Treated lumber Use Categories (UC1 through UC5) are the American Wood Protection Association's ratings for where a treated board can safely go: the higher the number, the harsher the exposure and the more preservative in the wood. For most residential work, UC3B covers above-ground outdoor uses like deck boards and railings, and UC4A or higher is the call for anything touching soil or hard to replace later.
Pick the wrong category and a deck post or sill can rot years early, even though it looked like "treated lumber" on delivery day. Here's how to read the rating and match it to the job.
When lumber is pressure-treated, preservative is forced deep into the wood to resist rot and termites. How much preservative it gets depends on where it's headed. The AWPA groups those exposures into Use Categories, and the category is printed on the end tag of every treated board.
That tag is the only reliable way to know what you've got. Color isn't a rating: modern treatments like MCA are lighter than older ones, and two boards that look alike can carry very different ratings.
| Category | Exposure | Typical uses |
|---|---|---|
| UC1 | Interior, dry | Interior framing protected from moisture |
| UC2 | Interior, damp | Framing that may see occasional moisture but stays out of the weather |
| UC3A | Exterior, above ground, coated | Painted trim that sheds water quickly |
| UC3B | Exterior, above ground, uncoated | Deck boards, railings, joists well off the ground |
| UC4A | General ground contact | Deck and fence posts, wood that's hard to replace |
| UC4B | Heavy-duty ground contact | Permanent wood foundations, critical structural posts |
| UC4C | Extreme-duty ground contact | Utility poles and pilings in severe conditions |
| UC5A/B/C | Marine | Salt or brackish water |
Want the short definitions for every rating? They're all in our Builder's Glossary.
On a typical deck or porch, the real decision is UC3B ("Above Ground") versus UC4A ("Ground Contact"). Lumber labeled Above Ground should never be buried, set in soil or used where it stays wet. Go with Ground Contact when the wood:
Wood that touches concrete or masonry, such as sill plates on a slab, should also be treated, so check your plans and local code for the required category.
Builder's Insight: When in doubt on a deck frame, step up to Ground Contact. The price difference on a few joists and beams is small next to tearing out a soft rim or ledger in year eight. And most deck failures start at the ledger, so flash it and fasten it exactly as your code requires.
Modern copper-based treatments like ACQ and MCA are more corrosive to metal than older CCA lumber, which was phased out of most residential uses at the end of 2003. Plain steel nails, screws and connectors can corrode quickly. Use hot-dip galvanized (look for G185 on connectors) or stainless steel hardware rated for treated wood, and follow the connector manufacturer's recommendations.
KDAT (kiln-dried after treatment) lumber is re-dried after treating, so it's lighter, more stable and ready for paint or stain sooner. It shrinks less after install, which keeps deck gaps and trim joints tighter. FRT (fire-retardant-treated) wood is a different product entirely: it slows flame spread and is required by code in some commercial and multifamily applications. It isn't a substitute for rot-resistant treated lumber outdoors.
No. Posts set in soil or concrete in the ground need Ground Contact lumber, UC4A or higher. Critical structural posts may call for UC4B.
No. Color depends on the preservative, not the rating. Always read the end tag for the Use Category.
Yes. UC1 and UC2 cover interior uses, and treated lumber is commonly required where wood touches concrete or masonry. Follow your plans and local code.
Planning a deck? See our deck products, browse lumber, and read 7 Things to Look for When Inspecting Your Deck.
Tell us what you're building and a Wilson Lumber specialist will help you match the right treatment, sizes and hardware before you order.
General guidance only. Always follow your plans, the engineer of record, manufacturer instructions and local building codes.