Durability- Coated steel cladding
Coated
steel cladding provides a cost-effective weathering envelope for buildings.
Peter Mayer of Building LifePlans examines the specification options and
whole life costs.
Introduction
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 manufacturers 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 0.6 – 2mm 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.
Specification options
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. This standard confirms steel quality, minimum
thickness, surface quality and adhesion criteria for zinc and zinc–aluminium
alloy coatings.
A roll of steel is cut to form cladding sheets, at these cut edges bare
steel is exposed. 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 microns
on each side of the steel. This is obtained by coating masses of 275g/mœ
(Z275) for zinc and 255 g/mœ (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 preparation of the steel play an
important role in the long-term performance of steel cladding.
Organic coating
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 200microns. Evidence
shows these can last up to 40 years.
Polyvinylidene–fluoride (PVDF or PVF2)
PVDF coatings are thinner at 25microns with expected performance in excess
of 20 years.
Polyester
A thin organic coating 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.
Durability issues
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 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
gives 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.
Specification options
| (Based on mild steel sheet minimum thickness 0.7mm; hot–dip galvanized to BS EN 10327, 275g/mœ zinc coating weight) | Capital cost £/m² |
Net present value for 60 years£/m² | Design life Years |
| PVC (Plastisol) coating, 200 microns nominal thickness. | 24 | 48 | 10-30 |
| PVC (Plastisol) coating, 120 microns nominal thickness. | 23 | 49 | 10-25 |
| PVDF or PVF2 (Polyvinylidene fluoride) coating, minimum 25 microns nominal thickness. | 22 | 54 | 10-15 |
| Polyester based coating, minimum 25 microns nominal thickness. | 19 | 53 | 5-10 |
| Polyurethane based coating, minimum 50 microns nominal thickness. | 22 | 54 | 10-15 |
Table notes
• Costs include cleaning, maintenance, recoating and repairs. Cost
and frequency of maintenance are generic and are based on 1000mœ of simple
cladding. Repainting is modelled at an average of the range given. The
model assumes the steel cladding is not replaced during the 60 year period.
• A discount rate of 3.5% is used to calculate net present values.
• A cost analysis based on project specific information is essential
for a realistic best value appraisal.
First published in Building 2005
Further information
Building LifePlans provides latent defects warranty
for all buildings.
BLP Construction Durability Database at www.componentlife.com
provides durability information for building components.
Further information contact peter.mayer@buildinglifeplans.com
or telephone: 020 7204 2441.
