Timber bridges have been built for decades all around the world. The hygroscopic material behavior of wood leads to the change of the moisture content of the wood and the dimensions depending on the climate. Therefore in regular inspections following questions arise: what happens with the wood due to the climate changes? Are there major changes of the moisture content? Are there differences between the natural material axes or within the cross section of the structural members? To answer these questions, traffic timber bridges with big cross sections are long-term monitored within a research project. The results of the moisture contents measured and a comparison between the different measuring groups and positions are presented. The analyses confirm that the moisture content in the wood follows the climate changes delayed and with smaller amplitude against the calculated equilibrium moisture content. In first steps, a different behavior of the change of the moisture content could be determined over the cross-section and along the span of the member.