This paper seeks to determine the trigger of faint heat generation, which was one of the causes of fires in the piles of rubble produced after the 2011 Great East Japan Earthquake. The rubble piles consist of various combustible materials, such as wood, paper and plastics, as well as non-combustible materials from destroyed buildings. Multiple rubble outdoor storage areas were temporarily set up in the disaster areas to dispose of rubble after it was piled up and separated. However, fires have been occurring in these storage areas since the summer of 2011. One cause of such fires is thought to be the heat generated by fermentation of microorganisms that easily proliferates within the piled up rubble, ultimately resulting in spontaneous ignition. In this study, we collected samples from rubble outdoor storage areas where fires have occurred. We then conducted thermal analyses to obtain basic data, and finally, we developed safety measures to prevent the recurrence of fire. The results revealed that the heat generated during fermentation of rotten tatami and wood chips most likely acts as a trigger causing the fires, given the adiabatic conditions are good. Copyright © 2014 John Wiley & Sons, Ltd.