Cladding - Steel Structures.

Cladding is required to be weathertight, to provide insulation, to have penetrations for daylight and access, to be aesthetically pleasing, and to last the maximum time with a minimum of maintenance consistent with the budget.

The requirements for the cladding to roofs and walls are somewhat different.

The ability of the roof to remain weathertight is clearly of paramount importance, particularly as the demand for lower roof pitches increases, whereas aesthetic  considerations tend to dictate the choice of walling.

Over the past 30 years,metal cladding has been the most popular choice for both roofs and walls, comprising a substrate of either steel or aluminium.

Cladding with a steel substrate tends to be more economical from a purely  cost point of view and, coupled with a much lower coefficient of thermal expansion than its aluminium counterpart, has practical advantages. However, the integrity of the steel substrate is very much dependent on its coatings to maintain resistance to
corrosion. In some ‘sensitive’ cases, the use of aluminium has been deemed to better serve the specification.

Typical external and internal coatings for steel substrates manufactured by Corus (formerly British Steel plc/Hoogovens) are detailed below.

Substrate – steel
• Galvatite, hot-dipped zinc coated steel to BS EN10147: 1992. Grade Fe E220G with a Z275 zinc coating.
• Galvalloy, hot-dipped alloy-coated steel substrate (95% zinc, 5%  aluminium)  to BS EN 10214. Grade S220 GD+ZA with a ZA255 alloy coating.

Coatings – external
• Colorcoat HPS200 – 200mm coating applied to the weatherside of the sheet on Galvalloy, above. Provides superior durability, colour stability and corrosion resistance.
• Colorcoat PVF2 – 27mm, stoved fluorocarbon, coating on Galvatite, above.
Provides excellent colour stability.
• Colorcoat Silicon Polyester – An economic coating on Galvatite, above. Provides medium term durability for worldwide use.

Coatings – internal
• Colorcoat Lining Enamel – 22mm coating, ‘bright white’, with an easily cleaned surface.
• Colorcoat HPS200 Plastisol – 200mm coating, used in either a corrosive  environment or one of high internal humidity.
• Colorcoat Stelvetite Foodsafe – 150mm coating, comprising a chemically inert polymer for use in cold stores and food processing applications.

The reader should note that there is an increasing move towards whole life-cycle costing of buildings in general, on which the cladding element has a significant influence.A cheaper cladding solution at the outset of a project may result in a smaller initial outlay for the building owner.Over the life of the building, however, running costs could offset (and possibly negate) any savings that may have accrued at procurement stage. A higher cladding specification will reduce not only heating  costs but also carbon dioxide (CO2) emissions.

The construction of the external skin of a building can take several forms, the most prevalent being:

(1) single-skin trapezoidal
(2) double-skin trapezoidal shell
(3) standing seam with concealed fixings
(4) composite panels.

Further information on the above topics can be found by reference to the cladding manufacturers’ technical literature.

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