The cheapest components in a rooftop solar system are also its most numerous corrosion points. A 100 kW metal-roof install can put over a thousand self-drilling screws through the roof skin — and every one of them is a small hole in the building envelope, sealed by a washer and protected by a few microns of coating. When screws fail, they fail everywhere at once. This article explains coating classes, how they map to real environments, and what test evidence is worth asking for.
Table of Contents
- What AS 3566 coating classes mean
- Coating class vs environment: expected service life
- Our salt-spray proof point
- Salt spray is a quality check, not a life prediction
- Practical selection rules
- Specification wording
- Quick FAQ
What AS 3566 coating classes mean
AS 3566.2 (Self-drilling screws for the building and construction industries — corrosion resistance) is the reference most metal-roof markets lean on, directly or indirectly. It grades screw corrosion protection into classes, with Class 3 and Class 4 the ones that matter for roofing: Class 3 for general external exposure, Class 4 for severe (marine-influenced) environments. Manufacturers achieve the classes with proprietary mechanically-plated or polymer-composite coatings over the screw substrate; some publish an in-house “Class 5” tier above the standard for the harshest exposures.
The substrate matters too. Roofing screws are commonly case-hardened carbon steel or, for the bimetal type we use in our coated screw range, a hardened SUS410 stainless body that keeps drilling capability while the coating system carries the corrosion duty.
Coating class vs environment: expected service life
Coating manufacturers publish service-life guidance by ISO 9223 corrosivity category. The figures below are from the published guidance of Kelash, the coating system used on our roof screws — useful for comparing classes, provided you treat them as manufacturer guidance rather than a warranty:
| Environment (ISO category) | Class 3 | Class 4 | Class 5 |
|---|---|---|---|
| C1–C2 — rural / dry inland | ≈40 yr | ≈60 yr | ≈75 yr |
| C3 — mild marine / urban coastal fringe | ≈7 yr | ≈10 yr | ≈13 yr |
| C4 — heavy marine / surf coast | — | ≈10 yr | higher, project-specific |
Two things jump out of that table. First, environment dominates: the same Class 3 screw that lasts four decades inland is a seven-year part near the coast. Second, in C3 and worse, screw coating class becomes a genuine design decision — not a purchasing afterthought — and in C4–C5 the conversation should include stainless-substrate screws and isolation detailing, not just a thicker coating.
Our salt-spray proof point
Guidance tables are the start; test reports are the evidence. Our Kelash-coated SUS410 roofing screws (6.3-10×80) were tested by SGS to ASTM B117 neutral salt spray for 1,000 hours with no red rust on significant surfaces (white corrosion products present, as expected for zinc-based systems). The report is downloadable with our other test documentation.
Salt spray is a quality check, not a life prediction
A number like “1,000 h NSS” is often misread as a lifetime. It is not. Neutral salt spray is a standardized, accelerated, comparative test: it verifies coating quality and consistency, and it catches bad batches. It does not replicate real atmospheres — no UV, no wet-dry cycling, no pollutants — so it cannot be converted into years of service. Galvanizing industry bodies make the same point about zinc coatings generally: rank products by NSS for quality control, but predict life from environmental corrosion rates and coating thickness.
That is why our procurement logic pairs both: a class/thickness specification matched to the site’s corrosivity category for life, and an NSS acceptance target for quality — the approach documented in the fastener chapter of our Corrosion Protection Guide (PDF).
Practical selection rules
- Classify the site first (ISO 9223 C-category, considering distance to surf, prevailing wind, industrial pollution) — then pick the class, never the reverse.
- C1–C2: Class 3 is comfortable. C3: Class 4 as the default; Class 3 only with a maintenance plan. C4 and beyond: Class 4/5 with stainless substrate preferred; review fastener strategy alongside the structure’s own corrosion spec.
- Match the washer: an EPDM-bonded sealing washer is part of the corrosion system — it keeps the cut edge of the roof sheet and the screw shank dry.
- Housekeeping matters: sweep drilling swarf off the roof daily. Carbon-steel swarf rusting on a coated roof (or embedding near stainless) causes staining and local attack no screw coating can prevent.
- Do not judge screws by shine or magnet — coating systems vary in appearance, and hardened stainless substrates are magnetic by design (see our note on stainless and magnetism).
Specification wording
Wording that keeps supplier quotes comparable and enforceable:
“Self-drilling roofing screws shall meet AS 3566.2 Class [3 / 4] corrosion resistance. Screws shall withstand [500 / 1,000] hours neutral salt spray per ASTM B117 / ISO 9227 with no red rust on significant surfaces; supplier to provide third-party test report. Screws shall be supplied with EPDM-bonded sealing washers. Substrate, coating system and installation torque limits to be stated in the supplier’s datasheet.”
Quick FAQ
Q: Are stainless screws always the safer choice?
A: Near the coast, a stainless-substrate screw solves red rust but introduces its own considerations — drilling capability, thread galling, and galvanic contact with the roof sheet, which needs isolation via the washer and sensible detailing.
Q: Our roof sheet is aluminium-zinc coated steel. Does screw choice change?
A: The screw must be compatible with the sheet coating — swarf and dissimilar-metal contact are the usual problems. Follow the sheet manufacturer’s fastener compatibility guidance and keep EPDM isolation in the joint.
Q: Can we mix screw classes on one building?
A: Technically yes (e.g. higher class on the weather side), but on most projects the cost difference is too small to justify the stock-management risk of mixing. Specify one class, sized for the worst face.