Add to ORKGThe results of creep and pseudo-creep tests, carried out on the masonry of an ancient structure, are presented and their interpretation by means of a probabilistic model is proposed, based on the definition of a random variable as a significant index of vulnerability, and on the solution of the classic problem of reliability in stochastic conditions. A comparison between vertical and horizontal strain-rate is put forward and the application of the proposed procedure to the Tower of Monza is attempted. Aim of the research is to provide a mathematical model able to predict possible failures of heavy masonry structures due to long term damage, allowing preventive repair interventions