What's below a deck footing — the textbook cross-section

Per IRC R403.1.4, the bottom of the footing must extend below the local frost line — the maximum depth seasonal freezing reaches in your zone. Typical depths: 0″ (subtropical Florida, southern Texas), 18-24″ (Mid-Atlantic), 36-42″ (Midwest, New England, Pacific Northwest mountains), 48-60″ (Northern Plains, interior Alaska excluded). DeckMath's Footing Calculator pulls the exact frost depth for your ZIP. Going 4-6″ deeper than the minimum is cheap insurance against an unusually cold winter.

Water in the soil expands ~9% when it freezes. If your concrete pier is cast within the frost zone, that expansion pushes the pier UP every winter — 1-3 inches per cycle. The pier never fully settles back when the soil thaws (water migrates back in differently), so the deck lifts a little more each year. After 5-10 winters the lift is enough to crack joist connections, pop boards, or pull the ledger off the house. Watch the Frost Heave Demo animation for the time-lapse version of this failure.

4-6″ of compacted gravel (clean #57 stone or 3/4″ minus). Three reasons: (1) it gives the pier base a stable bearing surface that doesn't pump up water during freeze cycles; (2) it lets groundwater drain AWAY from the pier base instead of pooling; (3) it spreads point loads from the pier into a wider footprint of the subgrade soil. Skipping gravel and pouring concrete directly on raw earth is a recipe for differential settlement.

Driven by tributary area (how much deck each post carries) and soil bearing capacity (PSF). Typical sizes: 8″ pier for a small ground-level deck (≤ 8 ft span), 12″ for standard residential decks (12-14 ft beam span), 16″ for large or 2-story-load decks, 24″ for hot-tub piers. The Deck Footing Calculator gives the exact diameter for your tributary area + soil class. Undersized footings cause settlement (deck droops), oversized footings waste money but never fail.

For typical residential deck piers under 4 ft tall: technically not required by the IRC, but recommended. A vertical #4 (1/2″) rebar cage in the upper 24″ of the pier resists tension forces from frost uplift and lateral wind/seismic loads. Without rebar, a pier can shear horizontally during a freeze-thaw cycle if there's any tilt. For tall piers (> 4 ft tall, common on sloped lots), rebar is mandatory per IRC R403.1.6.

Wood in continuous contact with concrete wicks moisture from the slab and rots from the base up. The Simpson CBSQ / EPB / equivalent stand-off post anchor lifts the wood 1″ above the concrete so air can circulate and water never sits against the end grain. End grain is the weakest part of a post against water — capillary action pulls moisture upward into the post like a sponge. Direct-contact mounting drops the post's service life from 25-30 years to 8-12.

Only for free-standing, ground-level decks under 30″ above grade where local code allows them — typically called "floating" or "deck block" construction. The blocks have a notch for the post and they sit ON the soil (or a leveled gravel pad). They do NOT carry the kind of vertical or uplift loads required for attached decks, raised decks, or anything with stairs to the ground. Most building departments reject them for anything over 30″. Pre-cast piers buried below frost (different product) are fine.