Preserving historical buildings is essential in the safeguard of the cultural heritage of any country. The need to carry out structural analysis with non-destructive methods gave structural monitoring a widespread fame in the world of diagnosis and control of historical buildings. The aim of this study is to introduce a standardized approach for the analysis of the data acquired from a monitoring system of an historical building, through the definition of specific reference quantities (extrapolated from the recorded time series) able to characterize the main features of the structural response and the preliminary identification of the order of magnitudes of these quantities. It is assumed that the recorded time series may be decomposed into two fundamental components: the first one related with the natural actions and characterized, in absence of extreme events, by a substantial periodic behavior, the second one related to the other factors such as the evolution of the state of the structure due to material degradation, soil settlements and others. Exploiting the properties of periodic functions, one may identify these reference quantities, which are based on the year and the day variability and allow to monitor the evolution of the phenomena under observation. These reference quantities may be collected in a database and may become fundamental for comparing the response of similar buildings. This methodology has been applied to the data obtained from the monitoring system of the Cathedral of Modena