Experiments to investigate interfacial heat transfer mechanisms during casting solidification were carried out by varying the surface roughness of a Cu chill used to bring about unidirectional solidification of an Al-4.5 wt.% Cu alloy. Little variation in interfacial heat transfer coefficient with varying chill surface roughness was found, confirming previously published results. Examination of the as-cast surface of the casting showed the presence of predendritic contact areas, and also that the casting surface roughness did not form as a replica of the chill surface, as has often been proposed. Rather, the casting surface roughness was consistently greater than that of the chill, but varied little in the experiments. A sum surface roughness parameter was devised to characterise the casting-chill interface that included the roughness of both surfaces. The value of this parameter was strongly influenced by the greater roughness of the casting surface, rather than the chill surface roughness, and therefore also varied little in the experiments. This lack of variation in the casting surface roughness and hence the sum surface roughness parameter showed how interfacial heat transfer should be more strongly influenced by the greater roughness of the casting surface than of the chill surface, and explains why the interfacial heat transfer coefficient was not strongly influenced by the chill surface roughness in these types of experiments.