COP v3.0:durability; materials

4.17 Materials 

Metals used in the roof and wall cladding industry in New Zealand are:

  • steel coated with zinc - Galvanised steel;
  • steel coated with an alloy of aluminium and zinc, sometimes with the inclusion of other minority elements;
  • aluminium;
  • copper;
  • zinc;
  • stainless steel; and
  • lead.

*Many of these can coated with an organic coating, including acrylic, polyester and PVDF.

4.17.1 Steel 

4.17.1.1 Metallic Coatings 

For most of the nearly 200-year history of lightweight steel cladding, the protective metal coating has been made from zinc (usually with minor additions of other metals), and this is called galvanised steel. It works by the zinc sacrificing preferentially to the steel.

In the second half of the 20th century research looked for metallic coatings which would provide longer life. Aluminium was tried as a coating material because of its passive surface, but it was not satisfactory on its own. However, aluminium alloyed with zinc and other metals produced more corrosion proof products than any metal on its own. (See 4.4.2 Barrier Protection.)

We now have two groups of metallic coatings for steel cladding products — zinc-dominant coatings, which primarily provide sacrificial protection; and aluminium-dominant coatings, which primarily provide a barrier protective coating of aluminium oxide. Coatings containing both aluminium and zinc are now the preferred coating for roof and wall cladding products, although zinc-based coatings continue to predominate for various other products.

The composition and weights of these coatings are described in detail in AS 1397:2011. The following sections discuss metallic coatings in the order in which they appear in AS 1397, not their rate of use in the market.

4.17.1.1.1 Coating Thickness 

Steel was zinc-coated for many years by dipping short lengths of flat or profiled sheet metal in a bath of molten zinc, and the steel was then hung to cool while the excess zinc coating drained off.

More than sixty years ago manufacturers developed a continuous hot dipping method. During the continuous hot dipping process, the steel coil is run through a bath of molten metal. The thickness is controlled by blowing off the excess coating with air jets applied to both sides of the strip as it leaves the molten metal bath. 

Continuous hot dipping, as opposed to the batch immersion process, is more cost-effective and allows for greater control of the consistency, thickness, and surface condition of the metallic coating.

It is a similar process to that for continuous paint coating, shown in 4.18.1.1 The Paintline Process, with priming, coating, and ovens replaced by the molten metal tank and blow-off section.

The atmospheric corrosion performance of a hot-dipped zinc coating is closely proportional to its thickness.

The thickness of coatings in micrometres (µm) can be measured with a non-destructive magnetic induction meter or similar device which can then be converted into grams per square metre (g/m²).

 

 

There is confusion about the method of describing the coating thickness of coil-coated sheet and strip products in g/m², compared to products that were hot-dipped after fabrication. The coating thickness of sheet and strip refers to the collective amount of coating on both sides of the sheet, effectively dividing the coating weight by half. It is invalid to equate the coating weight in g/m² of hot-dipped zinc coatings on fabricated products, such as nails and screws, with that of metallic coatings on sheet and coil; the coating thickness of the fabricated products relates to one surface only.

A micron (µm) is one-thousandth of a millimetre.

4.17.1.2 Galvanised Steel 

Zinc Coating, commonly called galvanising, is still one of the most common metallic coating processes for steel. Galvanising describes various methods of adding a metallic zinc coating to steel to give it cathodic protection; also known as galvanic protection.

Galvanised steel is classed as a “Z”-coating and has a bold crystalline pattern or spangle, a random geometric pattern that resembles frost on a window.

There are many processes for galvanising, but only products dipped or immersed in a bath of molten zinc can be called hot-dipped galvanised, the process used for the metallic coating of steel roof and wall cladding.

The thickness of the coating can be more precisely controlled on a continuous coil galvanising-line than it can be with other methods.

The standard coating weight for unpainted galvanised coil and sheet used for roof and wall cladding is 450 g/m², designated Z450, but other coating weights are available. The coating weight for products intended for painting is 275g/m², and it is designated Z275.

Since the advent of ZM coatings, minimised spangle zinc coated products, typically used for painting, are now designated with the “M” after the weight, e.g., Z275M.

