For the client the material and color of the exterior wall are important, because they affect the impression of the building [1]. However, discoloration of the exterior wall begins as soon as the building is completed, by various factors such as rainfall. We tried to quantitatively evaluate the changes of the exterior wood, and to organize the influences of climate conditions, in order to use the discoloration as design factor. It seems that the difference in the color changes in an exterior wood surface is remarkably influenced by what season the test begins. In order to confirm the difference in the color changes, the starting test month was shifted by one month for each specimen. Those plates were scanned at some intervals to get image data and the chromaticity was measured with a colorimeter. As a result, the following was clarified. The color change was divided into the following two patterns. A: Red → yellow → white → black, B: Red → yellow → black. Black discoloration occurred due to the plate material reaching severe climatic conditions after the ΔL * value reached the maximum value. Lignin is eluted from wood with rain and decolored with ultraviolet rays to turn white [2]. However, it seems that the plate material becomes black as soon as it reaches high-temperature and humidity (without passing through the white period). We believe that the white period is the period until it is hit by hard climate conditions. When mild conditions continue, the white period is extended. Initial discoloration is slow under mild conditions.

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