areas, mild coastal | HDG full (40-100 microns) or duplex zinc/aluminium coatings |
| Class 4 | Severe industrial, marine | Stainless 304 or specialised coating |
| Class 5 | Severe marine, splash zone | Stainless 316 |
A Class 3 coating typically means hot-dip galvanised (HDG) to a minimum 40-100 microns of zinc, or equivalent zinc/aluminium coating (e.g. Zincalume-coated steel in some applications).
Where Class 3 is required:
- Fastening into treated timber (H2, H3, H4): the copper and chromium actives in ACQ and CCA are aggressive on standard zinc-plated fasteners. Class 3 (HDG) is the standard residential requirement.
- Wet-area fixings: bathroom, laundry, toilet trim, where condensation and water spray reach the fastener. Class 3 minimum.
- Mild-coastal areas (within 5 km of moderate marine): residential roofing fasteners, fascia fixings.
- External non-marine work: cladding, fascia, gutter brackets in standard inland environments.
Where Class 2 is acceptable (lower spec): internal framing, dry roof framing, internal joinery in dry environments. Don’t substitute Class 2 for Class 3 in treated-timber applications; the actives will corrode through within years.
Where Class 4 or Class 5 is required:
- Severe marine: within ~1 km of breaking surf. Stainless 304 or 316.
- Pool decks and water-feature framing: continuous salt or chlorine exposure.
- High-end marine residential where 30+ year service life is expected.
Identifying Class 3 fasteners on site:
- Packaging label: “AS 3566 Class 3” or “Class 3 HDG” printed on the box.
- Head colour: HDG fasteners are silvery-grey (sometimes with a slight roughness from the dip); zinc-plated standard fasteners are smooth shiny silver.
- Manufacturer catalogue: spec class for each product line.
Common builder errors:
- Class 2 in ACQ-treated decking: rust streaks within years, joist-board bond fails as the screw corrodes.
- Class 3 in severe marine: works for ~10 years then fails; should have been stainless.
- Mixed classes on the same job: aesthetic continuity broken; inconsistent service life across the build.
- Confusing Class 3 with stainless: Class 3 is HDG zinc; stainless is Class 4+. Different metal, different cost (stainless typically 3-5x HDG cost).
For builders:
- Spec fastener class in the contract scope by application zone (internal, treated timber, external, coastal).
- Don’t buy cheaper standard zinc for treated-timber work. The 30-50% cost saving on screws translates to 10-20% rework cost on the building when the joist starts moving.
- Confirm packaging at delivery: a yard mix-up can put Class 1 or 2 in the bag labelled “for treated decking”. Read the box, not just the sticker.
Also known as: AS 3566 Class 3, Class 3 fastener, Type 3 corrosion class (informal).
Category: Materials / fixings / corrosion.
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Last updated: 2026-05-15. Verified: 2026-05-15.