The thermal performance of conventional flat roof systems is in practice normally assessed by the heat transfer caused by conduction. The effects of radiation and convection are in such cases ignored as being insignificant. Holistic flat roof systems are designed to better utilize the impact of radiation and convection on improving the indoor climate. However, to determine the influence of radiation and convection, an integrated assessment should be used. In this article, the effect of applying five different holistic flat roof systems is compared with a conventional roof system of an existing storage hall. With EnergyPlus the internal air temperature in the storage hall was simulated for a complete year using weather data of the moderate Dutch climate. The most efficient systems prove to be the systems that decrease the heat gain by solar radiation. These systems can decrease the number of overheating hours by up to 95%. Increasing the thermal resistance value of a roof could lead to adverse results. The study shows an integrated approach that could be implemented in policies and regulations for the thermal assessment of holistic flat roof systems