The green boards: a field guide to pressure-treated lumber
Pressure-treated lumber punishes anyone who treats it like regular wood. Here is what the green stuff actually is, when you actually need it, and the handful of rules that keep a deck standing.
Walk past the outdoor rack at any lumberyard and there it is: slightly green, slightly damp, heavier than it looks. Pressure-treated lumber is just ordinary framing lumber — almost always southern yellow pine, because its structure drinks the treatment deeply — that's been loaded into a giant pressure vessel and infused with copper-based preservative until the stuff fungus and insects eat is poisoned against them. That's the whole trick. It's not a coating; it goes into the wood, which is why a fresh cut shows a green ring around a pale center.
What it's for (and not for)
Use it anywhere wood meets weather, dirt, or concrete: deck framing, fence posts, sill plates on foundations, raised planters, stair stringers on grade, anything within splash distance of soil. Don't bother with it indoors where nothing stays wet — it costs more, weighs more, works worse, and the corrosion rules below follow it inside. And it's not furniture wood: it's wet, knotty, and moves like crazy while it dries. Build the deck frame from it and put something nicer on top.
Read the tag, not the shelf sign
Every treated board has a little stapled end tag, and it matters more than the price. The two ratings you'll actually meet are above ground and ground contact — ground-contact stock carries roughly twice the preservative. The rule that keeps decks standing: anything touching soil, buried, set in concrete, or closer than about six inches to the ground needs ground contact. Posts always do. When in doubt, buy ground contact; the upcharge is small and the failure is a rebuild.
A history note that still causes confusion: the old arsenic-based treatment (CCA) was phased out of residential lumber back in 2003–04. Modern treatments are copper-based — that's what the green tint is — which made the wood friendlier to people and absolutely vicious to fasteners.
It eats ordinary screws
All that copper turns damp treated lumber into a slow battery, and plain steel or electroplated screws are the sacrificial part. A deck screwed together with shiny interior screws can shed its fasteners in a few years. Use hot-dip galvanized or stainless fasteners and connectors rated for treated lumber — the box will say so — and never let aluminum touch it at all. Aluminum flashing against a treated ledger corrodes into lace; there's vinyl-coated and copper flashing for exactly this reason.
It's sold wet, and it shrinks on your deck
Treated boards leave the store still damp from the pressure vessel — sometimes literally dripping. Over the first months outdoors they dry and shrink, mostly across their width. Deck builders exploit this: install the boards tight together, and drying opens up the drainage gaps on its own. Fight it and you lose; screw down a soaked board with a pretty 1/4″ gap and you'll have a 1/2″ gap by fall. Same story for staining — finish won't stick to wet wood, so wait until the classic sprinkle test passes: water that beads means wait, water that soaks in means go. That usually takes a month or three. (If you hate waiting, some yards sell KDAT — kiln-dried-after-treatment — at a premium.)
The safety rules that are actually about safety
Modern treated lumber isn't the hazard its arsenic-era ancestor was, but a few rules stay non-negotiable. Never burn it — not offcuts, not old deck boards, not in the fire pit, not "just once"; the smoke and ash concentrate the preservative. Wear a dust mask when cutting and gloves for long handling sessions, and wash up before lunch. Brush a copper-based end-cut preservative on fresh cuts — the factory treatment doesn't reach the center of thick stock, and your cut just exposed the untreated core. And send offcuts to the trash, not the burn pile or the wood chipper mulch.
The raised-garden-bed question comes up constantly: manufacturers say modern copper-treated lumber is fine for vegetable beds, and most extension services agree the copper largely stays put. Plenty of gardeners line their beds with plastic anyway for peace of mind. Both positions are reasonable; pick your comfort level.
The two numbers to carry to the yard
A treated board is heavier than the same board dry — often half again as much (a 6×6 is a two-person post) — so plan the hauling. And treated dimensional lumber follows the same nominal-size rules as everything else: the 2×6 is 1-1/2″ × 5-1/2″, and the planner will happily total a whole deck's worth, waste and tax included.