“Wood will last virtually forever, if it’s kept dry.” Virtually all problems with wood-based building materials are moisture problems. Peeling paint, rot, warp, cracks and general shrinkage are all related to water in wood. Moisture problems also lead to rot issues caused by wood-decay fungus. A wood-decay fungus is any species of fungus that digests moist wood, causing it to rot.
Wood’s fiber saturation point is around 28% moisture content (MC). Excessive moisture above the fiber saturation point in wood is required for fungal colonization and proliferation. Fungi that not only grow on wood but permeate its fibrous structure and actually cause decay, are called lignicolous fungi. In nature, this process causes the breakdown of complex molecules and leads to the return of nutrients to the soil. Various lignicolous fungi consume wood in various ways; for example, some attack the carbohydrates in wood and some others decay lignin. The rate of decay of wooden materials in various climates can be estimated by empirical models.
Wood-decay fungi can be classified according to the type of decay that they cause. The best-known types are brown rot, soft rot, and white rot.
Natural durability is the inherent capability of wood to tolerate and resist fungal decay and insect attacks. This protective feature is attributable to specific biological compounds, called extractives that are toxic to wood-destroying organisms. The natural durability varies between tree species, geographic regions, environmental conditions, growth stage, and increases with the age. Generally, the heartwood of durable tree species is considered as very durable, whereas the sapwood of all tree species is considered not durable and the most vulnerable. A wide selection of timber preservation has been developed to give woods an improved durability and to protect it from decay. The more permeable the wood the easier to treat.