Pressure-Treated Lumber Grades for Deck Building

Not all pressure-treated lumber is the same. The lumber is treated to different retention levels depending on the application, and using the wrong one is a code violation and a maintenance problem. Here is how to match the retention level to the part of the deck you are building.

Above-ground applications (UC3B)

Decking boards, joists, beams and railings that are exposed to weather but not in contact with the ground use Above-Ground designation lumber, typically labelled UC3B. These members are protected from rain but have intermittent wetting. The required preservative retention level for UC3B is lower than ground-contact.

Ground-contact applications (UC4A/UC4B)

Posts that are embedded in the ground, set in concrete, or continuously wet must be UC4A (standard ground contact) or UC4B (severe ground contact near salt water or high soil moisture). Using above-ground lumber in a ground-contact application is a common cause of rapid post rot — the treating chemical retention is insufficient to resist the constant moisture and microbial exposure.

How to read the tag

Every piece of pressure-treated lumber has a tag stapled to the end. It states the preservative type (ACQ, CA-C, MCQ, etc.), the retention level in pcf, and the use category (UC3B, UC4A, UC4B). Match the use category on the tag to the application. If the tag is missing or illegible, don't use that piece for a critical structural application.

Fastener compatibility

Modern copper-based preservatives (ACQ, CA-C) are corrosive to plain steel. Use only hot-dip galvanized or stainless fasteners, or fasteners with a coating specifically approved by the lumber manufacturer. Joist hangers and post bases must also be rated for use with ACQ/CA-treated lumber — look for the G185 or ZMAX rating on Simpson and USP connectors.