Discoloration of building facades due to airborne algae is observed in our surroundings. The growth conditions of these algae are not fully clear yet, and efficient preventive measures have not yet been determined. This study was aimed at investigating the influence of ambient environment and building structure on algal growth. A residential building in the cold region of Japan was surveyed. The roof was a multilayered structure consisting of a semi-transparent film, an air layer, an outside insulation layer, and was supported by rafters. The soiled state was visually observed and recorded through pictures, and seemed to be particularly increased in autumn. Several black stripes appeared on the northeast (NE) roof four months after its cleaning. The soiling first appeared on the film backed by the rafter, and then extended to the film backed by the air layer. It rarely appeared on the southeast roof. The roof-surface temperature was measured and a stripe-shaped distribution was observed. The temperature of the film with rafter was higher and lesser than that of the film with the air layer during the night and in the early morning, respectively. Although condensation occurred nightly, its frequency showed no orientational difference. Algae can die when exposed to a temperature higher than 40 °C. The southwest roof had the longest period of a surface temperature over 40 °C, while the northwest (NW) roof had the shortest period of this surface temperature. These measurements corresponded well to the survey results according to which soiling mainly occurred on the NE and NW sides of roofs. The time for algal growth was estimated assuming that algae can grow at surface temperatures between 0 and 40 °C.
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