ACQ (Alkaline Copper Quaternary)
The most common pressure-treatment preservative system for residential lumber, using copper and quaternary ammonium compounds to prevent rot and insect damage.
In Detail
ACQ (Alkaline Copper Quaternary) replaced chromated copper arsenate (CCA) as the dominant residential pressure-treatment preservative system in North America after CCA was restricted in 2003. ACQ uses copper as the primary biocide (kills fungi and insects) and quaternary ammonium compounds (quats) to enhance performance against copper-tolerant organisms.
ACQ is more corrosive to metals than CCA was — this is why modern PT lumber requires hot-dipped galvanized (ASTM A153), stainless steel, or manufacturer-approved coated fasteners. Never use standard zinc-plated hardware with ACQ-treated lumber; the copper will aggressively corrode the zinc coating, leading to fastener failure.
MCA (Micronized Copper Azole) is a competing system with similar performance. Both ACQ and MCA are considered safe for residential contact once the wood has dried, complying with EPA and Consumer Product Safety Commission standards.
Related Terms
Pressure Treatment
A process that forces chemical preservatives deep into wood fibers using a vacuum and pressure cycle to resist rot, decay, and insect damage.
Use Category (AWPA UC System)
The AWPA Use Category system classifies pressure-treated lumber by the severity of its intended outdoor exposure, from UC1 (interior dry) to UC5C (saltwater immersion).