Deck Footing Size Guide

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What Tributary Area Means on a Real Deck

Tributary area is the patch of deck surface whose weight (live + dead, or snow + dead) rides down through one post into one footing. It is not the whole deck. It is not the hole diameter. IRC Table R507.3.1 footnote e says it outright: area, in square feet, of deck surface supported by post and footings.

On a rectangular dropped-beam deck the picture is a rectangle around the post:

• Tributary length along the beam: half the distance to the next post (or to the end of a cantilever) plus half the distance to the post on the other side.
• Tributary width perpendicular to the beam: half the joist span to the ledger (or to a second beam), plus the full cantilever if joists run past the beam.
• Area = length × width, in square feet.

A corner post on the same deck often carries about a quarter of an interior bay, not half of two bays. A stair landing post can be a 5 sq ft row. Guessing “the whole 12×16 deck is 192, divide by four posts” is how you undersize the interior footings and oversize the corners.

You cannot get tributary area until you know post spacing (use the deck post spacing calculator), and you cannot honestly pick post spacing until the beam span table says the built-up beam reaches. Load path: joists → beam → post → footing. Skip a step and the area you type is fiction.

How Soil Bearing Capacity Changes Footing Size

The same 40 sq ft of deck on 40 psf live load is a 14″ square / 16″ round pad at 1,500 psf soil, a 12″ square / 14″ round pad at 2,000 psf, and a 10″ square / 12″ round pad at ≥3,000 psf. That is Table R507.3.1 talking. Weaker soil, bigger pad. Stronger soil, smaller pad — until the 7″/8″ floor that shows up on the 5 sq ft rows.

Those three columns are not a menu of how optimistic you feel in the hole. They are presumptive allowable bearing pressures from IRC Table R401.4.1 (clay, sand, gravel, sedimentary rock, and so on). The footing table just consumes whatever number you claim. Claim 3,000 on fill and the pad is too small the day the inspector, or the first wet season, disagrees.

Thickness in the table is for plain concrete footings (footnote f). A beefy engineered footing with rebar is a different animal. The table also requires the pad to fully bear the post, and a 2-inch projection on all sides if the support is a brick or CMU pier.

How to Find Your Local Soil Bearing Capacity

Three layers, in the order most carpenters actually get an answer:

1. Ask the building department what they accept without a report. Many AHJs publish a default (often 1,500 or 2,000 psf) on the deck handout. Some will not accept anything above 1,500 without a soils report. That conversation is cheaper than a redesign after the first inspection.
2. Read IRC Table R401.4.1 as a presumptive list, not a lab result. Clayey soils land near 1,500. Sandy soils often 2,000. Gravelly soils 3,000. Crystalline bedrock is far higher and is not an excuse to skip frost depth. The table assumes you classified the soil correctly. Most of us are not geotechnical engineers with a jar test.
3. Site testing when the jurisdiction requires it, or when the site looks wrong. Fill, organics, a high water table, a cut slope, or a previous failed deck are reasons to stop guessing. A soils report or an inspector sign-off on the excavated hole is the actual answer. The calculator cannot see your backyard.

USDA soil maps and well-driller logs are clues, not a substitute for the AHJ. If two neighbors poured 10-inch tubes on “good gravel” and the town still wants 1,500 psf on the permit, the town wins.

Local amendments to this table are common

Footing-size tables get rewritten more often than joist or beam span tables. Pad size is soil- and climate-dependent, so states and cities often keep their own Table R507.3.1 instead of taking the model IRC row. The calculator prints the standard 2021 IRC cells. If your jurisdiction amended that table, the number on this page is the wrong number.

Two verified examples, not a complete list:

• Seattle / Washington, 2021. The 2021 Seattle Residential Code keeps the Washington amendment in lieu of the model table (WAC 51-51-0507). Table R507.3.1 is a single combined “60 live or 70 ground snow” row — not the standard IRC 40 psf residential row. On 40 sq ft at 2,000 psf soil, the model IRC 40 psf cell is 12″ square / 14″ round; the Washington combined row is 15″ square / 17″ round.
• Minnesota, MN Rules 1309.0507. Minnesota replaces Table R507.3.1 entirely and sets a floor: footnote c assumes a minimum 12″ × 12″ × 6″ square footing for a 6x6 post. At 20 sq ft on 1,500 psf soil the Minnesota cell is 12″ square / 14″ round; the model IRC 2021 40 psf cell for that same area is 10″ square / 12″ round. Minnesota also has no 5 sq ft row.

Ask the building department which table they actually enforce — model IRC, a state amendment, or a city overlay — before you order tubes. That question matters more for footings than for a 2-ply 2x10 beam span.

What the Number Is Not

It is not frost depth. R507.3.2 puts the bottom at least 12 inches into undisturbed ground. R507.3.3 / R403.1.4 put attached-deck footings below the frost line in Table R301.2 when the house is frost-protected. A correctly sized 14-inch pad that only goes 18 inches down in a 42-inch frost town will still heave. That story is troubleshooting.

It is not post size or post height. Table R507.4 is a different table for the same stick of lumber. A 6x6 that reaches 14 feet does not magically shrink the pad, and a 24-inch pad does not let a 4x4 run 14 feet under a 160 sq ft tributary. That split is the whole footing vs post height comparison.

It is not the beam or the joists. Those are R507.5(1) and R507.6. Work them first so the tributary area is real: beam calculator, joist calculator.

Layout after the pad: deck stairs, interior stringers, common rafters, board feet.

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