Pressure Treated Wood (AWPA Use Categories)

Glenn Stewart, P.E.

Civil/Structural Engineer

Thursday, August 9, 2018

When evaluating existing structures of wood construction, structural damage from fungal-based decay or insect infestation is often encountered. Provisions can be taken by the design and construction professionals to minimize this type of damage. One method to minimize the potential of decay and deterioration is by using pressure treated wood. Of course not all pressure treated wood is the same. As a result, as we go about our work of evaluating existing structures, it is important to understand what pressure treated wood is and how to select the proper type for a particular application.

Techniques for preserving wood have been employed since soon after the advent of its use. Historical techniques provided protection through the application of coatings or from the limited penetration of the wood by soaking the wood in a preservative agent. Modern techniques force the preservatives farther into the wood.

In order for wood to decompose, four conditions are needed: high moisture, favorable temperature, oxygen, and a food source(wood fiber). Pressure treatment eliminates the wood fiber as a food source and removes a condition required for decomposition. During the treatment process, wood is placed in pressurized vessels where preservatives are forced deep in the wood fibers. Three broad classes of preservatives are used: water-borne preservative, oil-borne preservative, and creosote preservatives. Under each of the broad classes are different preservative systems with different chemical compositions and trade names.

Sorting through the various preservative systems and classes to select the proper piece of lumber for a project can be a complicated process. Both the International Building Code and the International Residential Code reference the use of the American Wood Protection Association Standard, AWPA U1, Use Category System: User Specification for Treated Wood. Manufacturers often label their products with the appropriate U1-Use Category. Each U1-Use Category is suitable for different service conditions, environments, and is recommended for typical applications. Common U1-Use Categories that one may encounter are summarized in the below table.

AWPA Use Categories

U1-Use Category	Service Conditions	Use Environment	Typical Applications
UC3B: Above Ground, Exposed	Exterior construction, above ground, uncoated, or poor water run-off	Exposed to all weather cycles, including intermittent wetting but with sufficient air circulation so wood can readily dry	Decking, railings, joists and beams for decks, fence pickets, uncoated millwork
UC4A: Ground Contact, General Use	Ground contact or fresh water, non-critical components	Exposed to all weather cycles, including continuous or prolonged wetting	Fence posts, deck posts, guardrail posts, joists and beams for decks, crossties & utility poles (low decay areas)
UC4B: Ground Contact, Heavy Duty	Ground contact or fresh water, critical components or difficult replacement	Exposed to all weather cycles, high decay potential includes salt water splash	Building poles, horticultural posts, crossties & utility poles (high decay areas)
UC5A: Marine Use, Northern Waters	Salt or brackish water and adjacent mud zone which includes Long Island, NY and northward, north of San Francisco	Continuous marine exposure (salt water)	Piling, bulkheads, bracing
UC5B: Marine Use, Central Waters	Salt or brackish water and adjacent mud zone, south of Long Island, NY to southern border of GA, south of San Francisco	Continuous marine exposure (salt water)	Piling, bulkheads, bracing
UC5C: Marine Use, Southern Waters	Salt or brackish water and adjacent mud zone, south of GA, gulf coast, Hawaii, and Puerto Rico	Continuous marine exposure (salt water)	Piling, bulkheads, bracing

¹ Excerpts from AWPA U1-17, Use Category System: User Specification for Treated Wood, Table 2-1.

The use of pressure treated wood is one of the provisions available in order to minimize the potential for future decay and insect infestation of wooden structures. As we proceed with our evaluations of buildings and structures, we should remember that not all pressure treated wood is the same and that AWPA U1 Use Categories are a source of information that can assist with ensuring that the proper materials have been used.

Potential resources for further research on this topic are AWPA U1-17, Use Category System: User Specification for Treated Wood and Southern Forest Products Association, Pressure Treated Southern Pine: Standard, Specifications, Applications, 2010and 2017 Editions.

About the Author

Glenn Stewart, P.E. is a Consulting Engineer in our Columbia, SC Office. Mr. Stewart provides specialized consulting in the areas of construction evaluation, damage assessment, engineering design, safety consultation, and loss evaluation.