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.
COP v3.0:durability; accessories
All cladding fasteners must be compatible with the material, suitable for the environment and a durability equivalent to that of the cladding material. All exposed fasteners must have a minimum durability of Class 4. (See 17 Testing and MRM Standards)
Only aluminium or stainless steel screws and washers should be used on pre-painted aluminium roof and wall cladding. Stainless steel fasteners must not be allowed to come into contact with the cladding and should be installed through oversize holes.
Sealing washers must be non-conductive to prevent electrical contact between the screw, the metal washer and the cladding surface.
Steel cladding screws can be subject to hydrogen embrittlement when they are hot dipped galvanised. Alternative methods of galvanising, such as peen plating and other metallic coatings, are generally used in combination with an organic coating.
Care should be taken to minimise damage to the head of the nail or screw when using colour matched painted hot-dipped galvanised nails, bolts and screws.
Sandblasting in exposed conditions can significantly reduce the coating thickness and the longevity of the fastener.
Premature failure can result when the shanks of the screws and the eaves purlin are exposed to sea spray, and a high-fronted gutter is recommended to help prevent this.
The performance of the shank of the fastener is also affected by internal environments when the contaminant is inside the building, e.g., animal shelters, fertiliser works.
All fasteners should be easily identified by a code stamped on the head to identify the manufacturer and the coating class.
To avoid damage to the coating to the screw-head, drivers should not be of the impact type and should be fitted with a snug fitting driver bit.
Sealant should be neutral cure silicone or MS polymer.
Underlay should have durability no less than the cladding material, and be compatible in contact with the roofing material.
Galvanised wire netting or mesh can be used to support underlay where the internal environment is not aggressive, but is not to be used with painted aluminium.
Plastic mesh, tape or string may be used to give support at a maximum of 300 mm centres. These alternatives should be used when underlay support is required with aluminium cladding. Ensure that the underlay-support fasteners do not come in contact with the underside of the cladding.
For buildings with harsh internal environments, stainless steel, or PVC-coated mesh and netting are available. PVC-coated mesh may suffer degradation from UV radiation if used externally or when it is exposed to high levels of direct or reflected sunlight.