Deterioration of nail was graded by a five - rank numerical - rating. The rating of nail moisture content and decay of wood were inspected in a mortal-wall of 34-year-old house. The wood was mostly in decay, when the rating of the nail was above about 4, and the moisture content was above about 20%. Nailed wood specimens kept in several humidity room of 20°C for 4 years showed that the rating of the nail was about 3 in the wood with moisture content 20%. A curve of the rating of the nail against the service year was obtained from the inspection of 524 nails in the mortal-wall. This curve could be a standard life curve of the nail in mortal-wall in the central part of Japan. The curve obtained by connecting the points (4 years, rating 3) and (34 years, rating 4) may be a wood decay warning line in terms of high moisture content. Shear tests for deteriorated nailed joints showed that the load was expressed as function of the slip and the amount of the rust of the nail. The nail used for bevel siding wall is more likely to be influenced by the environmental factors. From the inspection of the nail, the northerly wind was found to be influential to the deterioration the nail in an area of Tokyo.

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