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Use classes for timbers

According to the European standard EN 335, use classes for timbers are defined as follows:

Use class 1internal, dry, u max. 20 %
Use class 2internal, u occasionally > 20 %
Use class 3.1external, no ground contact, protected, u frequently > 20 %
Use class 3.2external, no ground contact, not covered, u frequently > 20 %
Use class 4.1external, with ground contact, permanently > 20 %
Use class 4.2external, heavy duty with ground contact, with water contact, permanently > 20 %
Use class 5in salt water, permanently > 20 %

u = moisture content

The required protective measure (selection of preservative, retention amount impregnation process) is geared to the timber’s intended use class. The application field, retention amounts and processes for the preservative are specified in the official approvals.