There is a certain mystique surrounding the thickness of coated metal cladding. To begin with all coated coil producer mills refer to the thickness of their products as being ‘nominal’ with a permissible variation (tolerance). Tolerances are normally quoted as plus and minus, however, some mills will produce coil with only a plus or minus tolerance. Locally the tolerance for material up to and including 0.50mm is ±0.02mm, between 0.51-0.58mm ±0.03mm and 0.59-0.80mm ±0.05mm.
Another variant is whether the thickness is total coated thickness, excluding paint (TCT) or base metal thickness, i.e. the thickness of the steel core (BMT). Currently the thickness of locally produced coil is TCT. Following discussion during the writing of SANS 10237 (the new cladding code to replace the former SABS 0237) and the revised version of SANS 10400-L it was agreed to change to BMT when these codes are published in the near future. During the transition period coil will be branded with both TCT & BMT thicknesses. Future span tables for cladding profiles will be based on BMT. For clarification on branding refer to our article entitled “Are you getting what you specified” on our website.
With regard to the various types of protective coatings two systems of measurement are used:
- mass per unit area for metallic coatings
- Dry Film Thickness (DFT) for painted coatings.
Metallic coatings have an alpha-numeric designation, the alpha defines the type of coating for example Z for galvanized and AZ for 55% aluminium zinc alloy. The numeric portion defines the mass in g/m² for example a Z275 coating indicates a combined total (both surfaces) of 275g of zinc per 1m² of material. This translates into a thickness of approximately 19m per surface.
Painted coatings are measured in terms of the DFT of the total coating system (i.e. primer plus top coat) measured in m.