Top Coat and Backer

This is the outer skin and it must give the desired appearance. In terms of durability, it provides a measure of barrier coating while still being breathable, and prevents UV degradation of the primer.

Functionally the top coat must be hard enough to prevent excessive marring during profiling and installation; and when in use, it must:

  • be flexible enough to form to relatively tight bends without excessive micro-cracking;
  • be resistant to fade in NZ’s harsh environment;
  • withstand extremes of temperature; and
  • be a suitable surface for the collection of potable water.

More recent innovations include solar reflectance technology to minimise the amount of solar heat gain gathered by the cladding

Backer coats generally have the same primer and a thinner top coat than the upper surface. Double-sided systems can be specified for areas where the underside is seen, but in external environments these will be exposed to salt spray and other contaminants, and it cannot be assumed that the underside will last as long as the rain washed top surface — even with regular maintenance.

As paint formulations from different suppliers may have different performance characteristics, it is important that cladding and accessories are supplied from the same manufacturer as differing weathering characteristics may result in visible variance in appearance.

Surface coating must comply with AS/NZS 2728. Cladding and flashings must come from the same source and have the same coating specification, so that fade rates are similar. Fade rates must not exceed those stipulated in AS/NZS 2728. All coatings must be lead free and suitable for the collection of drinking water.

AS/NZS 2728 has been deemed ambiguous in that it can be interpreted as allowing accelerated testing to determine colour fastness and durability. Such tests have been found to be an unreliable indication of a system’s performance in real-life situations. MRM has therefore adopted an interpretation of this standard as the compliance standard for its members.  In the MRM standard, the four-year real-life testing for durability and colour fastness are clarified as being Normative (Compulsory).

Revision Category: 
1 - Minor Errata
Draft Clause: