Wood decks are used in outdoor condition and their durability, such as decay resistance, is most important property for safety use. To prevent biodegradation, wood should be maintained at low moisture condition, but surface checks occurring in long time use may make inside moisture content high. In this study, surface check observation and visual decay grading of sugi（Cryptomeria japonica ）, hinoki（Chamaecyparis obtusa ）and redwood （Sequia sempervirens ）decks exposed outdoor for 10 years. Water absorption and drying test of redwood with sawn slit（0.5, 1, 2cm depth）were also carried out. Number of checks were large in hinoki decks and checks depth of 2cm were observed in sugi and hinoki decks, in contrast with 1 cm depth in redwood decks. Sugi and hinoki decks were also marked higher rot level. As a result of water absorption test, deeper slit on flat grain caused higher inside moisture content. Only in the case of 2cm depth slitted specimens, moisture content at slit bottom was kept enough high for wood decay after 48 hours drying. Results of observation and water absorption and drying test indicate that over 2cm depth check on wood surface may increase risk of decay significantly.

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Appropriate preservation measures for lumber, at the time a house is newly constructed, and routine maintenance, after initial occupation, are essential steps for making wooden houses long-lasting. In this study, we conducted a survey on biodeterioration damage related to rot damage and termite damage in 37 houses that were scheduled to be dismantled in the Tohoku, Kanto, Kinki, and Kyushu regions of Japan. The results confirmed some type of biodeterioration damage in 36 houses. Of those, this type of damage had extended 1m or more above the ground level in 35 of the houses, and several instances were confirmed in parts of the house, which are not required by the Building Standards Act to have preservation measures taken to prevent deterioration or termite damage, such as roof trusses and second-floor apertures. Furthermore, the results suggest that, in cases in which waterproofing function was lost with the passage of time, there was the possibility for biodeterioration damage. This was true even for wood that had been surface-treated with medical substances and for durability classification D1 tree species that matched components indicated for preservation measures to prevent deterioration or termite damage, as established in laws related to the promotion of maintaining the quality of residences （“quality maintenance laws”）. Furthermore, we also observed several cases of living organism deterioration damage that would not have been discovered in an ordinary inspection. When biodeterioration damage occurs in construction components, repair is often time-consuming and expensive. Reapplying or reinjecting preservation and termite-proofing substances is pragmatically impossible, so in order to make wooden houses long-lasting, we believe that it would be effective to take measures for preservation treatments that can improve the long-term effectiveness of preservation and termite-proofing measures.

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