Building a ground-level deck with 4x4 posts in the dirt and joists a few inches off grade.

Lumber:

• Posts: Use UC-4B treated 4x4s (ground-contact rated) — rated for direct burial.
• Joists/ledger: Use UC-3B treated lumber (above-ground, ground-contact exposure).

Hardware:

• Post anchors: Use hot-dip galvanized (HDG) or stainless steel post bases (e.g., Simpson PBSQ-SDS). Keep posts 1" above grade minimum.
• Joist hangers: Use HDG or stainless steel (Simpson LUS or A35).
• Fasteners: Use HDG or stainless steel nails/bolts rated for treated lumber (ASTM A653/A153 coating).

Key details:

• Concrete footings: Below frost line (per local code). Use Sonotubes with proper embedment.
• Ventilation: Leave 2-4" air gap between joists and soil. Install a vapor barrier (6-mil poly) under the deck area.
• Slope: 1/8" per foot away from structure for drainage.

Code references:

• IRC R318.1 (deck foundations)
• AWPA U1 (treatment standards)
• Local building codes may require permits and inspections.

Avoid: Regular steel or aluminum hardware — will corrode rapidly with treated lumber.

If your local code requires engineered plans or permits, consult a licensed contractor.

Lumber:

• Posts (in ground): UC4B (Ground Contact, Heavy Duty). Standard UC4A is insufficient for direct burial. Look for the end tag: "Ground Contact" + "UC4B" or "Heavy Duty." Use 6x6 posts, not 4x4—4x4s lack bearing area and rot faster.
• Joists/Beams (near grade): UC4A (Ground Contact, General Use) minimum. Since they are "a few inches off grade," they are in the splash zone. Do not use Above Ground (UC3B) lumber.

Hardware (Critical):

• Fasteners/Connectors: Hot-Dip Galvanized (HDG) per ASTM A153 Class D or Stainless Steel (Grade 304/316).
• Do NOT use: Standard G90/G185 galvanized (ZMAX, Triple Zinc), electro-galvanized, or "exterior" screws. The copper in modern preservatives (ACQ, CA, MCA) corrodes thin zinc rapidly.
• Post Bases: Use HDG or SS post bases (e.g., Simpson CB44/66 HDG or CBSQ) with a 1" stand-off to keep post end grain out of standing water/mud. Embed base in concrete pier, not dirt.
• Joist Hangers/Straps: HDG (Simpson ZMAX/HDG) or SS.

Installation Details (IRC R507 / AWPA U1):

1. Footings: Pour concrete piers (min. 12" dia.) above grade (min. 6" above grade per IRC R403.1.6). Set post bases in wet concrete or anchor with HDG/SS epoxy anchors (Simpson SET-3G) after cure.
2. Drainage: Backfill piers with washed gravel, not soil. Slope grade away.
3. Flashing: Apply self-adhering flashing tape on top of beams/joists before decking.
4. Field Cuts: Brush copper naphthenate (2% Cu) on all cuts/holes (IRC R317.1.1, AWPA M4).

Verdict: UC4B 6x6 posts on HDG/SS stand-off bases on concrete piers + UC4A joists + HDG/SS hardware. If within 300 ft of saltwater, specify 316 Stainless Steel for all hardware.