Standards which are relevant to Roof and wall cladding products.

There are basically four properties or metal roofing associated products that we are concerned about, which are covered by Standards, either joint New Zealand/Australian or Australian. There are several European (EN) or ISO standards which cover the same properties and are similar to AS or AS/NZS, but since metal roofing is such a major aspect of all types of building in New Zealand and Australia compared to Europe, and since we are in New Zealand, not Europe, we only use Australasian Standards.

Inadequate quality of any of these properties can cause failure, either sooner or later. These properties are –

Physical performance provided by a combination of -

• Thickness of the base steel;
• Strength of the base steel; and

Durability performance provided by the base and any coatings -

• Corrosion resistance/durability of the metallic base; and
• Durability of any paint coating, either pre- or post-forming.

Physical performance

Thickness (and other dimensions) of the base steel

In NZ, for any kind of dwelling (i.e., other than sheds), the NZBC requires a minimum thickness of 0.40 mm for roll-formed cladding. In fact, currently, 0.55 mm is more commonly used. The thickness determines the possible span and the ability to withstand imposed loads (and, of course, the cost). For metal tiles, the minimum thickness is 0.39 mm. 

The tolerances for this property are covered by AS/NZS 1365:1996 – Tolerances for flat-rolled steel products. This Standard also specifies tolerances for width, length, flatness, edge camber, and out-of-square compliance, all of which are necessary to roll-form cladding properly (or at all).

These properties for aluminium coil are covered by AS/NZS 1734:1997. Aluminium and aluminium alloys—Flat sheet, coiled sheet and plate.

 Strength of the metallic-coated steel

This property is assessed by the tensile strength of the material expressed as minimum mPa (megaPascals) GXXX to failure. It is measured by methods described in AS 1391:2020  – Metallic materials – Tensile testing at ambient temperature. Steel-based products used for cladding in New Zealand are either nominally G300 or G550 ("high tensile") mPa. G300 is used for forming tiles, curved roof sections, or where greater ductility is required.

Most steel used for roof and wall cladding in NZ is nominally G550, although it often has an actual strength of up to 700 MPa. This allows the use of thinner, cheaper sheet – 0.40 mm at G550, supposedly equal to 0.55 mm at G300.

Aluminium coil is softer and has lower tensile strength, as covered in AS/NZS 1734:1997.

Thickness and strength go together in determining the actual physical performance of the cladding, and failing to meet both thickness and strength standards can lead to premature wind uplift failure of the roof cladding and/or installation damage and foot traffic damage. Typically, these are specified together, e.g. 0.55mm G550 - "Product name".

Physical properties of finished products are covered in AS 1562.1:2018 – Design and Installation of Sheet Roof and Wall Cladding – Part 1: Metal. Actual performance testing of finished products for wind load resistance and trafficability is conducted in accordance with AS 4040.1 (Concentrated load) and .2 (Wind load), and the NZMRM test laboratory can test products to these standards or other methods and provide performance outcomes.  

Durability Performance

Corrosion resistance – for steel, this is achieved by applying a metallic coating to the base metal (or by using a metal with greater natural corrosion resistance, e.g., aluminium). 

The predominant material for roof and wall cladding, used in NZ and made here, is Zincalume®, which has a coating of aluminium and zinc (and, more recently, also magnesium). Product cannot legally be called Zincalume® if it is not coated in a licensed plant. Zincalume ® is made by NZ Steel and painted by both NZ Steel and Pacific Coil Coaters. The coating weight of Zincalume® made in NZ has been 150 g/m² or 200 g/m². The new painted magnesium-containing product is called COLORSTEEL Maxam, 150 g/m².

The corrosion resistance of this coating is achieved by a combination of the inert oxide coating formed on the aluminium (which makes up nearly 80% of the metallic coating by volume) and the sacrificial protection provided by the zinc content, now supplemented by additional edge protection from magnesium.

In New Zealand, we also see galvanised steel (with a zinc coating) and a product called "ZAM" (zinc-aluminium-magnesium), which Pacific Coil Coaters also paints. 

Metallic coatings and their properties are covered in AS 1397:2013 – Continuous hot-dip metallic coated steel sheet and strip – coatings of zinc and zinc alloyed with aluminium and magnesium. This designates steel that can be coated under the Standard and then lists six coatings with a one or two-letter code, and specifies acceptable coating weights to be used for external exposure -

• Zinc (Z) – which is what we call “Galvanised” – not now used for roofing
• Zinc coating as a zinc/iron allow (ZF) – also called Galvannealed
• Zinc/Aluminium (ZA) – also called “Galfan”, not used in New Zealand
• Zinc/Aluminium/Magnesium (ZM) – which is used in NZ under the name “ZAM”
• Aluminium/Zinc (AZ) – which we call in Australasia “Zincalume ®” (elsewhere “Galvalume or Aluzinc”)
• Aluminium/Zinc/Magnesium (AM) – launched in New Zealand recently

Some other imported products may have different names but will be one of the above.

