One of the most discussed flat roof structures during the last 20 years is a structure called inverted roof, where the main thermal insulation layer is placed above the main waterproofing system. The reasons why this type of flat roof is or could be chosen are more less clear. Usually it is the intention to protect the main waterproofing system, usually of synthetic or bituminous membranes, against the impact of outdoor air thermal changes, against any prospective mechanical damages and also to reduce risk of water vapor condensation in the structure. This type of structure could help to solve the vapor/thermal difficulties during the design of the flat roof over the space with higher indoor air humidity like swimming pools or specific industrial processes. Due to the higher rate of safety against mechanical damage it is also used quite often in the case of the design of the roof terraces or roof gardens. Nevertheless the correct attitude during the design of the structure is to take into considerations all possible aspects including the defects and problems which are most typical for each one type of structure. This paper is willing to give the brief overview of the typical defects for inverted flat roofs and to contribute a little to the understanding of commonly discussed effect of undergoing water beneath the thermal insulation itself and decreasing thus the thermal protection efficiency as well as the inner surface temperature.