<p>Underground coal fires (UCFs) have caused disasters in most coal-producing countries, and their dominant burning mode is smouldering combustion which is controlled by oxygen supply. Cracks and fissures in the overlying layer(s) provide air (oxygen) to sustain the smouldering propagation underneath. The effects of multiple cracks’ spacings and their distribution on smouldering characteristics have not been addressed in the literature. With some reasonable simplifications and assumptions, a lab-scale experimental setup comprising a packed coal bed and an overlying layer with multiple cracks was constructed to fill the research gap. Three experimental cases were designed to investigate the effects of cracks’ spacings in order to identify the limiting conditions. The coal bed temperature and gaseous emissions were measured, based on which it was found that the limiting crack spacing (<i>L</i>_c) for self-sustained smouldering propagation is about 6&#xa0;mm. It is interesting to observe that the propagation was not in a sequential manner, that is, positions further from the ignition source might enter the smouldering mode earlier under proper thermal (temperature) and chemical (oxygen content) conditions. The cracks played an important role in determining the smouldering intensity underneath, depending on two main properties of each crack, they are, its spacing and oxygen concentration in it.</p>