ACQ treated lumber is wood that has been pressure-treated with alkaline copper quaternary, a preservative that protects against rot, decay, and insect damage. It became the dominant treatment for residential lumber in 2004, after the EPA phased out the older arsenic-based preservative (CCA) for consumer products. If you’ve bought pressure-treated wood from a lumber yard in the last two decades, it was almost certainly treated with ACQ or a closely related copper-based formula.
What’s in ACQ Treatment
The preservative is a combination of two active ingredients: copper oxide, which makes up about 67% of the formula, and a quaternary ammonium compound (sometimes called “quat”), which accounts for the remaining 33%. Copper is the primary defense against fungi and wood-eating organisms. The quaternary ammonium compound works as an additional fungicide and insecticide, similar in chemistry to certain disinfectants and cleaners.
There are a few ACQ formulations on the market. ACQ Type B uses ammonia as a carrier to dissolve the copper, while ACQ Type D uses an amine-based carrier. Type C can use either. The differences matter mainly to manufacturers and specifiers. For most homeowners, the key point is that all ACQ formulations are arsenic-free, which was the whole reason for the switch.
Why ACQ Replaced CCA
For decades, chromated copper arsenate (CCA) was the standard wood preservative in the United States. It worked well, but it contained arsenic, a known human carcinogen. In 2002, EPA Administrator Christie Whitman announced that the wood treatment industry had voluntarily agreed to stop using CCA for residential products by December 31, 2003. Starting in January 2004, CCA was no longer allowed for decks, playsets, picnic tables, fencing, or any other residential application.
ACQ was one of the primary alternatives that filled the gap. It offers comparable protection against decay and insects without relying on arsenic or chromium. The tradeoff, as many builders quickly discovered, is that ACQ’s higher copper content creates new challenges with corrosion and copper leaching.
Use Categories and Grades
Not all ACQ lumber is treated to the same level. The American Wood Protection Association (AWPA) assigns use categories that describe where and how the wood can be used. The ones you’ll encounter most often are UC3B (above-ground, exposed to weather, like deck boards and railings), UC4A (ground contact, like fence posts and landscaping timbers), and UC2 (interior, protected from weather). Each category requires a different concentration of preservative to be forced into the wood during pressure treatment.
When buying ACQ lumber, look for an end tag or stamp that lists the AWPA use category. Using wood rated for above-ground exposure in a ground-contact application means it won’t have enough preservative to resist the moisture and organisms in soil, and it will fail much sooner.
Corrosion and Fastener Requirements
This is the single most important practical detail about ACQ lumber. The higher copper content that replaced arsenic is significantly more corrosive to metal than the old CCA formula. Research from the USDA Forest Products Laboratory found that ACQ-treated wood is roughly 2 to 19 times more corrosive to galvanized steel than CCA-treated wood, depending on the specific conditions and retention levels.
Standard zinc-plated screws and nails will corrode quickly in ACQ lumber, sometimes within a year or two. You need hardware rated for use with ACQ-treated wood. The minimum recommendation is hot-dip galvanized fasteners (look for a G185 coating designation). For longer-lasting results, stainless steel fasteners are the better choice. Because stainless steel is nobler than copper on the electrochemical scale, it resists the corrosion mechanism that eats through regular steel and even galvanized coatings.
This applies to everything that touches the wood: screws, nails, joist hangers, post brackets, and lag bolts. Using the wrong hardware on an ACQ deck is a common and expensive mistake. The wood will look fine while the fasteners quietly dissolve.
Copper Leaching and Soil Impact
ACQ lumber releases more copper into the surrounding environment than the old CCA formula did. Research published in Building and Environment found that ACQ-treated wood leaches 4 to 10 times more copper than CCA-treated wood. That higher copper release is what makes the treatment more corrosive to metal, but it also matters for soil.
Copper is an essential nutrient for plants in small amounts, but at elevated concentrations it becomes toxic to soil organisms and plant roots. Studies at sites with ACQ-treated structures found higher copper contamination in surrounding soil compared to background levels. That said, risk assessments at those sites found the hazard index for human exposure through soil ingestion remained below the threshold of concern.
For practical purposes, this means ACQ lumber is fine for most outdoor construction, but you should think twice about using it for raised garden beds where edible plants will grow in direct contact with the wood. Many gardeners line the inside of treated-wood beds with plastic sheeting as a precaution, or choose naturally rot-resistant species like cedar for food gardens.
Finishing and Staining
ACQ-treated wood can be painted or stained, but the wood needs to be dry first. Experts at the USDA Forest Products Laboratory advise that you can apply finish right after installation, as long as the moisture content has dropped to appropriate levels. Freshly treated lumber from the store is often still saturated from the pressure-treatment process, so it may need several weeks of drying time before it will accept a finish properly.
You can test this by sprinkling water on the surface. If it beads up, the wood is still too wet. If it soaks in, you’re ready to apply stain or paint. Many builders recommend waiting two to four weeks after installation, though this varies with climate and sun exposure. Applying a water-repellent finish or UV-blocking stain within the first few months helps prevent the gray weathering and surface checking that untreated ACQ lumber develops over time.
Safe Handling and Disposal
ACQ lumber is safer than the old arsenic-treated wood, but it’s still pressure-treated with pesticides and deserves basic precautions. When cutting or sanding, work outdoors and clean up sawdust afterward. Wear gloves when handling freshly treated wood for extended periods, and wash your hands before eating.
Never burn ACQ-treated lumber. The smoke and ash can release concentrated copper and other chemicals. Don’t chip or compost it either, as this can leach those chemicals into your garden soil. Treated wood scraps can go in regular construction waste in most areas, but check with your local waste management program for specific rules. The EPA classifies treated wood as a “treated article” rather than hazardous waste, so disposal isn’t heavily regulated at the federal level, but some states have stricter requirements.