Pressure Treated Decking: Complete Guide

Roughly three out of four decks in America still get built with treated lumber, and the reason is arithmetic: at $2 to $5 per square foot for boards, nothing else comes close. But pressure treated decking is also the most misunderstood material at the lumberyard — buyers grab wet, twisted #2 boards with the wrong treatment rating, fasten them with the wrong screws, and then blame the wood when the deck cups and the hardware corrodes. Treated decking done right is a different product from treated decking grabbed off the top of the stack. This guide covers the treatment chemistries, grades, sizes, ratings, and movement realities that separate a 25-year deck from a 8-year eyesore.

Part of our Decks & Outdoor guide — explore the full guide →

What the Treatment Actually Is: CA and MCA

Pressure treating forces waterborne preservative into southern yellow pine (or hem-fir and Douglas fir in the West) inside a pressure vessel at around 150 psi. The chemistry has changed twice in a generation. Arsenic-based CCA left the residential market in 2004. Its replacements, and what you will find on today’s tags:

• Copper azole (CA-C, sold as Wolmanized and others) — dissolved copper plus azole co-biocides. Slight green-brown cast.
• Micronized copper azole (MCA, sold as MicroPro, Ecolife-adjacent lines, ProWood) — copper ground to sub-micron particles rather than dissolved. Now the dominant residential treatment. Advantages: lighter, more stainable color; less corrosive to fasteners than dissolved-copper formulas; certified for many environmentally sensitive uses.

Copper is the workhorse in both — it is toxic to fungi and the insects that eat wood, which is the entire point. Neither modern chemistry contains arsenic or chromium. Sawdust is still not something to breathe: cut with a mask, and never burn offcuts.

Reading the Tag: Use Category and Retention

Every treated board carries an end tag with an AWPA Use Category. For decks, two matter:

• UC3B — Above Ground. Retention around 0.06 pcf copper azole. Fine for deck boards with 6+ inches of ventilation below, and only if cut ends get field-treated.
• UC4A — Ground Contact. Retention around 0.15 pcf, roughly double the preservative. Required for posts, joists near grade, and any board within 6 inches of soil, over poor drainage, or buried in vegetation.

Here is the pro move: buy ground-contact for everything, including the deck surface. Since the 2016 AWPA standard revision pushed most framing to UC4A, many yards stock ground-contact as their default anyway, and the price difference is small — often 10 to 20 percent. The extra retention is cheap insurance for boards over shaded, damp, or low-clearance framing, exactly where above-ground boards fail early. Always brush cut ends, notches, and drilled holes with a copper naphthenate end-cut solution (Coppercoat, about $20 a quart); the treatment envelope only penetrates roughly half an inch to an inch, and untreated heartwood is exposed at every cut.

Grades: Where the Money Is Well Spent

Treatment prevents rot; grade determines whether the deck looks decent. Southern pine decking comes in:

• #2 grade — the commodity stack. Allows large knots, wane, and pith. Cheapest, most prone to twist and shell out at knots.
• #1 grade — fewer, smaller knots, straighter grain. Usually 15 to 25 percent over #2 and worth every cent for the walking surface.
• Premium / “Select” decking — the appearance tiers many treaters brand (e.g., radius-edge premium decking): tight knots, eased edges, better face. This is the stack to hand-pick from.
• KDAT (kiln-dried after treatment) — any grade, re-dried to ~19 percent moisture after treating. Costs 30 to 50 percent more, arrives dimensionally stable, takes finish immediately, and shrinks far less. If the budget allows one upgrade, KDAT is it.

Whatever the grade, sight down every board at the yard and reject anything with twist, crook, or boxed pith. The five minutes of sorting outperforms any product upgrade.

5/4×6 vs. 2×6: Picking the Board

Two profiles dominate deck surfaces:

• 5/4×6 radius-edge decking (actual 1 x 5-1/2 in.): the purpose-made deck board with rounded edges. Spans 16-inch joist spacing comfortably; at 24 inches it feels bouncy and most manufacturers disallow it. Typical cost: $1.60 to $2.50 per linear foot.
• 2×6 (actual 1-1/2 x 5-1/2 in.): 50 percent thicker, spans 24-inch framing, feels rock solid, and resists cupping better simply by mass. Costs 25 to 40 percent more than 5/4 and needs longer screws.

