Creep and the creep-fatigue interaction have shown to strongly influence the mechanical behaviour of ancient masonry and long-term heavy loads proved to cause a continuous damage. The problem of achieving a reliable life-time estimate of historic masonry toward the effects of persistent loading has been dealt with by a probabilistic approach. The results of pseudo-creep tests on ancient masonry of different ages and their interpretation through a probabilistic model are presented, aimed to the individuation of a random variable as a significant index of vulnerability, and to the solution of the classic problem of reliability in stochastic conditions