AbstractFlat-to-slope roof retrofitting transforms the flat roofs of multistory buildings into the sloping roofs to improve the building function and landscape effect provided that the building structures and the foundation bearing capacity meet the requirements, which normally commits to improve the thermal insulation performance of the top layer and to solve the problems such as the top layer leakage. This study developed an optimal structural design protocol for flat-to-slope roof retrofitting through comparative analysis. The case study used an existing building in Hangzhou city and in the process of energy consumption simulation on flat-to-slope roof retrofitting, control variables were used to keep consistent building materials, building environments, and meteorological conditions such as the hourly temperature, monthly heat gain and loss, and monthly energy consumption. The indoor environment of the building top floor with different gradients settings was simulated in groups, and the hourly temperature, monthly heat gains and loss, and energy consumption of each model were analyzed. This study provided both theoretical and practical references for future flat-to-slope roof retrofitting projects in terms of building energy efficiency.