The five IRC zones a deck inspector actually checks

Five structural zones plus stair geometry. (1) Footings: size + depth below frost line per IRC R507.3 and R403.1.4. (2) Ledger board: bolt size + spacing + flashing per R507.9.1. (3) Joist hangers: every joist + Simpson-brand nail in every hole per R507.6. (4) Guard rail: height ≥36″ and 4″ sphere baluster spacing per R312.1.2 + R312.1.3. (5) Stairs: tread + riser geometry per R311.7. Most failed inspections trace back to ledger flashing (missing) or footing depth (too shallow).

Improper ledger attachment — usually missing or wrong flashing, undersized lag bolts, or lag bolts in band joist instead of structural framing. This is also the #1 cause of catastrophic deck collapse in real-world data (CPSC + ASHI studies). The ledger carries 50% of the deck's load directly into the house wall, so when it fails the whole deck pulls away. Inspectors flag missing z-flashing (covering the top edge of the ledger), back-flashing (between ledger and siding), and rim-joist bolting (vs structural-stud bolting) most often.

In most US jurisdictions, yes — for any deck over 200 sqft, more than 30 inches above grade, or attached to a dwelling. Some jurisdictions exempt small (<100 sqft) freestanding ground-level decks. Even when permits are exempt the IRC structural requirements still apply (and your homeowner's insurance can deny coverage on non-permitted structures after a failure). The local building department is the authority — call before designing, not before pouring footings.

Below the local frost line — varies by state and city. Examples: Florida coast 12″, Atlanta 18″, Denver 36″, Minneapolis 48″, Anchorage 60″. The Footing Depth Calculator gives the exact depth for every US zip code. Going shallower than your local frost line is the #2 cause of deck failure (after ledger issues) — frost heave will lift the post seasonally, eventually breaking the deck-to-house ledger connection.

Simpson Strong-Tie's load ratings assume every nail hole is filled with the specified Simpson-brand nail (typically 10d × 1-1/2″ for residential joist hangers). Studies have shown that filling only half the holes drops the hanger's allowable load by 60-70% and shifts the failure mode from gradual to catastrophic. The cost of nails is trivial; the cost of a half-nailed hanger failure under a backyard party load is not. Always every hole, always Simpson nails.

IRC R312.1.3 requires that a 4-inch rigid sphere not pass between any guard infill members (balusters, cables, slats). For traditional vertical balusters that means ≤4″ on-center spacing measured between the balusters. The rule is enforced specifically because it correlates to the size of a small child's head — the original code intent was preventing kids from getting their heads stuck (which is more dangerous than falling through). Cable railings, glass, and infill panels all measure against the same sphere.

IRC (International Residential Code) is a national model code that ~95% of US jurisdictions adopt with local amendments. Local amendments mostly tighten requirements (deeper footings for frost, higher wind / seismic loads, additional permits for waterfront construction) — they almost never relax IRC minimums. When IRC and local code conflict, the stricter wins. The animation reflects IRC 2021 (current edition as of most jurisdictions in 2025-2026). The Footing Depth + Deck Beam Span calculators include the local amendments by state.