The process of zinc coating by electro-plating gives a much thinner protective film and is not considered suitable for painted or unpainted cladding materials exposed to the weather.

4.17.1.3 ZA Coatings 

ZA coatings are a zinc-aluminium alloy coating consisting of 95 % zinc and 5% aluminium by mass, with the addition of lanthanides.  It is commonly known as Galfan® and is as designated ZA in AS/NZS 1397. As an European product it generally conforms to EN 10214..

ZA serves the same purpose as galvanised Z and AZ coatings, but has different corrosion characteristics than both.

ZA coatings are not currently available in NZ or Australia.

4.17.1.4 ZM Coatings 

ZM coatings are zinc-aluminium alloy coatings with a majority of zinc and a small amount of magnesium. Steel with a continuously hot-dipped coating of zinc with 5 -13% aluminium and 2 - 4% magnesium is designated in AS 1397:2011 as ZM. In New Zealand, it is commonly marketed as ZAM.

The coating weights are similar to Zinc coatings, with ZM 240 used for products which will later be coil coated and ZM 450 for unpainted products.

Unpainted ZM products have been used for roofing accessory and rainwater cladding applications in New Zealand, but are more commonly found in factory pre-painted products.

4.17.1.5 AZ Coatings 

AZ coatings are zinc-aluminium alloy coatings with a majority of aluminium. AZ coating, marketed as Zincalume® steel, is an alloy of zinc and aluminium which is now the most commonly used coating in New Zealand for protecting steel roof and wall cladding.

AZ coating is applied in the same way as other coatings, but with a pot temperature at about 140˚C higher than galvanised coating, and it is rapidly cooled to provide a dual-phase microstructure.

The alloy consists of 50 to 60% aluminium, zinc, and a small addition of silicon. In New Zealand, the ratio is nominally 55:45. These percentage ratios are by mass; by volume, the percentage ratio changes to approximately 80% aluminium and 20% zinc. Volume is probably a more realistic measure of its nature.

The alloy coating thickness generally used for steel roof and wall cladding is 150 g/m² (AZ150). This coating is approximately the same thickness (0.04 mm) as Z275 zinc. AZ200 coatings are available as a substrate for organic coated products to be used in very severe environments.

An AZ coating protects steel both as a barrier and sacrificially, as the aluminium content provides a barrier, while the zinc content of the coating will sacrifice itself to protect the base steel.

The AZ coating is finer grained than zinc alone and has a silver matt hue with a lightly visible spangle. This finish has a relatively high level of initial reflectivity, which darkens over time.

A thin acrylic film is applied during manufacture in New Zealand. The acrylic film acts as a roll forming lubricant and minimises finger marking and surface discolouration.

4.17.1.6 AM Coatings 

Adding magnesium to an aluminium dominant zinc-aluminium alloy coating improves the cut edge corrosion resistance to a similar level as zinc coating, but still confers the improved surface protection and slower erosion rate of AZ coatings.

Steel with a continuously hot-dipped coating of 47 - 57% aluminium and zinc, with the addition of 1 - 3% magnesium by mass is designated in AS 1397:2011 as AM. It is not currently sold in NZ but is under test.

Coating weights may be AM100, AM 125 or AM150.

4.17.2 Stainless Steel 

 

Stainless steel is a durable, corrosion-resistant material used in harsh environments when a non-weathering finish is desired. Chromium forms a tenacious oxide protective film on stainless steel that is transparent and self-healing, as it will repair itself on exposure to the atmosphere.

Stainless steels are resistant to most chemicals, but are subject to crevice and pit corrosion (see Wet Storage).

Some light surface staining known as tea staining may appear, but it is not damaging to the product.

Most stainless steel roof and wall cladding, flashings and panels in New Zealand are made from the 300 series of austenitic non-magnetic stainless steel, which contain chromium, nickel, and manganese, with 304 and 316 being the most common grades.

Grade 304 stainless steel is an alloy of 18% chromium and 8% nickel that provides high corrosion resistance and is known as an all-purpose alloy.

Grade 316 stainless steel should be specified where tea staining must be avoided. It contains 16% chromium, 10% nickel, with 2% molybdenum added, which increases resistance to staining and corrosion.

