The service life of structures depends on many factors including the current status of the structures and the age, as well as the expected future use. In order to be able to make decisions on possible interventions, an overview is given of a number of methods available to determine the current state of a structure. Data taken from existing structures (foundations) with and without biological decay as well as literature have been analysed. It has been used to extent an existing damage model for timber structures that takes the load history into account into a model that allows for service life calculations on partly deteriorated structures. This comprises structures or structural elements that suffer for instance from (biological) decay or from drying cracks as is often found in foundations and historic timber structures.