The paper considers the reinforced concrete monolithic floors, realized like beam-type construction. The slab is represented as a complex construction system of some intermediate level, which is a constructive element of the next higher level - the old industrial buildings of urban area in St. Petersburg. Overlay consists of the main beams (Gk), secondary beams (Wk), slabby sections (Fk) and columns (Lk), which are considered as subsystem elements.

The methodology, based on technical diagnostics theoretical apparatus using probabilistic methods of recognition of complex technical systems states, is proposed for the analysis of elements condition and for determination the categories of the technical state. Diagnosis is performed by statistical methods using the generalized Bayesian formula.

The analysis of the obtained results is executed using the apparatus of information theory, in particular using the concepts of information entropy, after calculating the posterior probabilities of subsystem elements states and the constructive system "overlapping" state by the indicated dependence. The category of the technical condition and the probability of this condition are appropriated to the constructive system "overlap" by the survey and probabilistic analysis results. Then the degree of definiteness of the system V (U) is calculated. The value of V (U) is an input of the "overlap" state in the state of the next, higher level system - building as a whole.