•TRNSYS Type 1270, PCM component was validated with experimental data in literature.•Validated component was used to simulate PCM as a roof layer in a residential house.•PCM roof layer can reduce both the annual cooling and heating loads.•PCM roof layer can reduce both peak cooling heating demands.•The optimum phase transition temperature for Melbourne and Sydney is 23°C. This paper assesses the effectiveness of Phase Change Materials (PCMs) for the improvement of the thermal performance and the thermal comfort of a residential building in Melbourne. The incorporation of PCMs in buildings with their significant heat storage capacity can delay the heat transfer and reduce the cooling and heating loads. Numerical simulation is a useful tool for comprehensive assessments and optimization of PCM applications in buildings. Thus an available TRNSYS component, PCM Wall: Type1270, was implemented with Type56 (Multi zone component). PCM Wall TRNSYS component has been validated with some experimental data published in the open literature. The validated model was then utilised to simulate the thermal performance of a residential building which has a PCM roof layer. The building is a typical single-storey, three bed room residential building in Melbourne. It was found that the PCM roof layer can reduce the cooling and heating loads whilst providing better thermal comfort for occupants with reduced indoor temperature fluctuations.