What is the difference between ACQ and MCA pressure-treated lumber for NB decks?

MCA (micronized copper azole) has largely replaced ACQ (alkaline copper quaternary) as the standard pressure treatment for residential lumber in New Brunswick, offering comparable rot protection with significantly less corrosion to metal fasteners and connectors. Understanding the difference matters because the treatment type directly affects which fasteners you can safely use, how long your structural connections last, and what maintenance your deck will require over its lifetime.

ACQ was the first major replacement for CCA (chromated copper arsenate) when that arsenic-containing treatment was phased out of residential use in Canada in 2004. ACQ uses dissolved copper and a quaternary ammonium compound to protect wood against decay, fungus, and insect damage. It works effectively for these purposes, and ACQ-treated lumber installed properly will resist rot for 20 to 30 years in New Brunswick's climate. However, ACQ has a significant drawback: the alkaline copper solution is highly corrosive to metals. The dissolved copper ions in ACQ create an electrochemical reaction with standard galvanized fasteners and connectors, eating through protective zinc coatings roughly two to three times faster than untreated wood would. This means that decks built with ACQ lumber and standard galvanized hardware experience accelerated fastener failure, with screws and joist hanger nails potentially losing structural integrity within 8 to 12 years in New Brunswick's wet climate.

MCA technology addressed this corrosion problem by changing how the copper is delivered into the wood. Instead of dissolving copper in an alkaline solution, MCA uses micronized copper particles, essentially copper ground to an extremely fine powder and suspended in water. These tiny particles are driven into the wood's cellular structure during the pressure treatment process, where they become physically lodged rather than remaining in a chemically reactive dissolved state. The result is wood that contains the same protective copper but in a form that is far less aggressive toward metal fasteners. Independent testing has consistently shown that MCA-treated lumber corrodes galvanized fasteners at rates comparable to untreated wood, which represents a dramatic improvement over ACQ.

For New Brunswick deck builders, the practical implications are straightforward. With MCA-treated lumber, standard hot-dipped galvanized fasteners meeting the G185 coating specification are considered compatible and should provide normal service life. This is the combination most building supply dealers in Moncton, Fredericton, and Saint John stock and recommend. You still benefit from upgrading to coated screws from manufacturers like GRK or SPAX at $0.08 to $0.15 per screw for better long-term performance, and stainless steel remains the gold standard for coastal locations like Saint John, Bathurst, and Shediac, but MCA does not demand stainless steel the way ACQ arguably does.

When shopping for pressure-treated lumber at New Brunswick suppliers, you can identify the treatment type from the end tag stapled to each board. MCA-treated lumber is typically stamped with designations like MCA-C or bears the Wolmanized brand name from Lonza, the primary manufacturer. ACQ lumber will be marked as ACQ-C, ACQ-D, or similar variants. Most major suppliers in the province, including Kent Building Supplies and Home Hardware locations, have largely transitioned their residential inventory to MCA, but ACQ stock still circulates, particularly at smaller yards or when older inventory is being cleared.

The colour difference between the two treatments is subtle but noticeable. ACQ-treated wood tends to have a stronger green tint due to the higher concentration of dissolved copper, while MCA-treated lumber appears more naturally brown or tan with a lighter greenish cast. Both will weather to a similar grey over time if left unstained, and both accept stain and finish products equally well. For New Brunswick homeowners planning to stain their deck, the treatment type has no meaningful impact on stain adhesion or colour outcome, though allowing the standard four to eight weeks of drying time before staining applies to both.

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