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.
Because aluminium roof and wall cladding are often used in severe and very severe marine environments, direct contact with an absorbent permeable underlay must be avoided in areas subject to salt spray ingress.
Corrosion at the interface of the cladding and the underlay is likely to cause pitting where marine salts can gain access to the roof cavity and the time of wetness (TOW) is high. This restriction applies at all exposed ends of the sheets, such as at spouting, gutters, and valleys; in most cases, it extends to the penultimate purlin. In these places underlay as described in 10.7.5 Non-Residential Buildings is recommended as the separating medium between the aluminium sheeting and the purlin.
Wire netting (including plastic coated wire netting) must never be used as an underlay support without separation to the netting.
Self-supporting permeable underlay should be fixed at the roof underlap with stainless steel staples, avoiding contact with the aluminium, and should not extend more than 20 mm into the gutter to avoid wicking.
If the pitch is less than 8°, it is recommended to use a self-supporting synthetic underlay laid horizontally and stretched tight between the purlins at less than 1100 mm with durable synthetic string or strapping attached to the face of the purlins.
Note: As corrugate is not laid at a pitch of less than 8°, self-supporting underlay can always be used for this profile, without any additional support.
Aluminium sheeting, flashings and cappings must not be fixed directly to butyl membrane gutters or roofs for the same reasons as given above, but must have ventilation and underlay as described. Impermeable separators such as DPC can also retain moisture and ultimately cause underside corrosion.
As required with other profiled metal roof cladding, ventilation is necessary to avoid moisture build-up in the cavity, particularly where skillion roofs or cut-in purlins are used. A minimum gap of 20 mm is required between the underlay and insulation, and if necessary deeper purlins or a counter batten should be used to get this clearance. See 14.6 Installation
Fasteners for fixing aluminium sheeting can be either aluminium, which are suitable for timber construction, or Grade 304 stainless steel. However, to obtain maximum durability, both are required to have a 10 mm oversized hole to avoid bimetallic contact and expansion noise.
Load spreading profiled washers are required when oversized holes are used, and these should be made from aluminium or grade 445M2 stainless steel used in conjunction with an EPDM 36 mm isolation washer. Aluminium embossed washers can also be used on some profiles and flashings.
When fixing aluminium corrugate or trapezoidal cladding, an aluminium gutter apron flashing is recommended to be used. See 188.8.131.52 Eaves Flashing.
To comply with this COP an apron flashing is required where the pitch is less than 10° and the site exposed.
A secret fix roof cladding system minimises the entry of salt-laden air, but when using a corrugated profile, an eave profile closure as discussed in 8.7.1 Soft Edging should be used. The use of high fronted spouting profiles is also recommended to help minimise entry of salt-laden air into the roof cavity.