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Burning characteristics of botanical roofs, which are often used for cultural assets buildings in Japan, have been investigated through a series of experiments. Based on the experiments, suitable fire detecting system for such kind of fires are discussed and proposed. The rate-of-rise heat detectors are installed in the attic, on the ceiling and under the eaves, for cultural assets buildings. However, in 30% of fires at cultural assets buildings, fires break out from their roofs and those detectors beneath the roofs are not proved to be suitable for the early detection of such kinds of fires. For the purpose of earlier detection of the roof-fires, a series of fire experiments, in which the roofs are ignited from outward, are conducted and the rate of the fire spread are observed. In the experiments, sensors of various kinds are installed and the response of them is also evaluated. Those experiments reveal that fires on the roof surface can be detected in early smoldering stage by smoke detectors with wind, and temperature rise is only detectable at a point just opposite to the fire point at that time. Flame sensor is proved to be effective to detect roof-fires in very early stage.