In course of the conservation and renovation of historical timber buildings in Wenzhou, Zhejiang Province, we found serious decay at the bases of wooden pillars. By analyzing the decay problem, consulting relevant domestic and overseas specifications and experiences, as well as conducting field research, instrumental tests, material analysis and also statistical data analysis, we identified the main cause of the decay:condensed water. In addition, we determined that the traditional methods of waterproofing the wooden pillars could effectively solve the problem of decay.