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.
Areas on a building that seldom receive rain-washing gather salt, dust and other contaminants. When condensation, dew or humidity moistens these particles, they react with metal cladding. The reaction is often noticeable as a white zinc corrosion effect, which will precede more serious corrosion.
An unwashed or sheltered area is any surface that is above or inside a line drawn at 45° to any weathertight overhang. Such areas require special consideration, particularly in severe environments.
Unwashed areas include: unlined soffits, roof overhangs, canopies, sheltered walls, and the upper part of garage doors.
Wall claddings receive less effective rain washing than roofs, and may be harder to maintain or replace, so materials for wall cladding should be selected accordingly.
Solar energy collectors consisting of solar water heaters or photovoltaic (PV) panels may be attached directly to the surface of profiled metal roof cladding or mounted on a frame above the roof plane.
For direct fastened applications, a complete seal between the PV cells and the cladding is essential to prevent water ingress resulting in wet storage corrosion.
Solar frames installed above the roof surface must have a sufficient gap underneath to allow manual washing of the roof surface. The minimum gap is 100 mm, but greater clearance may be required for more extensive installations.
The unit must be connected to the roof at the rib adjacent to a purlin line, have a waterproof seal, and be made of materials compatible with the roof product. Discharge, run-off or contact with copper must be avoided with both plain and pre-painted roofing.
The combination of hot water and copper is detrimental to all types of roofing, and hot water from copper pipes will exacerbate the corrosive effect of copper itself.
Run-off from inert surfaces such as glazed tiles, aluminium and aluminium-dominant metallic coatings, fibreglass, pre-coated metals, glass or any painted surface can cause corrosion of unpainted galvanised steel and other zinc-dominant metallic coatings. This is known as 'drip-spot corrosion' or inert catchment corrosion.
Water sitting on a surface absorbs carbon dioxide forming carbonic acid, which is reactive with zinc. On a galvanised surface, the carbonic acid reacts with the zinc and becomes neutral. On an inert surface discharging into an unprotected zinc surface, the carbonic acid is not neutralised, and reaction will be concentrated on the drip points of the inert surface onto the zinc surface.
As the formation of carbonic acid takes time to occur, inert catchment corrosion is normally seen at specific drip points of dew off a roof, rather than below rain washed painted walls and windows.