Decay and drying cracks affect the strength of historic timber structures. Because of the traditional timber dimensions used, drying cracks are often large, decreasing the effective cross section of members. Deterioration can also be caused by biological decay, depending on wood species, climatic conditions, structural detailing, etc. Decay can affect the load carrying capacity, having considerable economic and cultural consequences. However, when the decay is no longer active and the damage is limited, repair is not always necessary. When the reliability of the structure is above a certain minimum acceptance level, it is still able to fulfil its task and can be maintained in the original state as much as possible. A method to determine the safety of deteriorated structures is lifetime modelling using damage accumulation models. It allows for the assessment of structures, combining both the influence of mechanical loads (live, wind and snow loads) as well as reduced strength values caused by cracks or decay. Some of the parameters of the model have been determined on the basis of time to failure tests on timber and timber joints, already lasting 40 years. The influence of cracks and decay on the strength of timber members is presented and the consequences for the lifetime expectancy of structures are shown. The model accounts for the level of decay, which is generally determined via in-situ measurements, the rate of decay, as well as the residual strength of decayed timber. The latter two are generally determined in laboratory or field experiments. Different safety factors can be used in the model and future maintenance schemes can be developed. Practical examples are given dealing with cracked beams and with the residual lifetime of decayed piles.
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