Typically, the geometry in most compartments is such that the ceiling is flat (hor- izontal): when a fire develops and grows, it can reach the ceiling creating a ceiling jet where flames extend horizontally beneath it. Nowadays, advances in architec- tural and building design see continuous innovation: this includes the utilization of curved structures, curved ceilings and structures such as tunnels. Extensive lit- erature is present regarding tunnels [6], but not much focus is put into the flame extension aspect. For this reason, in this thesis the study of flame extensions un- der curved ceilings will be undertaken further and a comprehensive analysis of the various factors that contribute to their flame’s behaviour in such settings will be conducted. Furthermore, a comparison with the same phenomena occurring under flat ceilings will be also outlined in order to provide greater knowledge regarding similarities and differences between the different ceiling geometries and their effect on flame extensions.