This paper reviews the key publications available on service life prediction of exterior wood applications, critical conditions for decay initiation and progression, and the impact of decay on wood strength. While it is helpful to use climatic data to predict general trend of decay potential, more precise decay initiation and progression information is required by hygrothermal modeling for its damage functions. Most literature is not in conflict with the conclusion that the time required for kiln dried wood products to show significant colonization from unsterile conditions, once they reach a marginal MC for decay (about 26%), appears to be in the region of 6 to 12 months. It may then take a further 3 to 6 months for detectable strength loss to occur. These times vary with sapwood or heartwood, wood species and dimension. They may increase considerably if other conditions are not favourable for decay fungi growth such as low temperature or low oxygen availability, or the moisture level is below the minimum requirement, or simply the wood is decay-resistant itself. This explains why untreated wood can stand cyclic wetting and drying in service to a certain degree. However, under ideal conditions including ideal temperature and moisture (40 to 80% MC) conditions, wood infected with mycelium of decay fungi can lose significant strength in as little as 3 to 6 months. All studies confirmed that the key to prolonging the service life of wood products is to keep them dry and prevent wetting. The use of 20% as a safe moisture level for wood construction certainly accommodates a considerable margin of safety as well as tolerance for moisture control of wood products.

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