Grade 445 ferritic stainless steel is now available in New Zealand, which combines the corrosion resistance of grade 316 with formability approaching that of carbon steel. As the work hardening of 445 is much lower than with austenitic grades, it can be formed in a similar way to carbon steel and is more easily sheared.

Grade 445 stainless steel contains 22% chromium and 1.2% molybdenum and no nickel. It has lower thermal expansion than other grades, so it is less likely to distort in the heat of the sun. The yield stress and hardness of 445 is higher than 304 and 316, but the tensile strength and elongation properties are lower.

The corrosion resistance grade of 445 is similar to grade 316 in most marine and aggressive industrial environments.

Stainless steel is available in various mill finishes from dull matt to highly polished. The most common finishes for roof cladding and sheet metal flashings, are those designated as 2B and 2D.

The 2B finish is a bright, cold-rolled finish that is highly reflective and 2D is a dull finish that is less reflective. BA is a bright reflective surface only suitable for decorative cladding in thicker gauges. Embossed patterns are available that reduce visible distortion and minimise glare and reflection.

Stainless steel should not be cleaned with steel wool, but stainless steel wool or synthetic abrasive pads can be used. Cleaning should be done with care as roughening the surface may promote further stains.

Stainless steel fixings should be used with stainless steel sheet to avoid dissimilar metal corrosion. The fastener grade must match the grade of the cladding.

There is no well-defined yield point for stainless steels. Fully annealed or standard annealed tempers are used for ease of forming with 304 and 316 having an approximate yield strength of 290 mPa.

Austenitic stainless steels require different forming techniques than other metals, and are known to be tougher and more difficult to form than carbon steel of the same thickness, e.g.,m when shearing stainless steel the equipment capacity should be increased between 30% - 50%. Because of the toughness of the metal, sharp cutting edges dull more quickly than when used with carbon steel.

Although stainless steel is not much harder than mild steel, increased power is necessary to form it because of its high ultimate strength and its higher work hardening rate. As most forming machines are rated for the heaviest gauge steel this capacity should be de-rated by 40%.

Precautions should be taken not to contaminate the surface of the metal by inclusions from roll forming or folding equipment. It can appear as rust spots on stainless steel, which is detrimental to performance. Stainless steel coil and sheet can be supplied with a strippable film on both faces to avoid this contamination.

4.17.3 Aluminium 

The aluminium alloys used in New Zealand for roof and wall cladding are included in the 5000 series.

  • Aluminium 5005 has excellent workability, weldability and corrosion resistance.
  • Aluminium 5052 is a higher strength marine grade alloy with exceptional resistance to corrosion in marine or industrial environments.

Following strain-hardening of aluminium alloys, tempering increases the ductility by low-temperature heating, and their description regarding hardness relates to the last number, e.g. H12 or H32.

The description of tempers given to aluminium alloys can be confusing because the different alloys are strain hardened in different ways. As a result, different alloys with the same hardness description may have significantly different yield strengths.

 

Pure aluminium (99%) can be used as a soft edging for ridge or apron flashings required to act as a wind barrier.

Aluminium alloys are available in three surface finishes.

  • Mill finish: A smooth, lustrous finish which will dull relatively quickly.
  • Stucco Finish: An embossed mill finish, which reduces the specular reflectance of mill finish sheet.
  • Painted Finish: A range of painted finishes are available similar to those offered in painted steel.

The high reflectance and emissivity of unpainted aluminium can reduce heat transmission considerably.

Aluminium develops a thin oxide film on the surface that is impermeable to most airborne contaminants, except for strong alkalis and acids.

4.17.3A Aluminium Hardness End-use

Note: Typical stocking of 5052 alloy, H36 allows for rollforming both corrugate and trapezoidal profiles. Trough section may require H34 material. H36 material can be used to manufacture most flashings, except those requiring soft edging or hemming.
Alloy;Yield MinimumTypical Use
5005 – H32 Quarter Hard85Lockseam
5005 – H34 Half Hard105Folding
5052 – H32 Quarter Hard160Lockseam
5052 – H34 Half Hard180Folding and curving
5052 – H36 Three Quaters Hard200Rollforming and folding
5052 – Fully Hard220220Rollforming

 

4.17.4 Zinc 

Zinc is a traditional roof cladding material and weathers to a dark grey patina after environmental exposure, like galvanised steel, except there is no spangle effect on the surface. Zinc roof panels and flashings are commonly 0.7 mm thick, although heavier gauges can be used. Zinc roofs are usually fully supported on sarking.

