Re-evaluation of the Assessment Done by FFPRI to Elucidate the Natural Durability of Heartwood

Approximately 50 years ago, the Forestry and Forest Products Research Institute (FFPRI) conducted field tests (stake tests) to assess the natural durability of heartwood of various wood species. FFPRI examined the damage of heartwood stakes during the testing period and determined the average service life of each stake. The natural durability of each species was classified according to the average service life, and this classification has influenced the Japan Agricultural Standards for sawn lumber (JAS 1083) and the Housing Quality Assurance Law. However, inspection of the classification process revealed ambiguity because the durability of wood species was determined only by their average service lives. Differences in the number of stakes and variations in service lives in each species were not considered. Hence, we applied survival analysis to this existing data and evaluated the classification more precisely to consider these factors. The evaluation showed that there are differences in the tendency of deterioration depending on the wood species, i.e., there are species in which the durability of all stakes is low, ones in which this value is high, and others in which this value depends on the stake. The re-evaluation of the durability by a survival analysis revealed that the durability of kouyamaki is higher than that of Japanese cypress and hiba, both of which are considered to be highly durable. It also revealed that differences in the durability of species considered durable, such as Japanese cedar and Japanese larch, and that of species considered highly durable, such as hinoki and hiba, are not significant.