Wood decay is caused by small plants called fungi. These plants cannot live on wood that has a moisture content of less than about 30 percent. The wood in most well-built homes is safe because the moisture content is rarely about 15 percent. The way to stop decay is simple. Keep wood dry.
Keeping wood dry is not always that easy to do. The outside walls of a house are open to rain that blows against them. Soil carries water. It can wet any wood that touches it. Also, if there are plumbing leaks in your home, the inside weed can get wet.
Wood must be kept dry at all times because wood soaks up water and holds it for a long time. From time to time heavy rains can supply enough water for decay to begin.
If you know that wood will become wet, it can be protected. Wood can be treated with preservatives. For long term safety, the preservative should be put on under pressure. If the wood comes in touch with the ground, only pressure treatment will do the job. Some pieces of wood that only get wet from time to time can be treated to stop decay by brushing on a preservative. The correct chemical to use for treatment depends upon what the wood is used for. Some chemicals give off a terrible smell. These are not good for use indoors. Others cause paint to peel. You should mention what the wood is used for when you buy the preservative.
Mold and stain fungi can attack wood. Molds grow mainly on the surface but may get inside the outer sapwood. The dark color caused by mold on wood can be removed by light sanding. Stain fungi go beyond the outer layers and causes a dark color that cannot be removed. Both molds and stain feed on wood. By themselves, they do not decay or weaken the wood. But, they do increase the wood’s ability to take on and hold moisture. Thus, they increase the possibility of future decay. If you see signs of mold and stain fungi, you may have a problem.
Wood decays slowly at temperatures below 40F. So, decay is more rapid in the South than in the North. Decay is also more rapid in humid than in dry regions even though the plants which cause decay are present everywhere throughout the United States.