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Organically coated steel sheeting

NBS Technical Author, Charles Stirling, draws on his experience at the 'cutting edge' of steel sheeting to advise on the installation and maintenance of this ubiquitous material.

Introduction

For many years, profiled metal sheeting has been specified extensively throughout the UK providing external wall cladding and roof covering to all manner of building types. A range of materials and finishes are available but by far the most popular is organically coated zinc-galvanized steel. Instances of this coated metal sheeting prematurely failing have raised concern over its performance and long-term durability. This note is to provide guidance on installing and maintaining this material.

Site cutting and durability

Guidance is available from material suppliers on the minimum period to first maintenance of their coated products. In general, this is colour and location specific, i.e. dark colours on walls and roofs in polluted industrial or marine environments are typically 10-15 years to first maintenance. Lighter colours on walls and roofs in less aggressive environments are typically 20-25 years till first maintenance.

However, there have been instances of coated metal sheeting failing before this recommended period primarily due to damage incurred during installation, e.g. where profiled sheets have been cut to accommodate site variations or penetrations. Cutting by burning with torches, hacksaws and abrasive discs is recognised as universally unacceptable as localized high temperatures can damage the coating. Suppliers recommend using a sheet nibbler, which provides a clean edge and an effective method of cutting complex shaped holes. All site cuts should be treated with a proprietary sealer to prevent corrosion of the cut edge.

Cut edge corrosion

This can arise where an inappropriate method of cutting or trimming of the metal sheet has been used during assembly leading to damage or disruption of the coating along the edge. This coating damage will lead to corrosion of the underlying metal and delamination of the coating. In some instances this can extend some distance from the cut edge and, where the corrosion is left unchecked, the metal sheeting may become perforated. In assessing the extent of damage along cut edges, it is important to assess the extent of damage to the underside of the sheet. This may extend (unseen) beyond the level of the external manifestation due to condensation or water penetration on the underside.

Localized corrosion and delamination

At the lower end of the damage spectrum this may manifest itself as a gradual change in appearance, e.g. chalking, loss of gloss or colour change, which arises due to natural exposure to the atmosphere and to UV light. At a more serious level this can involve fine crazing over a large area of the coating leading to some slight corrosion of the galvanized protection or metal substrate and subsequent flaking of the coating. If crazing and preliminary flaking are left untreated, the underlying metal will corrode leading to extensive delamination of the coating. The random appearance of this delamination is often the result of stressing of the coating either during manufacture or site assembly, e.g. pin holes as a result of roll forming or damage resulting from cutting swarf.

Mechanical damage

Dents and scrapes to sheeting may result from deliberate vandalism, or from accidental damage during assembly or access during maintenance. In many cases this mechanical damage results in no more than a slight deterioration in the aesthetics of the sheeting. However, where the coating is punctured or impaired, then the underlying metal is at risk from corrosion.

Water penetration at fixings and laps

The weather resistance of a wall cladding or roof covering system is largely dependent on the correct selection and subsequent performance of the fixing system and joint sealants. The fixing system requires to provide a structurally sound connection between components and to provide a weather resistant seal throughout the life of the sheeting.

A number of defects can arise during assembly, which may compromise the effectiveness of fixings. Poor initial alignment can result in additional holes being drilled, which are subsequently left uncovered or are filled with an inappropriate sealant. In addition, over or under tightening of fixings may result in compressible weather seals subsequently failing.

In many instances, compressible sealing washers at fixings provide an isolation medium to prevent contact between dissimilar metals and to reduce the risk of bi-metallic corrosion. Where the fixing seal is ineffective, there is a serious risk of corrosion.

Water can also penetrate at laps in sheets as a result of capillary action. It is drawn up into narrow joints, usually resulting in deterioration of the underside of the sheeting, which may have a less robust coating. Manufacturer guidance is available on the recommended sealing of laps dependent on sheeting inclination and degree of exposure to wind-driven rain.

Condensation at rooflights

This often manifests itself as dripping moisture inside the building, giving the impression of penetrating rain. If allowed to continue unchecked, it can lead to the corrosion of adjoining sheeting. Condensation occurs where warm, moist air from inside the building comes into contact with colder components of the building fabric. It is vitally important to the durability of sheeting components, including roof lights, that this moist air is prevented from penetrating the building fabric. Manufacturers' recommendations should be followed for the effective installation of roof lights, and guidance is given within BS 5250 (Code of practice for control of condensation in buildings) on the effective specification of ventilation, vapour control layers and breather membranes.

Maintenance

Routine inspections of the external fabric provide an early identification of any sheeting defects and also identify those building defects which often aggravate or accelerate the corrosion process, e.g. blocked gutters, damaged flashings, damaged cut edges, impact damage, disrupted fixings. Experienced personnel should carry out these inspections at least annually and, in particular, following severe storms. A coordinated programme of maintenance may include the following key remedial processes:

Cleaning of external surfaces: Often rainfall provides sufficient wash-down to keep external surfaces looking clean and bright. Where there is a concentrated build-up of grime and detritus, dedicated washing down should be undertaken with fresh water and a soft bristle brush. Power washing and aggressive scrubbing should be avoided. Heavy industrial deposits can be removed using household detergent materials or proprietary cleaners. Affected surfaces should be thoroughly washed down with fresh water following these treatments.
Touch-up painting: Where minor scratches to the coating are identified, and these expose the underlying protection or metal, it is possible to carry out minor 'touch-up' repairs to these small areas. The area of applied touch-up should be kept as small as possible to reduce highlighting the slight colour differentials that can arise.
Treatment of cut edges: Remedial treatment to damaged cut edges involves the complete removal of all disrupted organic coating and all products of metal corrosion (rust), to exposed firm bright metal. This prepared area can then be over-painted using a proprietary primer system and topcoat application. At laps, additional sealant may be required to reduce the risk of water penetration.

Conclusion

Coated steel sheeting can provide an effective and durable cladding or covering material. Poor site installation, however, will impair its performance. Where the organic coating is compromised at cut edges or within plain areas of sheeting, remedial repairs should be undertaken swiftly to reduce the risk of further damage to the sheeting. Regular inspection is recommended to identify defects and plan maintenance.