Close stacked sheets may deteriorate quickly if water enters the pack. This happens because the metal exposed to moisture without air forms bulky, unstable, and loosely adherent, hygroscopic compounds.
When wet sheets are delivered or they get wet in storage, they must be used immediately or dried. Sheets can be dried by filleting sheets or cross stacking them on a slope to allow water to drain and air to circulate between the sheets.
Only use a dry, well-ventilated environment for long term storage.
Failure to follow these handling and storage precautions could result in spoiling the surface appearance of the products and severely reducing their service life. On galvanised material this will appear as a white corrosion product (white rust); on Aluminium/Zinc steel the corrosion shows up black.
This should not be confused with fretting. On pre-painted steel, the result of wet storage damage could be a bubbling of the paint surface. Damage resulting from such failure invalidates the warranty. See 4.11.7 Crevice Corrosion/Wet Storage Stain.
The extent and severity of wet storage stain is usually proportional to the length of time the sheeting has been wet. In the case of superficial attack, exposure to the atmosphere and careful cleaning will sufficiently reduce surface imperfections, but heavier deposits can damage the sheets enough that they require replacement.
Where it is likely that the roof or wall cladding will be stored on site for more than one week before installation, the contractor (or person responsible for doing ordering material) should consider including waterproof packaging from the manufacturer in the order.