The Nordic Wood Preservation Council (NWPC) has since 1976 had a standard for classification of preservative treated wood, documents describing conditions for approval of preservatives for industrial use and requirements for quality control of preservative treated wood. Last year, 2017, the NWPC extended the quality system to include natural durability (heartwood) and modified wood. The objective of this paper is to present and discuss some examples of premature failure caused by decay of exterior wood structures exposed above ground, use class 3, in Norway that could have been avoided by following NWPC’s requirements for preservative treated wood, heartwood and modified wood. Five different examples of premature failure caused by decay of external claddings, window joinery and decking are presented and discussed. Based on the examples it is concluded that inclusion of wood with considerably lower durability (e.g. sapwood), insufficient treatment or removal of protection layers through machining/crosscuts, or by using protected wood in use classes with higher risk of deterioration than it is intended for will often lead to premature failure, increased costs and larger environmental impacts. End grains of cladding and decking boards need extra protection due to moisture ingress and accumulation in end joints. Heartwood products need to be carefully sorted to avoid inclusion of non-durable sapwood. Treated wood products (both preservative and modified) need a quality scheme that comprises internal quality procedures as well as third party audits to assure that the performance of the products comply with the requirements of the end use/use class. The presented examples of premature failure caused by decay would have been avoided if the products had been produced and treated in accordance with the requirements described by NWPC.

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