The present paper deals with the assessment of several realized construction details within the renewed flat roof of the multifunctional building. The new roof structure has the composition of a single-layer flat roof with a classic order of layers, which is mounted on the original roof structure. The problematic details are closely related to the incorrect realization of the waterproof membrane and its fitting to the individual building structures within the detail. Individual factors play a significant role in the assessment: absence of additional elements (cover and anchor strips), improper drainage of rainwater from the roof surface and improper handling of accessories within the waterproof membrane. The assessment of the individual details revealed significant shortcomings that can cause rainwater penetration into the roof structure and thus endanger the reliability and durability of the roof.