Abstract Roof with outer-layer shape-stabilized phase change material (RSPCM) is a building construction that incorporates shape-stabilized phase change material (PCM) into the out layer of the roof. The decrement factor of the roof and the peak temperature of the inner surface can be greatly reduced by using the PCM in summer. CFD numerical simulation is used to investigate the thermal performance of RSPCM. The effects of the phase transition temperature, layer thickness and phase transition temperature radius of PCM are studied numerically. Results show that the suggested PCM thickness is 30 mm and the temperature radius should be as small as possible. The optimum phase transition temperatures are 31–33 °C, 34–36 °C, 36–38 °C, 34–36 °C, and 29–31 °C respectively in severe cold region, cold region, hot summer and cold winter region, hot summer and warm winter region and mild region. The decrement factors at the corresponding optimum phase transition temperature are about 0.030, which are lower than that of the roof without PCM by over 85%. The peak temperatures of the inner roof surface are all decreased by over 3.7 °C. These results can be used as a guideline for optimum design to improve the dynamic thermal performance of roof structure in summer.