What is the difference between AC2 and MCA pressure treatment for NB decks?

AC2 and MCA are both copper-based pressure treatments, but MCA (micronized copper azole) is the current standard that replaced AC2 around 2013-2015. For New Brunswick decks, you'll almost exclusively find MCA-treated lumber at retailers today, and it's actually superior for our Maritime climate.

MCA treatment uses microscopic copper particles suspended in an azole co-biocide, creating more uniform protection throughout the wood fiber. This micronized copper penetrates deeper and more evenly than the older AC2 formulation, which is crucial for NB's harsh freeze-thaw cycles that can open up wood grain and expose untreated areas. The azole component provides additional protection against the fungi and mold that thrive in our humid Maritime climate.

AC2 (Alkaline Copper Quaternary) was the previous generation copper-based treatment. While effective, it didn't penetrate as uniformly as MCA, and the quaternary ammonium compounds were less effective against certain fungi common in wet climates like ours. You might still find some AC2 lumber in stock at smaller suppliers, but major retailers have largely switched to MCA.

For New Brunswick deck builders, MCA offers several advantages over AC2. It handles our 100+ annual freeze-thaw cycles better due to more uniform copper distribution. The treatment is less likely to leach out during our wet springs and humid summers. MCA-treated lumber also tends to have fewer surface deposits and stains more evenly when you're ready to apply your first coat of stain (remember to wait 3-6 months for the wood to dry below 15% moisture content).

Both treatments are safe for residential use and far safer than the old CCA (chromated copper arsenate) that was banned in 2003. Neither AC2 nor MCA contains arsenic or chromium. However, always wear gloves when handling pressure-treated lumber and avoid burning the scraps.

When shopping for deck lumber in NB, simply ask for "pressure-treated lumber rated for ground contact" if you're buying posts, or "above-ground PT lumber" for joists and decking. The supplier will provide MCA-treated lumber as standard. Look for the grade stamp showing "MCA" and the appropriate use category - UC4A for ground contact posts, UC3B for above-ground framing and decking.

The bottom line: If you're building a deck in New Brunswick today, you'll get MCA treatment, which is actually better suited to our climate than the older AC2 formulation. Focus on proper installation, adequate ventilation, and that crucial 3-6 month drying period before staining rather than worrying about treatment types.

Need help finding a deck builder who understands proper pressure-treated lumber selection and installation? New Brunswick Decks can match you with experienced local contractors for free.