Reduction of energy demand in residential buildings is an issue of permanent research interest. Every building component has a role in the energy performance of a whole building, especially the building envelope - walls and roof. The problem related to flat roofs in Novi Sad, Serbia is recognized and will be addressed in this thesis. Typical residential buildings built in two prefabricated systems were selected and analyzed using a simulation tool - EnergyPlus. Models were simulated in the original and post refurbishment state, followed by an evaluation and comparison of obtained results. The aim of the thesis was to verify the assumption that the green roof system can be a better measure for the refurbishment of flat roofs in Novi Sad in terms of energy efficiency, as well as in the sense of architecture, than currently offered options. In the interest of obtaining unbiased, applicable and comparable results of green roof performance, another two roof types were additionally simulated. These are a refurbished roof with new thermal insulation, that was analyzed for both heating and cooling performance, and a white roof without insulation, that was analyzed only for its cooling performance. Furthermore, the performance of a green roof depending on the building type was analyzed. Final results showed that green roofs have the potential for improving a building's thermal performance, although there was no substantial difference between results obtained in a case of green and refurbished roof. However, other advantages of a green roof, that were outlined in the thesis, can outweigh in its favour when choosing the option for retrofit.