General Environment
The durability of galvanised zinc coated products is dependent on:
- the environment it will be installed in.
- the grade or weight of the zinc coating used.
- the degree and extent of the maintenance that will be undertaken over the life of the product.
Performance of galvanised zinc coated products is affected by:
- the cumulative effects of the weather.
- the amount of dust that settles on the product (which can hold moisture).
- any other wind-blown deposits that may settle on the product, promoting corrosion.
If these deposits are not removed, they will greatly lessen the durability of the product. Regular maintenance should be carried out on these areas.
Standard zinc coating weight is used on most buildings where components are kept dry, protected from exposure to moisture and corrosive environments. Inside the building the galvanised zinc coated products can be used in the temperature range of +60ºC and down to a minimum of -30ºC.
In high risk areas such as the underside of canopies, exposed purlin systems used above underslung canopies or exposed purlin systems around large door openings facing the prevailing wind direction, attention should be given to specifying a suitably protective paint coating on the purlin and bracing. The special grade Z450 material may also be specified for the purlins. Bracing Channel and cleats are supplied standard as Z450 galv weight.
Limitations on Use
Avoid the use of galvanised steel purlin systems without the additional protection of an appropriate coating in the following environments:
- Swimming pool covers, where high concentrations of chlorine are combined with a high humidity environment. In this situation the purlin system remains wet for long periods of time, causing a rapid consumption of the galvanised zinc coating and eventual red rusting of the base metal.
- Any use where the galvanised surface is being exposed to continuous moisture, without a chance for the surface to dry out.
- In or near marine environments, where the prevailing wind may deposit marine salts on the galvanised surface.
- In areas surrounding chemical or industrial storage buildings where any chemical attack may lessen the life of the structure or wind-driven chemical fumes may attack the galvanised coating.
- When in contact with the ground (e.g. soil or clay) or where embedded in concrete.
Avoid the use of galvanised steel purlin systems:
- When in contact with timber and especially treated timber such as CCA (copper chrome arsenic) without the use of an isolating material such as Malthoid (DPC) between the timber and galvanised steel flooring sheet. This avoids any moisture or chemical reaction between the two materials.
- When in contact with the ground (ie soil or clay) or where embedded in concrete.
- When used in sub-floor areas with less than 450mm ground clearance.
- When used in sub-floor areas where ventilation does not comply with NZS 3604 Clause 6.14.
- When used within 50mm of the concrete ground slab.
