“Rot” or “rotting” is not simply the presence of fungi, but rather the damage or disintegration (and ultimately dissolution) of the wood substance as the result of the presence (and growth) of fungi within the wood tissue. Wood does not decay simply because it is wet or ill maintained! The process may happen only if the wood has been attacked by certain fungi under some very specific environmental conditions of oxygen, temperature and moisture. As far as decay prevention is concerned, generally it is necessary to identify the attack, know the nature of the particular fungi, and the conditions necessary for their survival. Only then it may be possible to modify the wood’s service conditions or perhaps treat the wood to prevent the development of fungal growth.
The microscopic organisms that discolor and decay wood belong to a huge group of primitive plants known as fungi. Unable to produce their own food, fungi feed instead on natural substances that make up organic materials like leather, cloth, rattan, paper, and wood.
Mushrooms that spring from lawns and tree trunks are fungal “fruits”. They release millions of dust-size spores that are scattered helter-skelter by wind. When the conditions of the surface they eventually settle upon are right, spores germinate, sending out thread-like filaments called hyphae. Enzymes secreted by hyphae break down organic matter so fungi can use it for food.
Before fungi can colonize wood, four requirements must be met: an oxygen supply, temperature in the 40 to 100 (F range, a supply of sufficient moisture, and a food source (wood). Infection can be prevented by eliminating any one of the requirements. Obviously, it’s hard to limit oxygen. Temperature control is tough too, since most living things thrive in this range. And even at subfreezing temperatures, many fungi don’t die, they just go dormant.
The most effective “method” of preventing fungal deterioration of wood is to keep it dry. Most fungi need a wood moisture content of at least 20% to carry on. With the moisture content of wood indoors over most of the United States cycling annually between 6% and 16%, it’s too dry for most microorganisms to get started.
In exterior or other situations where wood can’t be kept dry, decay has been delayed traditionally by using naturally rot-resistant woods like Western red cedar and redwood. Nature has partially protected these woods from fungi by depositing toxic extractives in their heartwood. But supplies of naturally durable woods are too small to meet today’s demand at an ecologically and economically acceptable price. In imitation of Nature, less naturally durable woods are impregnated with pesticides like CCA (chromate copper arsenate) that extend their service life by 30 to 50 years or longer.
One of the most common ways to prevent exterior wood from rotting is to wrap or cover the wood with a maintenance free material like PVC coated aluminum. It is important to note that any rotten wood covered up will continue to rot, so it is important to make sure the wood is not decaying before it is covered.
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