According to DIN 18807, part 1, corrosion protection class II is sufficient for the underside of trapezoidal steel profiles, this is e.g. a 12µm thick polyester coating on top of a galvanised layer.
Since the underside of canopies are not subject to any precipitation and thus washed off like normal exterior walls and roof surfaces, aggressive, acidic dirt settles over time. The same applies, by the way, to wall surfaces under canopies that are not exposed to precipitation.
The acidic dirt formation is favoured by heavy vehicle traffic and the corresponding exhaust gas pollution.
Temporary condensation, caused by sudden changes in temperature, leads to the formation of sulphurous acid or sulphuric acid, which strongly attacks the corrosion protection of the construction elements
Unless undersides of canopies or wall surfaces under canopies are regularly washed once a year with clear water, we recommend corrosion protection class K III for these areas at risk of corrosion, e.g. a 25µm thick strip coating on strip or alloy galvanising (Z 275 or ZA 255).
Netphen, July 2001