In DIN 18807-1 (trapezoidal profiles in building construction - steel trapezoidal profiles) from June 87, tables 1 and 2 list the required corrosion protection classifications for both sides of the trapezoidal profiles when used in various building systems for roof, wall and ceiling.

The side of the trapezoidal profiles that must have corrosion protection class I, II or III can be read under each construction system.

Interestingly, the following note is written under both tables: 'For corrosion protection class II, the usual backside coating of 10µm thickness is sufficient for coil coated material'. This additional note was neither necessary nor useful at that time.

This is because the definition of corrosion protection classes is basically only regulated in the relevant, valid DIN 55928-8 (corrosion protection of steel structures by coatings and overlays - corrosion protection of thin-walled load-bearing components).

However, at the time of the publication of DIN 18807-1, DIN 55928-8 from March 80 for KII specifies a required nominal strip coating thickness of only 10µm. (Note: nominal coating thickness, explained in DIN 55928-5, is now called nominal coating thickness; for both terms there are similar permissible tolerances with regard to coating thickness).

However, the revised DIN 55928-8 from July 94 requires a nominal coating thickness of the coil coating of 12µm to achieve this corrosion protection class II. (Remarks: With the publication of the revised DIN 55928-8 from July 94, DIN 55928-8 from March 80 became automatically invalid. For example, for the tolerance measurements of the coils today, DIN 59232 from July 78, which has since been overhauled, is no longer applied, as mentioned in DIN 18807-1, but DIN EN 10143 from March 93).

DIN 18807-1 (trapezoidal profiles in building construction - steel trapezoidal profiles) from June 87 only applies to trapezoidal profiles, but not, for example, to cassette profiles or sandwich elements. For these construction elements, the current DIN 55928-8 clearly applies.

If the 'advocates of the 10µm theory' were right, then in order to achieve corrosion protection class II for trapezoidal profiles, a coil coating with a nominal coating thickness of only 10µm would be required in addition to the metallic coating, and a coating with a nominal coating thickness of 12µm for all other components.