The use of wooden parts in roofs has been a common practice for centuries, mainly as rafters or as wooden cladding or sheathing. With the increase of thermal resistance of the roof, the use of more and more insulation combined with air and vapor barriers, more problems occur. In the last few years, there started problems by trying to use the evaluated structure ventilated pitched roofs as a single ply flat roof, which led to the complete decomposition of the sheathing and also to the rotting of wooden rafters. Lately, a similar problem has also occurred within the pitched roof. A common problem of these roofs is the ignorance of the layers with low water vapor permeability. This paper deals with a case study of the apartment building's pitched roof, in which condensation occurs. The required evaluation of building envelope parts is mandatory by the Slovak standard, but unfortunately a lot of authorized engineers are not able to understand the results. Based on the experience from a building survey, non-steady simulation in WUFI Pro software was carried out. This confirmed the inappropriate composition, which needs complete reconstruction of the roof.