Many historical buildings of the historic city of Venice (Italy) are founded on wooden piles which are very short and typically driven at very small spacing into the soft silty clay in order to improve the mechanical properties of the ground. Being permanently under the groundwater level does not prevent wood decay, indeed some recent evidences show that anaerobic bacteria can deteriorate the wood even in anoxic conditions. On the basis of some experimental observation of the decay of wood mechanical properties, a simple material model is proposed to describe the evolution of strength and stiffness over centuries in a simplified manner. This material model is then applied in finite element analyses to investigate the effect of such degradation on the overall wooden pile foundation response, accounting also for viscous soil behavior. To achieve this aim, at first the behavior of a single pile is analyzed, and then the model is expanded to a pile group. The numerical approach is finally used to analyze a case history: the wooden pile foundations of the ancient Church of Santa Maria Maggiore, which showed high differential settlements along its external walls. Parole chiave: historical foundation, wooden piles, wood degradation, pile modeling.