The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
Where the underside of metal roof cladding is subject to contaminants such as salt at the gutter line, an over-flashing should be inserted between the roof and the gutter to prevent deposits which lead to 'inside out' corrosion. This flashing can become a sacrificial one where dissimilar metals are used for gutters or spouting, and where splashes and contaminants can corrode the roof cladding. See 18.104.22.168 Eaves Flashing.
Where the gutter line micro-environment is likely to be severe, the underside of metal cladding should be painted using a two pack epoxy primer or elastomeric polyurethane paint before installation.
Building underlay alone can provide this protection if carried down into the gutter, but does not have a 50-year durability if exposed to UV and can give rise to flutter noise if lapped too far into the gutter.
Where roof cladding is exposed to the elements from the underside, it should be included in the maintenance schedule for unwashed areas.