Coated steel cladding provides a cost-effective weathering envelope for buildings. Peter Mayer of Building LifePlans examines the specification options and their whole life costs
Coated steel cladding comes as profiled steel sheets or composite panels, which are used to clad walls and roofs. It is used for cladding a large proportion of industrial, warehouse and out-of-town retail units.
The challenge for coated steel cladding makers is to make a product that both resists corrosion and looks good. To achieve this, coated steel cladding is a composite laminated product comprising:
- Steel sheet 0.4-2 mm thick, which on its own would rapidly rust in the UK climate
- A corrosion protection layer; zinc or zinc-aluminium alloy applied by a hot-dip process
- A paint finish: an organic coating to provide an attractive finish, usually applied by a roller, typically based on polyvinyl-chloride (PVC or plastisol), polyvinylidene-fluoride (PVDF or PVF2), polyester or polyurethane formulations.
The challenge for specifiers is how to determine which coating system offers the best performance.
Steel sheet and corrosion protection
Steel sheet should be to BS EN 10327, the European standards for continuously hot-dip coated steel sheet. These standards confirm steel quality, minimum thickness, surface quality and adhesion criteria for zinc and zinc-aluminium alloy coatings.A roll of steel coil is cut to form cladding sheets. The exposed steel face is prevented from corroding by zinc layers, which corrode sacrificially in preference to the steel. The sacrificial zinc layer initially protects the cut edges but eventually these will corrode, leading to delamination of the organic coating layer.
Typical corrosion-protection-coating thicknesses are 10 micons on each side of the steel. This is obtained by coating masses of of 275 g/m² (Z275) for zinc and 265 g/m² (ZA 255) for aluminium-zinc alloy. Aluminium-zinc alloy gives better edge performance with a reduced corrosion rate and better paint adhesion.
The thicker the corrosion protection layer, the less likely and the longer the time before for the onset of delamination. But it is not as simple as this, as production processes and the preparation of the steel play an important role in the long-term performance of the cladding.
The properties required for a durable organic coating are good adhesion, resistance to ultraviolet light, resistance to chalking, and abrasion resistance. Part three of the European standard EN 10169 will eventually provide a useful method to compare components made from organically coated steel sheet. Unfortunately the standard is still in draft form.In the meantime, specifiers have to rely on test data from manufacturers or third-party certification. Alternatively a detailed assessment of manufacturer's guarantees may provide assurance of performance.
Organic coatings have a finite life depending on how these are maintained. Failure is generally an issue of appearance as the weatherproofing function of the cladding is rarely compromised because of the underlying corrosion-resistant layer. However this is thin and corrosion will eventually set in unless the cladding is regularly repainted.
- Polyvinyl-chloride (PVC or plastisol) PVC-based coatings are applied in thicknesses up to 200 microns. Evidence shows these can last up to 40 years.
- Polyvinylidene-fluoride (PVDF or PVF2) PVDF coatings are thinner at 25 microns with expected performance in excess of 20 years.
- Polyester A thin organic coating of 25 microns, with expected performance in excess of 15 to 20 years.
- Polyurethane An organic coating of 50 microns thickness, with expected performance in excess of 20 years.
Some durability issues apply to all types of organic coated cladding. The amount of time the coating lasts before painting is needed is largely related to the ultraviolet light dosage:
- Colour Lighter colours can have longer periods between redecoration than darker colours.
- Building element Cladding on flat roofs tends to deteriorate faster than claddings on pitched roofs. Cladding on walls has the longest intervals between repainting.
- Orientation South facing claddings tend to deteriorate slightly faster than east, west or north facing claddings.
- Environment Polluted or marine environments will tend to accelerate paint deterioration.
- Cladding profile shape Flexible organic coatings are less likely to deteriorate and crack at shaped edges of profiled sheeting.