Types of Pressure Treated Wood: UC Ratings, Grades, Decks

For a deck you need two grades of treated wood, and the end tag tells you which is which. The American Wood Protection Association assigns UC3B to exterior wood above ground that is uncoated or has poor water runoff - that is your decking, rails and most framing - and UC4A to general ground contact, which covers posts, anything buried, and anything sitting on or near soil. UC4B is the heavy-duty ground-contact grade.

AWPA warns specifically that treated lumber two inches thick or less may be treated only for above-ground use, so it will not last long in the ground. Colour is not a grade. Two boards with the same green tint can be UC3B and UC4A, and only the tag separates them.

AWPA lists four things to look for on the stapled end tag.

• "AWPA U1" - the wood was treated to Standard U1, the primary specification for treated wood.
• The Use Category - UC2, UC3B, UC4A and so on.
• The preservative name or abbreviation - ACQ, CA-B, CA-C, MCA, CuN-W and others.
• The nominal retention - a number such as 0.060 or 0.31, in pounds of active ingredient per cubic foot.

For code-regulated work you should also see a checkmark logo and the quality mark of an ALSC accredited agency. AWPA is blunt about the failure case: if you cannot find "AWPA U1" and a Use Category on the tag, the wood probably does not meet AWPA standards.

Retention is how much preservative stayed in the wood, in pounds of active ingredient per cubic foot, and every preservative has its own scale. AWPA's Southern pine table shows why you cannot compare the bare numbers across chemistries:

• ACQ (Type B or C): 0.25 for UC3B, 0.40 for UC4A, 0.60 for UC4B
• Copper azole Type B (CA-B): 0.10 for UC3B, 0.21 for UC4A, 0.31 for UC4B
• Copper azole Type C (CA-C): 0.060 for UC3B, 0.15 for UC4A, 0.31 for UC4B
• Micronized copper azole (MCA): 0.060 for UC3B, 0.15 for UC4A, 0.31 for UC4B
• Waterborne copper naphthenate (CuN-W): 0.070 for UC3B, 0.11 for UC4A

A CA-C board at 0.15 and an ACQ board at 0.40 are both rated UC4A. Read the Use Category first and treat the retention as a cross-check against the preservative named on the same tag.

Copper-based preservatives corrode plain steel, so the American Wood Council's prescriptive deck guide requires fasteners and hardware in contact with preservative-treated wood to be hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze or copper. Where the deck is exposed to salt water, it calls for stainless steel grade 304 or 316. Hardware must be hot-dipped before any coating.

Every cut, notch and bored hole exposes untreated wood inside the board. The same guide requires field cuts to be treated with an approved preservative before assembly. Cut ends that will sit on a post or in a joist hanger are exactly where rot starts, so brush the end grain and let it soak in, do not just wipe it.

DCA 6 sizes residential deck framing for a 40 psf live load plus 10 psf dead load, No. 2 grade, wet service. Its maximum joist spans, measured face of support to face of support:

• Southern pine at 16 inches on centre: 2x6 9 ft 0 in., 2x8 11 ft 10 in., 2x10 14 ft 0 in., 2x12 16 ft 6 in.
• Douglas fir-larch, hem-fir or SPF at 16 inches on centre: 2x6 8 ft 4 in., 2x8 11 ft 1 in., 2x10 13 ft 7 in., 2x12 15 ft 9 in.
• Redwood, western cedars, ponderosa pine or red pine at 16 inches on centre: 2x6 8 ft 0 in., 2x8 10 ft 7 in., 2x10 13 ft 0 in., 2x12 15 ft 1 in.

Decking is attached with two 8d threaded nails or two #8 screws per joist, with boards spaced about 1/8 inch apart. Ledgers must be at least a nominal 2x8. Always confirm against your local building department, which is the authority that signs off.

Wood treated with chromated arsenicals (CCA) was used for residential decks and playsets before 2004; manufacturers voluntarily stopped producing it for homeowner uses in December 2003. EPA does not require existing structures or the soil around them to be removed, and says a regularly renewed penetrating oil- or water-based stain may reduce leaching. Do not burn CCA-treated or any preservative-treated wood in a residential setting, wear goggles and a dust mask when sawing it, and wash your hands afterwards. If someone swallows treated-wood sawdust or ash, or has a reaction after exposure, call Poison Control at 1-800-222-1222 in the US or see a doctor.

