WoodWorks Blog

FAQ - When is preservative treated wood required?

An engineer recently contacted WoodWorks regarding the classification system for preservative treated wood detailed in the American Wood Protection Association (AWPA) standard, AWPA UC1. He was updating his company’s specifications and was unsure how to determine which products fall into which categories. The question is particularly relevant since WoodWorks technical directors occasionally come across design professionals who are using outdated standards in this area, specifically the AWPB LP1, AWPB LP2 or AWPA “C” standards, which have been replaced by the AWPA Use Category System’s U1 Standard.

For those of you with similar questions, this was our answer:

With a little effort, the AWPA system is fairly simple and intuitive. All you need to know is where the wood product will be used and the anticipated exposure to biodeterioration hazards. Start by asking yourself the following question:

Is the product being used for an interior or exterior application?

If it’s interior, then it’s going to be classified as UC1 or UC2. UC1 applies to products only exposed to insects (i.e., termites). Your local building official should be able to tell you if termites are common at the project location and to what extent the framing materials may need to be protected. For example, in specific areas of the U.S. where Formosan termites are found, the entire framing package is typically treated. The UC2 classification is for interior materials exposed to insects and/or moisture (e.g., sill plates and columns in contact with concrete, bathroom floors, etc.) or high humidity (greenhouses and swimming pools). In most northern regions of the U.S., termites pose little or no concern, and building materials in interior applications don’t need to be treated unless they’re exposed to moisture.

If the product is being used for an exterior application, ask yourself the following:

Is it an exterior, above-ground situation?

If yes, it falls into the UC3A or UC3B category. UC3A covers materials that are coated (e.g., painted or stained) and used in vertical applications, such as siding, trim, window frames and other products associated with rapid water run-off. UC3B is for uncoated products used for above-ground applications, such as decking, deck rails, fence pickets or millwork, where water runoff is not considered rapid.

Is the wood in an exterior application and in direct contact with the ground or fresh water?

If yes, it falls into category UC4A, UC4B or UC4C. The difference depends on the severity of the biodeterioration hazard and how critical the component is to the structure (i.e., what it’s supporting and how hard it would be to replace). For example, a deck or guardrail post or a utility pole in a lower hazard region would fall under UC4A. Permanent wood foundations, building posts and utility poles in high decay hazard regions fall under UC4B. Building pilings and railroad crossties and utility poles in severe decay hazard regions would be categorized as UC4C.

Will the product be placed in a salt or brackish water environment?

More specifically, will any part of the component be frequently submerged in salt or brackish water (i.e. extending below mean high tide level)? If yes, it should be categorized as UC5A, UC5B, or UC5C. UC5A is for coastal marine environments north of New Jersey or north of San Francisco. UC5B is for coastal marine environments between New Jersey and Georgia on the east coast and south of San Francisco on the west coast. UC5C is the severest biodeterioration classification and is for materials in coastal areas south of Georgia, the Gulf Coast, Hawaii, and Puerto Rico.

For detailed information on specifying treated wood products, see Section 6: Commodity Specification in the AWPA U1 Standard ($50). Other good sources include the following:

Websites – AWPA • Southern Pine Council • APA • American Institute of Timber Construction • Western Wood Preservers Institute (WWPI)

Publications – Excerpt of the AWPA U1 Standard (AWPA)  • How to Specify Treated Wood Products (AWPA) • Guide to Pressure Treated Wood (WWPI) • Pressure Treated Southern Pine (Southern Pine Council)

Post submitted by:

Tom Milton, WoodWorks Technical Director, North-central region