“Suriawase”and“Kigoroshi”are traditional constructive skills to improve joint adhesion and prevent rain water from entering joint of deck boards, which are applied to some of the traditional wooden bridges in Japan. In order to verify their effect, we exposed deck board samples with four different joint surfaces（control, Kigoroshi, Suriawase, and combination of Kigoroshi and Suriawase）for about four months. We measured the gap between the deck boards and moisture content of the joint surface. We also monitored ingress of water to the joints by original moisture sensor. As a result, it is suggested that Kigoroshi is effective to prevent rain water from entering joint of deck boards. It is because restoration of joint surface compressed by Kigoroshi improves joint adhesion. On the other hand, Suriawase makes joint surfaces fit each other and improves joint adhesion, but it does not appear to be effective for keeping water out. When it comes to large deck boards, using both of them keep rain water out for a long time.

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