At this point, all the wood's water is bound within the cell wall. As moisture content changes above fiber saturation, the cell cavities take on or lose unbound water but the wood cell walls do not change dimensionally. Below the fiber saturation point, however, the wood will change dimension with changing moisture content. Water, Water Vapor Effects Shrinking and swelling of wood occur whether the water is in the form of vapor or liquid. As discussed, wood can swell until it reaches fiber saturation point.
Rain water can cause wood to quickly reach, or even go beyond, fiber saturation.
Problems with poor performance of wood or finishes usually occur when the moisture content of wood reaches or goes beyond fiber saturation. This degradation leads to eventual checking and cracking of the wood, increasing water uptake, thus accelerating the degradation process. These discolorations, although not harmful to the wood, offer an objectionable appearance to the wood surface. As this moisture is absorbed through the wood, extractives are dissolved and then deposited on the exterior surface as the water evaporates. The deposited tannic acid appears as a reddish brown stain.