Deck Footing Requirements — IRC 2021 R507.3 + R403

Footing diameter is sized by tributary load (deck area each footing supports) and soil bearing capacity. R507.3 Table 1 gives you the sizes:

Tributary area = (post spacing along beam) × (joist span ÷ 2 + cantilever). For a typical 16×20 deck with 8 ft post spacing along the beam and 12 ft joists, tributary per footing = 8 × 6 = 48 sqft. On 2,000 psf sandy soil, that's a 12-14″ diameter footing.

If you're not sure about your soil, IRC R401.4.1 lets you assume 1,500 psf as the default — the most conservative, lowest bearing capacity. Most US residential soils are 2,000-3,000 psf, but verify with your AHJ; some require a soil test.

• Clay (organic / silty): 1,500 psf — most conservative, larger footings
• Sandy clay loam: 2,000 psf — typical Midwest farmland
• Sandy gravel / well-compacted fill: 3,000 psf — common urban backfill
• Sand or sand-gravel: 4,000 psf — coastal deltas, glacial outwash
• Sedimentary rock / partially weathered: 6,000+ psf — bedrock zones

Footings must extend BELOW the local frost depth + at least 6 inches of bearing soil. Frost heave (water in the soil freezing and expanding) lifts shallow footings every winter; footings that reach below the frost line don't move.

State-by-state frost depths (median; verify with your AHJ):

Total footing depth = frost depth + 6" minimum bearing. Examples:

• Houston TX (frost 6"): 6 + 6 = 12" minimum, but most build 24-36" for hurricane uplift safety
• Atlanta GA (frost 6"): 12-24" depending on soil
• Indianapolis IN (frost 36"): 36 + 6 = 42" minimum
• Boston MA (frost 48"): 48 + 6 = 54" minimum
• Minneapolis MN (frost 60"): 60 + 6 = 66" minimum

Sonotube cardboard form lengths: 4 ft, 6 ft, 8 ft, 10 ft, 12 ft. For shallow frost zones, use 4-6 ft tubes. For deep frost (60"+), use 8-10 ft tubes. Cut to grade once concrete is poured.

IRC R402.2 specifies minimum concrete strength: 2,500 psi for residential footings. Common ready-mix specs:

• 60-lb pre-mix bags (Quikrete Mix or generic 5,000 psi): typical residential, just add water and pour
• 80-lb pre-mix bags: cheaper per cubic foot, heavier to lift
• 3,000 psi ready-mix delivered (truck): for large projects with 20+ footings

Bag count math: 1 cubic foot of footing = 1.0-1.1 60-lb bags or 0.7-0.8 80-lb bags. Round up generously — leftover concrete is cheaper than running short. DeckMath's deck-footing-calculator computes exact bag counts for your dimensions.

IRC R507.5 requires a mechanical connection between post and footing — direct contact with concrete is not enough. Two accepted hardware:

Simpson ABU post bases (most common)

Set into wet concrete, holds the post 1" off the concrete to prevent moisture wicking into the post. Sized to the post:

• ABU44Z for 4×4 posts — $18 each
• ABU66Z for 6×6 posts — $26 each

Simpson PB post brackets (retrofit)

Bolts to top of cured concrete. Cheaper but slightly less robust. Used when concrete is already poured.

1. Footing too shallow — frost heaves the deck, joints crack. Verify your frost depth + 6" bearing.
2. Footing diameter too small — soil compresses, deck settles. Use the R507.3 table with conservative bearing capacity.
3. Skipping rebar in deep / large footings — typically required at 8"+ diameter for tension reinforcement. Verify with AHJ.
4. Concrete not below frost line in cantilever zone — sometimes contractors set the cantilever-side footing higher to grade. The footing depth still applies regardless of where on the deck.
5. Drilling post anchor bolts AFTER concrete cures — anchor pull-out strength is much weaker than wet-set. Always set anchors in wet concrete.
6. Pouring concrete in subfreezing weather — concrete won't cure properly below 40°F. Use insulated blankets or wait for warmer weather.