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.
High-risk areas such as around flues, near fumes from exhaust fans, under television aerials or overhanging trees, sites prone to mould, lichen and bird droppings or debris, all need to have extensive manual washing. Proximity to a motorway can cause the collection of fine dust, as can earthworks in the vicinity, and shavings from wood processing plants are often blown into crevices under flashings which retain moisture and can create 'poultice corrosion'.
Bird droppings should be cleared away regularly to avoid premature failure at ridges or bird perches. Canopies above load-out doors have large unwashed areas on their undersides and service station canopies are subject to a harsh environment so they should be treated as a special maintenance cases.