Of these, the majority of roof and wall cladding, rainwater goods, and tiles in New Zealand are made from Zincalume ®, either painted or unpainted.

Lack of corrosion resistance can be caused by inadequate weight, inferior coating quality, or a coating not being what it is claimed to be, and any of these can cause premature corrosion failure even within the modest 15-year life required by the NZBC, well below market expectations.   Some BIEC-licensed overseas plants produce lower-metallic-coating-weight materials for less demanding applications, and such products should not be used for external applications, even if the actual coating material is correct. 

The coating weight of metallic coatings can be measured relatively easily by methods described in AS 1397, and this is a measure of the likely corrosion resistance. But the actual corrosion resistance of a coated product can also be confirmed by exposure to a corrosive environment, either natural or accelerated, as described in AS/NZS 2728:2013, below. 

Paint durability

The final property is paint quality. Much of the product used for roof and wall cladding and rainwater goods is coated with paint or other coatings, which enhance appearance and may be purchased specifically for that purpose. This property is the most difficult to measure, but the quickest to fail and the most noticeable when it does. The vast majority of failures attributed to painted steel products are paint failures, leading to very rapid visible fading, chalking, and colour change, and complaints, sometimes within as little as 12 months.

The joint Standard for coating performance is AS/NZS 2728:2013 - Prefinished/pre-painted products for internal/external building applications – Performance requirements. It has a number of performance requirements, but for paint colour durability, this only specifies 4 years' exposure at nominated sites and then assessment of colour change. This is, in any case, only an informative test in 2728, and no accelerated test is detailed, although these do exist as ASTMs.

There are two ways to assess paint performance – real-life exposure at a severe site (normally the commercial site at Allunga near Townsville or others listed in 2728) or accelerated UV exposure using one of the accepted standard methods, e.g. a QUV cabinet and one of several test protocols.

AS/NZS 2728 also includes normative corrosion testing of painted products, in real or accelerated environments.

Other standards

In addition to Standards, published by Standards organisations, there are other published composition or performance requirements. Unlike Standards which need to be purchased, these other standards are usually free to download.

NZ Building Code

While adoption of Standards may not be mandatory, compliance with the NZBC is required for most buildings. The controlling entity, MBIE (Ministry of Business, Innovation and Employment) – which now covers Building controls and Standards New Zealand- issues methods of compliance called Acceptable Solutions, aligning with which is deemed to comply with the NZBC. Clause E2 of the NZBC, which covers the external envelope, including roof and wall cladding, has a massive AS E2/AS1, which is effectively a set of standards. Some of these call up actual Standards, some don't.   The NZBC and Acceptable Solutions are available as free downloads from the MBIE website.

(The Australian Building Codes Board – ABCB – also has non-Standard compliance requirements).

NZMRM Standards 

To simplify all of the above, NZMRM has produced industry specifications covering metallic-coated steel and pre- and post-painted metallic-coated steel. These generally include a reference to the Standards listed above in a single package.

In particular, the UV levels in New Zealand and Australia cause paint that performs well elsewhere to fail quickly, and, coupled with the high use of coil-coated steel products, the use of painted coil that has not actually been tested and proven adequate has serious market implications for all such products.

The NZMRM specification for pre-painted coil has modified AS/NZS 2728:2013 by, among other things, requiring the UV exposure test to be Normative, not Informative.  

The NZMRM Code of Practice discusses all of these properties in detail.

These specifications and the CoP are available as free downloads on the NZMRM website.

Quality Assurance

It's all very well having Standards for product performance and testing, but unless compliance is regularly assessed and certified, users have no idea whether their actual supply complies with the specified requirements. This is particularly true for the materials used for roof and wall cladding.   Samples may comply with the Standard, but production may not. Fortunately, this was recognised as needing attention back in the 1990s and the ISO 9000 family of standards - Quality Management Systems was the result. Suppliers should be able to offer certification of their quality systems to ISO 9001, which means their production is covered by an approved and inspected quality management system. Purchasers should ensure suppliers can offer this.

Summary

Despite many specifiers and users being unaware of it, the products of NZMRM members are covered by Standards. Demanding and ensuring compliance with these Standards, and properly specifying the correct product for the location, will provide assurance of maximum performance.   MBIE and the New Zealand building industry are currently expressing concern about the import and use of sub-standard (or even non-standard) building materials, potentially leading to the sort of catastrophic and very costly failures experienced in Australia. Demanding verified compliance with Standards is part of avoiding such issues.

Some MBIE control over imported products began in 2025, but it does not yet cover metallic cladding.