Wood used for exterior construction deteriorates primarily from decaying fungi, a lower form of plant life that consumes cellulose and other components. Eventually the wood becomes rotten and soft.
Fungi needs heat, moisture, oxygen and food to germinate. Their growth is slowed in hot and cold extremes, and wood with a moisture content of less than 20 percent won't decay.
Untreated wood that's in contact with the ground decays from the combination of soil moisture and oxygen. Insects and borers also attack wood's cellular structure.
To protect wood against decay, preservative chemicals are forced deep into it using a pressure process. The chemicals contaminate the food source -- wood sugars and cellulose -- for decay fungi.
Pressure is the most effective method for ensuring adequate retention, penetration and uniformity of the preservatives in the wood. Often incisions are made in the wood for uniform penetration of the preservative.
Retention values have been formulated that are specific for the intended use; they're stamped on the wood.
The sapwood of most species can be penetrated with preservatives, but the heartwood -- at the center of the tree trunk -- is variable. Most species of pine have heartwood that can be penetrated with preservatives with little or moderate difficulty; lodgepole pine is rated as difficult to penetrate.
Without treatment, the life expectancy of the heartwood of red and jack pines is 2 to 6 years and ponderosa pine is 3 to 4 years. Pressure-treated pine can last 20 to 25 years.
Preservative Chemicals for Pressure-treated Wood
Many preservatives for treating wood are arsenic and copper-based. Chromated copper arsenate (CCA) has been used for several years in wood treatments, but can leach into soil and exposure to it presents hazards to health.
In 2003 the use of CCA as a preservative was discontinued in pressure-treated wood for residential use, such as decking and children's play equipment. The chemical preservatives that are used are also toxic; follow the precautions on the consumer safety label carefully.