On cost, treated deck lumber is priced off the softwood market, which moves. NAHB, citing the Madison's Lumber Price Index, reported the framing lumber composite at $515.93 per 1,000 board feet for 18 September 2026, up 8.4% year over year. That is a wholesale softwood benchmark, not a retail shelf price for treated boards, which carry the treatment cost on top.

The types of pressure treated wood you can buy are named by the preservative on the end tag. The USDA Wood Handbook's preservatives chapter describes each:

• ACQ (alkaline copper quat) - 50% to 67% copper oxide and 33% to 50% quaternary ammonium compound, in several standardized types (A, B, C and D).
• CA-B and CA-C (copper azole) - amine copper (96%) with triazole fungicides; the handbook notes copper azole is commonly used for decking and dimension lumber at lumber yards and is also standardized for posts, poles and timbers.
• MCA and MCA-C (micronized copper azole) - the same chemistry with the copper ground to very fine particles and dispersed in the treating solution rather than dissolved.
• CuN-W (waterborne copper naphthenate) - standardized for above-ground and some ground-contact uses, according to the handbook.
• Borates (SBX) - very effective against decay, termites, beetles and carpenter ants, but readily leachable, so for above-ground uses protected from wetting only, such as framing.
• CCA (chromated copper arsenate) - the old 'green treated' wood that dominated from the late 1970s until 2004. Its EPA label now restricts it primarily to industrial uses such as poles, piles, round fence posts and permanent wood foundations, and it is generally not available at retail yards.

A note on names: AWPA's list of standardized preservatives uses codes like ACQ, CA-C and MCA. Names such as AC2 are not on that list; they are product names, so read the Use Category and preservative code printed on the same tag to know what you are buying.

• What is in them - CCA-C, the only CCA formulation in current AWPA standards, is 47.5% chromium trioxide, 34.0% arsenic pentoxide and 18.5% copper oxide. ACQ replaces the chromium and arsenic with a quaternary ammonium co-biocide; copper azole and MCA use triazole fungicides instead.
• Where you can use them - ACQ, CA and MCA are the retail deck and framing treatments; CCA is now limited mainly to industrial uses, and manufacturers stopped making it for homeowner uses in December 2003 (EPA).
• How they are rated - retention numbers are not comparable across chemistries. In AWPA's Southern pine table, UC4A ground contact means 0.40 lb per cubic foot for ACQ but 0.15 for MCA or CA-C, so compare Use Categories, not retention figures.

For an old deck or fence built before 2004, EPA does not require removing CCA wood or the soil around it, but says never to burn treated wood in a residential setting and to wear goggles and a dust mask when sawing it. If treated-wood sawdust or ash is swallowed, call Poison Control at 1-800-222-1222 in the US or see a doctor.

Colour. Copper-based treatments leave wood a light green-brown - the handbook uses that phrase for the alkaline copper formulations - and CCA was commonly called 'green treated'. Colour depends on the chemistry; it is not a grade, and a green board and a brown board can both be UC4A or both be UC3B. Only the tag tells you.

Lumber grade. The treatment rating and the lumber grade are separate stamps. The USDA Wood Handbook lists dimension lumber grades as Select Structural, No. 1, No. 2 and No. 3, with 2x4s also sold as Construction, Standard, Utility and Stud, and decking usually in Premium and Standard. A single set of grade names is used across the US, but the allowable design values vary by species. The handbook's Table 7-2 gives the minimum bending strength ratio each visual grade allows - the strength of a piece with its knots and other characteristics compared with the same piece if they were absent: Select Structural 67%, No. 1 55%, No. 2 45%, No. 3 26%, and for 2x4 light framing Construction 34%, Standard 19% and Utility 9%. The deck span tables earlier on this page assume No. 2.

KDAT. Kiln-dried after treatment lumber has been dried again after treating; Decks.com notes it is much more stable but also more expensive.