The dynamic behaviour of horizontal diaphragms plays a fundamental role in the seismic response of existing buildings. Indeed, it is well known that the floor diaphragms are responsible for the dynamic coupling and collaboration among the resistant elements, as well as for the redistribution of the seismic action in the nonlinear regime. The fulfilment of these fundamental structural tasks, which depends on the diaphragm in-plane stiffness and on the effectiveness of the boundary connections, can be particularly critical in existing masonry buildings, which are often characterised by timber floors or vaults that tend to exhibit a nonrigid in-plane behaviour. An in-depth knowledge of this subject is mandatory to guide the engineering judgment in the a priori assumptions governing the mechanical modelling, as well as to enable the a posteriori validation of the seismic assessment. To this purpose, the authors have developed a model-driven procedure to investigate the dynamic behaviour of the diaphragms by means of ambient vibration measurements. The procedure, employing at least two biaxial sensors for each floor diaphragm, solves an inverse kinematic problem to discriminate and estimate the diaphragm rigid rotation and macroscopic angular deformation from vibration data. A perturbation approach can be employed to run the procedure also in the typical operational case of minimal sensors availability. The rigid, quasi-rigid or deformable diaphragm behaviour of the observable structural modes is evaluated in the frequency domain, comparing the magnitudes of the power spectral densities associated to the rigid rotation and macroscopic angular deformation. Although the algorithm has already been successfully validated by means of laboratory data and tested through pseudo-experimental simulations against adverse field conditions, this paper investigates its application to full-scale measurements acquired on two existing unreinforced masonry (URM) buildings in Italy: the Pizzoli (AQ) town hall, in Abruzzo region and the Sanremo (IM) town hall, in Liguria region (Northwest Italy). The first application is supported by recordings from a permanent monitoring system installed by the Italian structural seismic monitoring network, whereas the latter by in-situ ambient vibration measurements. Emphasis is given not only to the capability of the procedure to support the calibration of numerical models, but also to its possible repercussions on the seismic assessment, in particular for simplified vibration-based approaches adopting the rigid diaphragm assumption. With this purpose, a more refined equivalent frame model of each structure \u2013 suitably calibrated to match the experimental modal parameters identified from the experimental measurements \u2013 is used to verify how finer rigid-diaphragm discretisations can improve the estimation of the mass participation factors, even for diaphragms commonly assumed as perfectly rigid
