For metal roofing and cladding in Australia and New Zealand, screws are one of the smallest components on site – but they carry a big share of the risk. If fasteners corrode, leak or snap, replacing them on a finished roof is expensive and dangerous. For this reason, more and more builders insist on Class 4 screws instead of basic coatings, especially in coastal or high-exposure environments.
Class 4 is not a marketing phrase. It is a corrosion-resistance classification defined in AS 3566, the Australian standard for self-drilling screws used in the building and construction industries. Part 1 of the standard covers mechanical properties, and Part 2 specifies corrosion resistance classes from 1 to 4.store.standards.org.au+1
This article explains what Class 4 coating means in practice, how it compares with lower classes, why builders prefer Class 4 screws for roofing and cladding, and how overseas buyers can specify and source Class 4 screws correctly.
1. What Does “Class 4” Mean?
1.1 AS 3566 and corrosion classes
AS 3566 defines requirements for self-drilling screws used in steel and timber, including dimensions, mechanical properties, and performance tests.store.standards.org.au+1
Part 2 of the standard categorises corrosion protection into several classes (typically 1–4), with Class 4 being the highest level for carbon steel screws. Each class is associated with minimum performance in accelerated corrosion tests such as neutral salt spray and cyclic exposure, intended to match different atmospheric environments.
In simple terms:
- Lower classes (1–2) – suitable for mild or internal environments
- Class 3 – for moderate external exposure
- Class 4 – for severe and very severe environments, including many coastal and industrial locations
Guides from fastener and coating suppliers confirm that Class 4 is designed for longer service life in aggressive atmospheres compared with Class 3, particularly for roofing and cladding exposed to wind-driven rain and salt.ramset.com.hk+1
1.2 Where Class 4 is required
In New Zealand, building guidance notes that fixings for profiled metal roofing should comply with Class 4 of AS 3566 Part 2 as a minimum in many applications, reflecting the country’s harsh marine environment and long-term durability expectations.BRANZ Build+1
Roofing and cladding manufacturers also recommend Class 4 or better screws to match the life of modern coated steel products in high-corrosion zones. In practice, many AU/NZ builders now treat Class 4 screws as the default choice for external roofing and wall cladding, especially within a few kilometres of the coast.
2. How Class 4 Screws Differ From Class 3 Screws
2.1 Coating systems
Class 3 and Class 4 screws usually share similar steel cores and mechanical properties; the difference is mainly in the coating system:
- Class 3 – often uses a high-quality zinc-based coating with moderate thickness, suitable for many suburban and rural environments.
- Class 4 – uses thicker or more advanced coating systems (e.g. mechanical galvanising or multi-layer zinc-aluminium / organic systems) that deliver significantly longer corrosion resistance.ramset.com.hk+1
A simple way to think about it: Class 4 is “Class 3 plus extra protection”, designed so the screw remains sound even when exposed to constant moisture, salt spray and temperature cycling.
2.2 Expected service life
Exact lifetimes depend on environment and design, but industry guidance consistently positions Class 4 as the preferred option for:
- Coastal C4–C5 environments by ISO 9223 categories
- High-exposure buildings such as coastal houses, farm sheds near the coast, and industrial plants
- Roofs and walls using high-performance cladding systems where owners expect 20–30 years of service
BRANZ’s fastener selection documents and similar guides show that higher specification fasteners (including Class 4 and stainless steel) are recommended as exposure severity increases.d39d3mj7qio96p.cloudfront.net+1
In short, using Class 4 screws helps align fastener life with the life of modern roofing and cladding materials.
3. Why Builders Prefer Class 4 Screws on Site
3.1 Reduced call-backs and warranty issues
Corroded screws are one of the most visible symptoms of roofing failure. Customers notice rusty fastener heads long before they see corrosion on the sheets themselves. When cheaper screws fail early, builders face call-backs, warranty debates and reputational damage.
