This part of EN 599 specifies for each of the five use classes defined in EN 335-1, the biological tests required for evaluating the efficacy of wood preservatives for the preventive treatment of solid timber, together with the minimum ageing tests required for the respective use class. It provides the method for calculating the critical value of a preservative. The critical value is the value that shall be used to calculate the recommended retention of the preservative appropriate for specific service conditions. The critical value is not necessarily the recommended retention or the minimum retention level for the preservative. The wide range of hazards, exposure conditions and service life requirements across Europe make it necessary to allow for local considerations in the calculation of the required preservative retention; EN 351-1 provides for the critical value to be adjusted to take account of these factors. This part of EN 599 is applicable to all wood preservative products supplied for application in liquid form for the preventive treatment of timbers (structural and non-structural) against wood-attacking fungi, wood-attacking insects and marine borers as described in EN 335-1. However, it is applicable to products for preventive treatments against fungi causing disfigurement (blue stain) of wood in service if this forms part of the overall preventive efficacy of the product.