The staining potential of zinc run-off onto other surfaces is less than that of copper and lead. Flat zinc panels must be adequately vented from underneath and are available with a high-build lacquer coating to help prevent corrosion of the under-surface.

Zinc has approximately twice the thermal expansion coefficient as steel, so allowance for expansion must be made accordingly.

Under 7 °C the metal becomes brittle and is difficult to form without fracturing.

Zinc used for roof cladding generally contains small percentages of titanium and copper, which add to the properties of pure zinc.

Zinc is also available in a range of pre-patinated surfaces.

4.17.5 Copper 

Copper is a naturally durable product.

Copper grades 122, 110, and 102 may be used in construction. Grade 122 is the most commonly used; it has been deoxidized with phosphorus, which makes it weldable. The other grades cannot be welded but can be soldered.

Copper darkens in reaction to the atmosphere. Near seawater or industrial sources of sulphur-containing gases, a green patina may develop in time. In different environments, the weathered colour may vary from dark brown to almost black.

Copper-containing alloys, such as brass and bronze, are available when different colours are requested or for accessories requiring greater strength.

Copper is more malleable than steel sheets, and annealed copper is used for hand folding or where a high degree of formability is required. Roll formed roofing, wall cladding, gutters, spouts, and flashings are typically made from half hard copper. Copper roofs are normally fully supported on sarking.

Copper must be protected from contamination when being processed with tools that have been used to process other metals because the resulting inclusions might cause pit corrosion.

Neither copper nor run-off from copper should come in contact with less noble metals, as it will cause galvanic corrosion. Avoid installing copper in contact with or receiving run-off from bituminous material or other acidic surfaces, because it prevents the formation of the protective patina, causing discolouration and a shortened lifespan.

4.17.6 Lead 

Historically, lead has been a popular choice for roof cladding and flashings, because it is naturally durable and is easily shaped using hand tools at ambient temperatures, without the need for softening or annealing.

Lead has an inherent lack of mechanical strength and is laid on solid sarking. It has high thermal movement and, over time, there is a risk of distortion and lead sheet cracking. Sheet lead is available in weights from 6 kg/m² to 40 kg/m².

The thinner the lead, the shorter the length should be. A maximum length of 1500 mm or less than 1.5 m² is ideal.

Run-off from a new lead roof can stain other metals with a white lead carbonate. Application of a proprietary product or boiled linseed oil and mineral turpentine mixture can avoid that happening.

A factory applied cured coating that inhibits the contact between lead and oxides with water is available. The lack of contact reduces the potential for run-off staining other metals, and of lead entering ground water systems. Avoid using lead roofs to collect potable water.

4.17.7 Translucent Sheeting 

Translucent sheeting should be manufactured from naturally durable products or have a protective surface film to avoid ultra-violet degradation. (See Natural light for more information.)

GRP

4.17.8 GRP 

GRP is a composite material made up of polyester resin, reinforced with glass fibres.  It is protected from UV erosion by a surface coating consisting of a gel or laminate.  The composite is extruded and set over forming moulds to match specific roofing profiles. 

GRP can often be used as translucent sheeting or it can be supplied with a clear, or opaque gel coat to the weather surface to provide a high level of corrosion resistance to aggressive atmospheres, where coated metal or even non-ferrous metals may not perform as required.

Examples of use can be found in extreme environments such as wool scouring plants, fertilizer stores, tanneries, acid plants and smelters, abattoirs, compost plants, galvanizing plants, and buildings in harsh geothermal areas.

Where an entire roof is clad in GRP, (rather than individual sheets separated by metal sheets as in a typical case of translucent sheeting), it affects the trafficability and safety requirements. If a GRP roof is required to be accessible, it can be manufactured incorporating woven roving reinforcement into the resin matrix to make it trafficable. Another option would be to install stainless steel safety mesh under the roof cladding, if required.