The durability performance of metal roof and wall cladding depends on the macro- and microclimates, airborne contaminants, and the material itself.

The macroclimate is the general environmental category where the building is situated.

The microclimate relates to the exact location of the building and the design or position on the roof or wall. Microclimate influences include geothermal fumaroles, rain sheltering, topography and ground roughness, prolonged wetness, and exclusion of oxygen. Internal microclimates can also occur as result of the particular use of the building.
Contaminants and pollutants are corrosive influences which can affect the cladding. These can include fertiliser, soil, leaf fall-out, exhaust fumes, industrial fumes, bird droppings and the build-up of debris. Influences such as chlorides near the sea, geothermal hydrogen sulphide (H2S) or man-made gases such as sulphur dioxide (SO2) accelerate the corrosion rate by increasing the conductivity of the electrolyte and changing its pH value.
Rain provides the moisture that acts as the electrolyte in corrosion cells. Rain varies in pH because it picks up various contaminants from the pollutants in the atmosphere. Acid rain can happen in geothermal areas due to the presence of hydrogen sulphide in the atmosphere.
At 0°C metal corrosion is minimal, because colder temperatures slow the reaction. The corrosion rate of some metals doubles with every 10°C rise in temperature given the same time of wetness and environmental conditions. However, in dry, warm environments the time of wetness is decreased by faster drying times, which has the opposite effect.

Designers should be aware of macro- and microclimates and the degree of contamination. They should design their building and select materials considering a combination of all these factors.

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