"What is Pressure Treated Wood and What do I need?"

Pressure treated lumber is commonly used in wood decks, raised flowerbeds, wood fences, and any application where wood is exposed to environmental conditions. What is pressure treatment? When wood is pressure treated, the wood is placed in a closed cylinder and chemical preservatives are forced deep into the cellular structure by pressure. This process allows the preserved wood to maintain a chemical barrier against termites and decay for long periods of time. Several manufacturers actually guarantee their treated wood to resist decay and termite attack for 40 years or more.

When selecting preservative treated wood it is recommended to select material that has met the approved treating industry standards set forth by the American Wood-Preservers' Association (AWPA) and the American Lumber Standards Committee (ALSC). This certification will be indicated by either an ink stamp (in addition to the lumber grade) or more commonly by a plastic tag stapled to the end of the lumber.

Information on the tag or stamp should include: Year of treatment, proper exposure conditions, if the wood was dried after treatment, the trademark of inspection, the preservative used, the treating company’s name and location, the retention level, and any other American Wood-Preservers' Association (AWPA) standards.

There are different levels of the amount of preservative required (retention), for a material based on its intended use. Retention is the amount of preservative placed in the wood, measured in pounds/ft³. For waterborne preservatives the retention level for specific applications is listed in Table 1.

Table 1. Preservative retention levels for specified applications.

What types of preservatives are used to treat wood? Today there are three common classes of wood preservatives used in the pressure-treating process: waterborne, creosote, and pentachlorophenol. Wood treated with water-borne preservatives are used in a wide variety of products and applications such as playground equipment, fences, gazebos, boat docks, sign posts, utility poles, and retaining walls. Common water-borne preservatives include: Chromated Copper Arsenate (CCA), Ammoniacal Copper Quat (ACQ), and Ammoniacal Copper Zinc Arsenate (ACZA). Creosote is primarily used for treating timbers for use in highway bridges and guardrail posts, railroad ties, docks, and seawalls. Pentachlorophenol is an oil-borne preservative. Utility poles and crossarms are commonly treated with pentachlorophenol.

While wood treated with these preservatives are available, the most common an easy to purchase locally is wood treated with CCA. By properly selecting treated wood with the correct retention level for the intended use, you can insure that your project will last for many years to come.

Brian Bond

Assistant Professor

Extension Forestry, Wildlife, & Fisheries