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.
Poultice corrosion or 'under deposit corrosion' is caused when a collection of fine dust — e.g., from earthworks and quarries, or sawdust and shavings from timber processing plants — collects in crevices behind laps or flashings.
These collections increase the time of wetness and retain contaminants.
Bird droppings can contain highly corrosive and adhesive materials that will affect metallic cladding. Birds can also deposit other injurious material, e'g', fish frames and chicken bones.