Lumber Buying Guide for DIY Decks: Grades, Sizes, and Actual Costs

Buying lumber for a deck project is more complicated than it looks. The wrong grade means a warped deck in two years. Here's what you actually need to know before you load up the cart at the lumber yard.

Pressure-Treated vs. Untreated: This Is Not Optional

Any lumber in contact with the ground, concrete, or exposed to weather must be pressure-treated (PT). PT lumber retention levels:

UC3B (Above Ground): For joists, beams, and decking boards not touching soil
UC4A (Ground Contact): For posts within 6 inches of soil
UC4B (Ground Contact — Critical): For posts in direct soil contact in high-decay-hazard areas

Current PT lumber uses copper-based preservatives. Requires hot-dipped galvanized or stainless hardware — not standard zinc-plated fasteners.

Actual vs. Nominal Lumber Sizes

Lumber is sold by nominal size but is actually smaller:

"2×6" is actually 1.5" × 5.5"
"2×8" is actually 1.5" × 7.25"
"2×10" is actually 1.5" × 9.25"
"4×4" post is actually 3.5" × 3.5"
"6×6" post is actually 5.5" × 5.5"

Always use actual dimensions in structural calculations. A joist span chart lists allowable spans for actual lumber dimensions.

Lumber Grades for Deck Framing

#1 Grade: Tight knots, high strength. Recommended for beams and load-bearing members.
#2 Grade: Larger knots allowed, still structurally sound. The standard choice for most deck framing.
#3 Grade / Economy: Avoid for structural use. Fine for blocking only.

2026 Lumber Pricing

2×6 × 8' PT: $8–$12
2×6 × 12' PT: $12–$18
2×8 × 12' PT: $16–$22
4×4 × 8' PT: $10–$16
5/4×6 × 12' PT decking board: $12–$18
2×6 × 12' cedar decking: $20–$35

Buy long boards and cut to length rather than exact-length pieces — fewer joints and often lower cost per linear foot. Use our deck lumber calculator to get the full materials list for your dimensions.

Hardware: The Part Beginners Underestimate

Hardware is 15–25% of total materials cost and critical to longevity:

Joist hangers: Must be PT-compatible (hot-dipped galvanized, ZMAX, or stainless): $1–$3 each
Post bases: Anchor posts to concrete without direct wood-concrete contact: $8–$20 each
Decking screws: 3" exterior-rated coated screws for PT lumber; stainless for cedar/hardwood to prevent staining