By specifying Class 4 screws from the start, builders reduce the risk that fasteners become the weak link. The initial cost difference between Class 3 and Class 4 is usually small compared with the labour cost of returning to replace fasteners several years later.
3.2 Better compatibility with modern cladding systems
Today’s roofing profiles and coated steel products (for example, high-build paint systems or zinc-aluminium alloy substrates) are designed for long life in demanding environments. Technical guides from roofing manufacturers emphasise the importance of using fasteners with compatible or higher corrosion resistance levels, often Class 4 or better, to avoid galvanic corrosion and staining.steelandtube.co.nz+1
Builders who consistently choose Class 4 screws are less likely to create mismatched systems where the screw coating fails before the roof sheet.
3.3 Flexibility across different job sites
Many contractors work across large regions that include inland, urban and coastal jobs. If they stock only Class 3 screws, they need strict rules to prevent these being used in high-risk locations. Stocking Class 4 as the default roofing screw simplifies logistics:
- One screw type covers more applications
- Less risk of the “wrong box” being used on a coastal job
- Easier training for staff and sub-contractors
This “one higher specification for all” philosophy is a major reason builders prefer Class 4 screws, even for some moderate environments where Class 3 might technically be acceptable.
4. Typical Applications for Class 4 Screws
Class 4 screws are most commonly specified for:
- Metal roofing and wall cladding in coastal or high-rainfall regions
- Buildings in industrial zones with airborne pollutants
- Exposed canopies, verandas, and fascia where fasteners are directly visible
- Agricultural sheds and farm buildings near the coast
- Cyclonic regions and high-wind areas where roofs are subject to severe weather
Many Class 4 products are self-drilling roofing screws with hex washer heads and bonded washers, used to fix metal sheets to timber or steel purlins. Others include Type 17 timber screws with mechanical galvanised or advanced coatings suitable for decks and external timber structures.Alibaba+1
On Linkworld’s side, similar applications are covered by products such as:
- AS 3566 Bugle Head Batten Screws for timber roof battens and cladding:
https://linkworldfast.com/product/as3566-bugle-head-batten-screw/ - T17 Ruspert Wood Screws for outdoor timber fixings where high corrosion resistance is needed:
https://linkworldfast.com/product/t17-ruspert-wood-screws/
Although not every screw will be formally tested to AS 3566 Class 4, these product types illustrate the style of fasteners builders use when they want high-performance coatings.
5. Key Features to Look For in Class 4 Screws
When you are sourcing or evaluating Class 4 screws for AU/NZ builders, pay attention to more than just the label.
5.1 Compliance with AS 3566.1 and AS 3566.2
Check that:
- The screw is designed as a self-drilling or self-tapping screw in line with AS 3566.1 (dimensions and mechanical properties).store.standards.org.au+1
- The coating system is tested against the requirements of AS 3566.2 for Class 4 corrosion resistance.steel.org.au+1
Ask suppliers to provide data or test reports summarising mechanical properties, drilling capacity, and corrosion tests. Even a short report showing salt spray hours and test method helps you compare products.
5.2 Coating type and colour
Class 4 performance can be achieved using different systems, for example:
- Mechanical galvanising with specific thickness
- Multi-layer zinc-aluminium flake plus organic topcoat
- Proprietary hybrid coatings developed by fastener manufacturersalzin.com.tw+1
You should confirm:
- Coating thickness range and typical salt spray test results
- Colour options (to match common roofing colours)
- Whether the coating is applied before or after drill point formation
For visible roofs and facades, colour-matched heads and washers are often important selling points for builders.
5.3 Washer and sealing system
For roofing screws, the washer is as critical as the screw itself:
- EPDM or similar high-grade sealing washers maintain elasticity over time
- The washer diameter and thickness must suit the roof profile
- Metal backing washers or pressure plates are needed for higher wind zones or fragile claddingsfixinz.co.nz+1
When comparing Class 4 products, look at the complete screw + washer assembly, not just the screw core.
