Influence of surface checks on wood moisture content during wetting and re-drying

Wood decks are popular outdoor wood products in Japan, and their durability is the most important aspect for their long-term use. To prevent decay, wood members should be maintained at a low moisture condition. However, surface checks occurring during long-term use may make the moisture content inside the wood members high, causing decay. In this study, water absorption and drying tests of flat grain redwood ( Sequoia sempervirens ) and Japanese cedar ( Cryptomeria japonica ) specimens with or without a sawed slit of 10 mm or 20 mm depth were carried out. Sectional moisture content distributions of the specimens were examined in detail with an X-ray densitometry method. Consequently, the influence of slit depth of an artificial surface check on the water absorption and drying processes was revealed. Moisture content mappings at 1 mm resolution around the slit showed that the water absorption region along the tangential direction from the slit was approximately 1 mm wide after 24 h, regardless of the species and slit depth. On the other hand, high moisture content after the drying period was observed only in a specimen which had a 20 mm depth slit. For the 20-mm depth slit specimens, the moisture content at the bottom of the slit remained more than 30% after drying for 8 h, which is a moisture level sufficiently high to cause decay. The results of the tests indicate that checks that reach 20 mm in depth from the surface may increase the risk of decay.