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Preservative Use Categories

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UC Guide

Dig deeper into how to use Use Categories for specifying preserved wood in the PreserveSpec - Specifying with Use Categories from Western Wood Preservers Institute. The 4-page guide includes a table listing the most commonly specified preserved wood products and the required preservative retentions for each Use Category.

Specifying with AWPA Use Categories

The Use Category System (UCS) was developed and is maintained by the American Wood Protection Association (AWPA). It's purpose is to provide a simple way of selecting the proper preserved wood product directly related to the appropriate AWPA Standard. The UCS defines a set of exposures for preserved wood in the environment. Each exposure has a different degree of deterioration hazard.

The UCS has five different categories, based on exposures and product performance. All preserved wood products can be placed in one of the five categories. The amount of preservative in the product, or the retention, and depth of penetration of the preservative in the wood are guided by these categories.

The smaller the Use Category number, generally the lesser amount of preservative is required to provide the necessary level of protection from fungal decay or termite attack. Likewise, the highest Use Category number signifies the highest degree of protection to wood used in the most severe service conditions.

Use Categories

Review the following Use Categories describing the exposure conditions the wood may be subject to in service as well as the typical applications.


UC1 wood products are used in interior construction, such as studs, joists, roof trusses, furniture and millwork. The preservatives are intended to protect against termites. These products are typically preserved with borates and may be ordered in higher borate retentions for areas where the Formosan termite is present.

Service conditions: Above ground, interior construction, dry. Protected from weather and interior sources of water such as leaking plumbing, condensate, pools and spans.

Typical applications: Interior construction.

Products in UC2 are used where there is potential moisture exposure and there is a need for protection against decay fungi as well as termites. Products available include lumber, millwork, glu-lams, flooring and furring strips.

Service conditions: Above ground, interior construction, damp.

Typical applications: Interior construction.

Products in UC3A may be exposed to the full effects of the weather, but are in vertical exterior walls or other types of construction that allow water to quickly drain from the surface.

Service conditions: Above ground, exterior, protected, coated and rapid water run-off.

Typical applications: Coated millwork, siding and trim.

Materials in UC3B are used where they are exposed to all weather cycles including intermittent wetting, but with sufficient air circulation so wood can readily dry

Service conditions: Exterior construction, above ground, uncoated or poor water run-off. Excludes above ground applications with ground contact type hazards.

Typical applications: Decking, railings, joists and beams for decks and freshwater docks, fence pickets, uncoated millwork

Products treated under UC4A are those that will be used in contact with the ground or with fresh water, where they may be exposed to all weather cycles, including prolonged wetting conditions that are favorable to deterioration.

Service conditions: Ground contact or fresh water contact, Ground Contact or Fresh Water. Non-critical components (includes above ground applications with ground contact type hazards or that are critical or hard to replace)

Typical applications: Sawn fence, deck and guardrail posts, joists and beams for decks and freshwater docks.

Wood in the UC4B category is typically used in contact with the ground in severe environments, in climates with high potential for deterioration and in critically important components.

Service conditions: Ground contact or fresh water contact, heavy duty, critically important components.

Typical applications: Permanent wood foundations, building poles, horticulture posts, utility poles, decking, above tidal piers and docks, and in areas with high potential for fungal decay.

Wood treated to UC4C can be found where there is extremely high potential for deterioration, with considerable exposure to moisture such as in rivers or lakes.

Service conditions: Ground contact or fresh water contact, extreme duty, extremely critical components.

Typical applications: Land or fresh water pilings, foundation pilings and utility poles in areas with a severe potential for fungal decay.

Wood treated to UC5 is preserved for the most demanding conditions such as salt water immersion where it may be exposed to marine borers and other salt water organisms. Unlike the other Use Categories, the subcategories are based on the location where the product will be used, reflecting the specific threats posed.

Wood treated to UC5A is intended for use north of San Francisco on the west coast and New Jersey on the east coast. UC5B is intended for wood used south of San Francisco on the west coast and New Jersey on the east coast. UC5C is set for use on the Gulf Coast and south of Georgia, as well as Hawaii and Puerto Rico.

Service conditions: Salt and brackish water and adjacent mud zones; continuous marine salt water exposure.

Typical applications: Marine pilings, bulkheads, bracing.