We analyzed of heat transmission across three roof structures ; bare RCC (reinforced cement concrete) roof, RCC roof with weathering coarse and RCC roof with PCM (phase change material) above RCC. A transient numerical procedure was developed. ANSYSFluent 12 finite volume method based software was used for solving the problem. The numerical procedure was validated against the available experimental data. The simulation was carried out on 365 days of the year for these three roofs. The PCM melted from 8 h to 18 h and solidified during the rest of the day. From March to August, the net heat entering into the room per day is positive, thus more and more PCM melts and becomes liquid. The liquid portion of PCM increases from March to August, during September to February, there is a net heat rejection per day, thus the melted PCM becomes solid progressively, and at the end of February, almost all PCM has become solid. Thus there is a melting cycle on a daily basis and over the year. It takes care of all the external climatic variations and keeps the roof bottom surface temperature almost constant and close to room air temperature. On yearly basis, about 56% reduction in heat transmission into the room is obtained with PCM roof when compared to the conventional weathering coarse laid roof.