What Is Treated Wood?

Treated wood is wood that has been appropriately treated with preservative chemicals with the intent of prolonging its intended usefulness lifecycle compared to untreated wood.

Different types of applications and preservatives are used to protect wood fibers from:

• structural degradation
• decay fungi
• termites
• marine organisms
• flames 

All chemical preservatives are registered pesticides and, as such, they are regulated by the US EPA. The American Wood Protection Association (AWPA) writes various standards that determine use levels for wood preservative formulations as well as their suitability for the intended end-use. Third party inspection agencies regularly perform audits to ensure that quality products are produced in accordance with the standards.

Advantages of Treated Wood

Life Cycle Comparison of untreated wood vs. treated wood

• Because treated wood is intended to extend the usefulness and life of wood, fewer trees are required to perform the same function that is required when untreated wood is used.
• Wood that has been appropriately treated can significantly reduce the number of trees that would otherwise need to be harvested.
• If untreated wood only lasts 5 years and treated wood lasts 25 years or more, it would take at least 5 times as many trees over the same period of time, as well as the incremental labor, to continually harvest, manufacture, replace and dispose of all of the untreated wood.
• Unprotected wood can begin to rot within 1-2 years depending on the environment.
• The structural integrity of untreated wood can be greatly reduced within weeks due to termites and certain marine organisms.
• Without chemical treatments, wood can be quickly consumed when exposed to flames.

Common Preservative Protection Use Categories

The Use Category System of the American Wood Protection Association (AWPA) designates what preservative systems and retentions have been determined to be effective in protecting wood products under specified exposure conditions. The Use Category is designated on the end tag of each piece of treated lumber.

• Fungal rot/decay and termite protection
• Outdoor, exposed applications
• Interior framing
• Marine organisms
• Dock/pier pilings
• Fire Retardant Treated Wood (FRTW)

Three Categories of Pressure Treatments

1. Waterborne treated lumber is generally used in building structures that are residential, commercial and industrial.
2. Creosote-treated lumber is mostly used for treating guardrail posts, railroad ties, and timbers used in marine structures.
3. Oil-borne treated lumber is used for treating utility poles and cross arms.

Pressure-impregnated Treatments

In pressure-treated wood, preservatives are infused into the wood, beyond just the surface.

Pressure Treatment (PT) is the general term to describe the process for infusing/impregnating the wood fibers with preservative chemicals and removing any excesses, leaving behind only enough chemical in the wood fibers (retention) to protect the wood. The AWPA sets appropriate chemical retentions depending on their intended use/requirements based on performance data derived from long-term scientific tests. The AWPA wood preserving standards are reviewed by their technical committees every five years to ensure that retention levels are appropriate and that a given preservative formulation is performing as expected.

Topical/Surface Treatments

• Topical/surface treatments usually limit protection to the surface area because it is applied by brushing, spraying or dipping.
• Although regularly coating a surface with a paint or sealer may help protect wood from the elements, it won’t necessarily prevent it from rotting or being attacked by insects.