On 16-inch joists, 5/4 premium is the standard, economical choice. Choose 2×6 for 24-inch framing, diagonal decking layouts (which shorten effective span support), stair treads (doubled 2×6 or 5/4 over closer stringers), and anywhere you want a heavier, quieter walk. Skip 2×4 decking — more gaps, more fasteners, no benefit.

Fasteners: Where Cheap Decks Die

Copper preservative plus steel plus moisture is a battery. Bright or merely zinc-plated screws corrode inside treated lumber in a few seasons, leaving black streaks and snapped shanks. Code and every treater’s literature require:

• Hot-dip galvanized (G-185 coating) nails and hardware, or
• Coated deck screws rated for treated lumber (polymer-over-galvanized types such as GRK, Deckmate, SPAX exterior), or
• Stainless steel (Type 304/316) — mandatory near saltwater, best-in-class everywhere.

Use 2-1/2 inch screws for 5/4 boards, 3 inch for 2×6, two per joist crossing, kept 3/4 inch off board ends (pre-drill ends to stop splits). Joist hangers and post hardware must be G-185 or stainless as well — ordinary electro-galvanized hangers are for interior use and will fail against MCA lumber.

Wet Wood Moves: Managing Shrinkage and Cupping

Most treated lumber sells “wet” — 35 to 75 percent moisture content straight from the vessel. It will lose that water in your backyard, and it will move while doing it. Plan for it:

1. Gap accordingly. Install wet boards tight together or with a 1/8-inch gap at most; they will shrink roughly 1/4 inch in width, opening the joint on their own. Only KDAT boards get deliberately gapped 1/4 inch at install.
2. Bark side vs. crown: the old bark-side-up rule matters less than fastening: two screws per crossing, snugged firm, is what actually restrains cupping.
3. Sticker and acclimate boards a week or two out of the rain if you can — and never let a wet stack sit banded in the sun, which invites instant checking and twist.
4. Expect checking. Small surface cracks are cosmetic and inevitable in southern pine. Grade and drying quality control their size.

Cost vs. the Alternatives

Board-only pricing per square foot of deck surface runs roughly: treated #2, $2.00–$3.00; treated premium/KDAT, $3.00–$5.00; cedar, $4.00–$8.00; entry composite, $5.00–$8.00; premium capped composite or PVC, $8.00–$14.00. On a 300 sq ft deck, treated premium saves $1,200 to $2,500 over composite in boards alone. The counterweight is time: figure a refinishing cycle every 2 to 3 years at $100 to $200 in materials DIY, and a realistic surface lifespan of 15 to 25 years with care versus 25 to 50 for composite. Treated wins on first cost and repairability; composite wins on lifetime labor. (Cedar and the major composite brands each have dedicated guides on this site.)

Maintenance Schedule That Keeps It Alive

• Months 1–6: let wet lumber dry. Sweep, keep gaps clear, nothing else.
• When water stops beading (sprinkle test), typically 2–6 months (KDAT: right away): clean with an oxygenated deck cleaner, brighten, and apply a penetrating finish. Details on products and technique live in our guide to staining pressure treated wood.
• Every year: wash off mildew and pollen; re-tighten any raised fasteners; clear debris from between boards — packed leaves hold the moisture that starts rot.
• Every 2–3 years: re-coat. Horizontal surfaces weather twice as fast as railings.
• Every 5 years: crawl underneath and inspect joist tops, ledger flashing, and post bases — decks fail from the frame first.

Bottom Line

Pressure treated decking remains the value king, but only when specified like a pro: ground-contact MCA retention, #1 or premium grade (KDAT if the budget stretches), 5/4 on 16-inch centers or 2×6 on 24, hot-dip or better fasteners, end cuts sealed, and finish applied once the wood dries. Do those things and a $2,500 deck surface honestly delivers two decades. Skip them and you will be shopping for boards again before the grill needs replacing.