Pressure-Treated Lumber
Affiliate link — we may earn a commissionPressure-treated (PT) lumber is wood that has been infused with chemical preservatives under pressure to resist rot, insects, and fungal decay. The treatment pushes preservatives deep into the wood fibers, not just the surface. Modern PT lumber uses MCA (micronized copper azole) or ACQ (alkaline copper quaternary) instead of the older CCA (chromated copper arsenate) that was phased out for residential use in 2004. PT lumber is standard for deck framing, fence posts, sill plates, and any wood that contacts the ground or stays exposed to weather. It comes in ground-contact and above-ground ratings.
Why It Matters
Untreated lumber in ground contact rots in 2 to 5 years. PT lumber lasts 15 to 40 years in the same conditions. Use ground-contact rated PT (stamped "GC" or "UC4A") for posts set in concrete and joists near soil. Above-ground rated PT works for deck boards and railings. Always use stainless steel or hot-dipped galvanized fasteners with PT lumber. The copper in the treatment corrodes standard zinc-plated fasteners.