How Effective is Pressure-Treated Wood?
Pressure-treated wood is wood that has had a chemical preservative forced into it, to help prevent it from being destroyed by wood-eating pests and decay fungi. Pressure-treated wood should be used whenever wood touches soil, or is close to soil.
The problem is that over a period of 7-10 years the chemicals slowly leach out of the wood. The outer inch or so of the wood might still be protected, but the inner wood might not be. If there are drill holes or cracks in the wood, or if the wood was cut, it often exposes unprotected wood in the center to pests and fungus decay.
Therefore, whether wood is pressure-treated or not, if it is in direct contact with soil, it will eventually rot and become susceptible to attack by termites and carpenter ants. In the long run, it is best to never let wood touch soil. Consider pressure-treated wood to be “termite-resistant”, not “termite-proof.”