5.4 Drill point and installation performance
Builders also care about how Class 4 screws behave on site:
- Drill capacity (maximum steel thickness for one-shot drilling)
- Consistent drilling time without burning coatings
- Good holding power in timber or steel purlins
- Stable drive performance with common tools
Products such as Type 17 timber screws, batten screws and concrete screws in Linkworld’s range are designed with specific drill points and threads to match their intended substrates:
https://linkworldfast.com/product/slotted-concrete-screw/
https://linkworldfast.com/product-category/screws/
6. Purchasing Tips for Overseas Buyers
6.1 Describe the environment and building type
When sending RFQs to your screw supplier, do not only write “Class 4 screws”. Provide context:
- Building type (residential roof, farm shed, school, commercial building, etc.)
- Distance from the coast and expected corrosion category
- Substrate (timber battens, steel purlins, aluminium framing)
- Roof and wall profiles being used
This helps the supplier select appropriate screw designs, washer combinations and coatings.
6.2 Specify standards and testing
In your specification or purchase order, include:
- “Self-drilling screws for roofing to AS 3566.1, corrosion resistance Class 4 to AS 3566.2”
- Required screw sizes, head types and drive types
- Coating description (e.g. mechanical galvanised Class 4, zinc-aluminium/organic multilayer Class 4)
- Any additional tests you expect (for example, batch salt spray samples or drill performance tests)
The clearer your requirements, the easier it is for factories and QC teams to control quality.
6.3 Pay attention to packing and labelling
Builders often store screws on site for extended periods. To help them manage Class 4 products correctly, you can request:
- Clear labelling on cartons and small boxes indicating “Class 4” and intended application
- Moisture-resistant packaging where possible
- Distinct colours or artwork to differentiate Class 4 boxes from general-purpose screws
From the supplier side, Linkworld can combine Class 4-type roofing screws, timber screws, concrete fasteners and related hardware into mixed shipments with small inner packing for retail or trade sales. A general overview of capabilities is available here:
https://linkworldfast.com/products/
7. How Linkworld Supports Class 4-Type Screw Projects
Jiaxing Linkworld Fastener has in-house cold forming, thread rolling and machining, combined with partner workshops for heat treatment, coating and assembly. This structure helps support projects that require higher-performance coatings and more complex assemblies.
For Class 4-type applications, Linkworld can:
- Produce self-drilling and self-tapping screws according to AS 3566 design principles for metal and timber fixing
- Coordinate mechanical galvanising or advanced multi-layer coatings through experienced surface-treatment partners
- Supply matching accessories such as bonded washers, concrete screws, structural bolts and rigging components in one container
- Offer flexible small packing (branded boxes, plastic boxes, bags) for builders’ merchants and distributors
To learn more about the company and manufacturing capabilities, visit:
https://linkworldfast.com/about-us/
8. Summary and Next Steps
Class 4 screws have become a preferred choice for many builders in Australia and New Zealand because they offer:
- Higher corrosion resistance than lower classes, especially in coastal and industrial environments
- Better compatibility with modern roofing and cladding systems
- Reduced risk of premature fastener failure, call-backs and warranty disputes
- Flexibility for contractors who want one screw type that safely covers a wide range of job sites
For overseas buyers and distributors, the key steps are:
- Understand the basic requirements of AS 3566.1 and AS 3566.2 and what “Class 4” means in terms of corrosion performance.
- Specify screw designs, coatings, and washer systems that truly match local environments and roofing products.
- Communicate clearly with suppliers about standards, testing, and end-use conditions.
- Choose partners who can combine high-performance screws with other fasteners and provide consistent quality and packing.
If you would like to discuss Class 4 screw requirements, develop a tailored roofing fastener range for AU/NZ markets, or send drawings for customised screws, you are very welcome to contact the Linkworld team. Visit the homepage at
https://linkworldfast.com/
see the product overview at
https://linkworldfast.com/products/
or send an inquiry via
https://linkworldfast.com/contact/
or by email at info@linkworldfast.com.