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Construction

The integrity and long-term performance of pre-finished steel cladding can be established or compromised by the way it is handled and installed on site.

Here practical advice and guidance is provided to help maintain the integrity of the cladding.

This includes information about the correct cutting of cladding sheets and panels, installing cladding sheets and panels using the correct fixings and positioning of any sealants as well as protecting cut edges and ensuring good junction details using flashings. It is suggested that the information here is used in conjunction with Site Safety, Site Preparation and Inspection at Handover.

Cutting cladding panels
Cladding panels should, ideally, be specified and manufactured to the correct size for the roof or wall to be constructed and require no further cutting on-site.

If cutting is required on-site then it should be done carefully to avoid damage which could affect both the appearance and performance of the cladding.

The following guidelines should be adhered to:

  • The appropriate tool for cutting cladding sheet is a sheet nibbler or snips, a tool developed especially for this application. This tool gives an accurate cut, with no damage to the coating and allows holes to be cut of varying size and complex geometry.
  • The use of an abrasive disk is unacceptable for the cutting of cladding sheet. This is because damage can be caused to the coating through frictional overheating or abrasion and leave an uneven, poorly finished cut.
  • Where possible, cutting should be carried out at ground level, rather than on the roof or over other materials which are to be used in construction. This will help satisfy Health and Safety guidelines and ensure swarf and debris are not deposited onto other panels, where it can cause discolouration and, potentially, corrosion. If it is not possible to cut at ground level, a scrap panel should be used to protect the roof cladding.
  • In the case of composite panels, the use of a reciprocating saw for cutting is necessary and must therefore be carried out with the utmost care. It is essential to use a cutting speed which will reduce the effects of frictional heating on the coating, and care should be taken to produce the best standard of cut possible. A number of saws and blades are now available specifically for the cutting of composite panels. However, caution should still be taken when using these tools to provide an even, high quality finish.
  • The use of an oxyacetylene torch to burn through sheet is also an unacceptable method. This is because not only does it produce a poorly finished and uneven cut, but, also, the high temperatures cause severe damage to the coating.
  • All swarf and other debris must be removed immediately from the coating surface since they can cause abrasive damage, discolouration and potentially corrosion.
  • Site-sawn edges may suffer accelerated cut edge corrosion or peel-back of the coating, and will be excluded from the warranty. All site-sawn cut edges must be protected from weathering with a suitable paint or lacquer. Best practice for protecting cut edges is given later on.

Fixing techniques and guidance
Fastener heads and washers act jointly to clamp individual components together and to resist uplift caused by the wind. Together with the seal, they weatherproof the hole and prevent water ingress. The following points should be remembered:

  • Fasteners must have a life expectancy comparable with the required life of the cladding, and must not cause rust stains on the sheet.
  • Fastener heads and shanks which are exposed to the weather or to aggressive internal conditions must be protected from corrosion. Specifying fasteners made from 300 series austenitic stainless steel (BS 5427,1996, clause 2.8.3) ensures long term performance over the entire effective fastener length. Visible heads may be factory coloured to match the sheeting. Fasteners with integral colour coded heads should be used in preference to push on caps.
  • Exposed heads of carbon steel fasteners may also be factory coloured for colour matching but this should not be relied upon as the sole protection against corrosion.
  • Any colour matched fasteners should be resistant to weather and UV light.
  • Any sheeting fastener that is exposed should have a resilient sealing washer. EPDM rubber is considered as offering the best overall performance and this should be bonded to the backing of the metal washer.
  • For the seals to be effective, do not under or overtighten the fasteners, since this will deform the resilient washers. Under tightening will not provide adequate compression and sealing.
  • Wind is responsible for the uplift on roof sheets, but it does not act uniformly across the entire roof. Different areas of sheeting will be subject to varying upward forces. Because the fasteners must carry these forces, it follows that the distribution of fasteners will not necessarily be uniform over the entire area. Some areas of the roof sheets may have more fixings than other areas.
  • Screw guns with correctly adjusted, depth-sensitive nose-pieces are recommended to ensure all fasteners are installed correctly, thus avoiding over-torque oblique thread-stripping from the supports or damage to the sealing washer.

The correctly specified sheet must be used for each area of the building, and all sheets must be aligned correctly. Roof panels should be fixed with the side lap away from the prevailing wind.

