This paper aims at establishing a model for the service life prediction of natural stone wall claddings directly adhered to the substrate, based on a survey of the degradation state of 142 buildings in the Lisbon area. The state of degradation is evaluated through visual inspections that allow the description of the extent of the cladding area affected by the different anomalies, thereby enabling the level of severity of the anomalies detected to be established. A numerical index of the degradation severity of stone claddings (Sw,rp) is established; this is intended to express the global degradation level of these claddings, thus allowing the study of the evolution of their degradation with time in order to estimate a reference service life for this type of cladding.