  • Selection of the fastener is the responsibility of the cladding system manufacturer and must be compatible with the building – this is usually specified within the design.

Guidance on very low-pitched roofs
Very low-pitched roofs are defined as roofs with an angle of 4° or less. This type of roof demands special attention to ensure long-term weather-tightness. All possible measures must be taken to avoid ponding on the roof. In particular:

  • Roof cladding contractors and maintenance engineers must ensure that any actions they take do not lead to excessive deflections developing in the sheets.
  • There must be no build-up of debris on the roof which could dam rainwater and cause ponding or poultice corrosion.
  • With very low pitches, there is an increased risk of water collecting on the underside of the cladding sheet at the eaves. Since this could lead to a premature breakdown of the coating, the edge of the sheet should be turned down to form a drip whose slope should not be less than 10°.

Installation of flashings
The design and construction of good junction details is vital in delivering a well finished building.

Good housekeeping is always important and extra care should be taken with flashings as their form tends to make them easy to damage.

Some important points to consider in the installation of flashings are:

  • The minimum steel thickness should be 0.7 mm (nominal thickness).
  • Care should be taken when setting-out flashings to avoid unbalanced joints. For example joints on a peaked verge should be symmetrical about the peak.
  • Flashings should not allow more than 250 mm of flat material between bends. Bends, folds, swaging or similar measures should be used to impart stiffening or extra support.
  • The longitudinal edges of long flashings should be returned to form a welted edge detail. Where flashings overlap, the welt should be trimmed off to the length of the over
  • Ridge flashings should be secured to the sheeting with appropriate secondary fasteners at intervals of 450 mm maximum or to alternate crowns and elsewhere with fasteners at the intervals used for side laps.
  • In preference to cutting and folding junctions on site, use prefabricated corner flashings and provide the fixers with details of the correct fixing sequence.
  • Sealants used with flashings should always be placed at the leading (exposed) edge.
  • Overlaps should be arranged down-slope, or away from the prevailing weather.
  • It is important not to over-tighten fasteners on flashings, since this will result in indentations and a quilted effect 

Position of sealants
Sealants are generally used to provide a weathertight seal between cladding sheets. However, the appropriate positioning of these sealants is vital to both weatherproofing and to ensuring the longevity of the cladding sheets themselves.

The Metal Cladding and Roofing Manufacturers Association (MCRMA) provides clear guidance as to the appropriate positioning of sealants, in Technical Paper 19, ‘Guidance for the effective sealing of end lap details in metal roofing constructions’.

Refer to cladding system manufacturer guidelines or local regional best practice.

Protecting exposed backing coats
The reverse side of roof and wall cladding will not generally be exposed except in circumstances such as canopies and overhangs. The reverse side of pre-finished steel products is not designed for external environments.

In the case where the reverse side is exposed, other products such as double-sided Colorcoat HPS200 Ultra® should ideally be specified. However, if this is not the case, it is good practice to ensure the exposed reverse side should have an additional coating applied as an extra protection.

Protecting site cut edges
A cut edge is an area of exposed substrate where a pre-finished cladding panel has been cut. To prevent problems with cut-edges, all exposed site-cut edges must be produced to an optimum standard.

Where it is not possible to protect the cut edge from weathering by means of a cover flashing, the site-cut edge should be treated with a suitable edge protection paint or lacquer.

Burrs at cut edges should be minimised by effective shearing practice. Where possible the down burr edge should be on the down slope edge of the roof pitch.

In the case of any cut-edges, the requirement for this sort of protection is particularly important in highly aggressive environments such as marine and industrial. It is also particularly important in the case of low and very low-pitched roofs, where the time of wetness is increased. 

Protecting penetrations in a sheet
Penetrations consist of any breach in the cladding, usually holes drilled or cut for pipework or other accessories.

A penetration made in a cladding panel effectively leaves a cut edge and an area where water may ingress into the cavity of the building.

To ensure that the penetration does not compromise performance, in the first instance the cut edge should be protected as described earlier. The area around the penetration must also be protected further by sealing using a suitable component to prevent ingress of water into the cavity of the wall or the building. In roof cladding applications, suitable sealing components should allow water to be channelled around any penetration to prevent ponding.





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