Deck Footing Size Chart: Diameter & Depth by Post Spacing

IRC R507.3 and AWC DCA-6 prescriptive footing tables for residential decks

On 1,500 psf soil (the IRC default), a deck post supporting 80 sq ft of tributary area needs a 24-inch-diameter footing. Drop that to a 16-inch footing on 3,000 psf soil. Drop it to 12 inches if the tributary area is only 20 sq ft (a small entry deck).

Footing diameter comes from IRC R507.3.1 (AWC DCA-6 Table 4). Footing depth must be at least 12 inches below grade and below the local frost line, whichever is deeper.

IRC R507.3.1 Footing Diameter by Tributary Area

Tributary area is the deck area that one post supports. Find your tributary area in the left column, then read across to your soil bearing capacity. The number is the minimum footing diameter in inches for a round (Sonotube-style) footing.

Tributary Area (sq ft) 1,500 psf 2,000 psf 3,000 psf 4,000 psf
2012"11"9"8"
4017"15"13"11"
6021"18"15"13"
8024"21"17"15"
10027"23"19"17"
12029"25"21"18"
14031"27"22"20"
16034"29"24"21"

Table assumptions

  • - Loads: 40 psf live + 10 psf dead = 50 psf total (IRC R301.5 residential deck load).
  • - Round footings cast against undisturbed earth or properly compacted granular fill.
  • - Soil bearing capacity per IRC R401.4.1. If no geotechnical report exists, use 1,500 psf (clay or silt presumed value).
  • - For square footings, the side length equals 0.886 × the round diameter, rounded up (same bearing area).
  • - These values are minimums. Local amendments may require larger footings.

Footing Depth Requirements

Depth is set by two rules. Use the deeper of:

Rule 1: 12 inches below grade (IRC R403.1.4)

The bottom of the footing must sit at least 12 inches below undisturbed ground surface. This applies even in regions without significant frost.

Rule 2: Below the local frost line (IRC R403.1.4.1)

In freezing climates, the bottom of the footing must sit below the maximum frost penetration depth. Frost depths range from 0 in (southern Gulf states) to 100+ in (northern Minnesota, North Dakota, Alaska).

Look up your local frost depth in the frost line depth guide. A 14-inch-diameter deck footing in Chicago (42-inch frost line) needs a Sonotube 42 inches deep — not the 12-inch minimum.

Exception: Frost-protected shallow foundations

IRC R403.3 allows shallow footings below heated, conditioned structures using rigid insulation. This rarely applies to open exterior decks because the deck is unheated. Open-deck footings must reach the frost line.

Above Ground Deck Footings: Pier Blocks vs Poured Footings

"Above ground" deck footings are precast concrete pier blocks (deck blocks) that sit on grade or on a few inches of compacted gravel with no excavation below the frost line. They are legal in more places than most people think, and illegal in more places than the product packaging suggests.

When code allows pier blocks

  • - The deck is freestanding (not attached to the house) and small: IRC R403.1.4.1 exempts freestanding accessory structures of 600 sq ft or less and 10 ft or less in eave height from frost protection, and many jurisdictions cap "floating" decks at 200 sq ft and 30 in above grade.
  • - The deck is less than 30 in above grade, so no guard is required and a small heave will not create a fall hazard.
  • - The block bears on undisturbed soil or compacted gravel, and its bearing area satisfies the tributary-load math in the table above (a standard 8 x 11 in block is about 0.6 sq ft; at 1,500 psf soil that is 900 lb, enough for a post carrying about 18 sq ft of deck).

When you must pour a footing below the frost line

  • - The deck is attached to the house with a ledger. If the outer posts heave and the ledger does not, the joists rack and the ledger connection fails.
  • - The deck carries a roof, hot tub or stairs more than 30 in above grade, or exceeds the local freestanding size limit.
  • - Your Table R301.2(1) frost depth is 24 in or more and the inspector treats deck blocks as unsupported footings. Ask before you buy the blocks.

Look up your frost depth on the frost line depth by ZIP code page. If you do have to pour, the footing calculator gives you the concrete per pier, and the post hole calculator converts that into bags.

How to Calculate Tributary Area

Tributary area is the deck area whose load transfers to one specific post. For a freestanding deck with a single row of posts down the middle, each post carries the area halfway to each adjacent post in both directions.

Formula

Tributary area = Post spacing along beam × Joist span / 2

"Joist span / 2" applies when the deck is attached to a ledger on the house at one end and rests on posts at the other end. The posts carry half the load; the ledger carries the other half. For a freestanding deck supported by posts on both sides, divide by 2 differently — each row of posts carries half the joist span.

Worked Example: 12 ft × 16 ft Deck

A 12 ft × 16 ft deck ledger-attached to the house. Three posts along the outer beam at 8 ft on center. Joists span 12 ft from house ledger to outer beam.

  • - Post spacing along beam: 8 ft
  • - Joist span: 12 ft
  • - Tributary area per outer post: 8 × 12 / 2 = 48 sq ft
  • - On 1,500 psf soil, the footing chart calls for ~18-inch diameter (between the 40 and 60 sq ft rows; round up).

The corner posts at each end actually carry half of that — 24 sq ft — because they sit at the end of the beam and share load with the house corner. For simplicity, size all outer-beam footings to the largest tributary area on that beam.

Worked Example: 10 ft × 20 ft Freestanding Deck

A 10 ft × 20 ft freestanding deck (no ledger). Three posts down each long side at 10 ft on center. Joists span 10 ft between the two beams.

  • - Post spacing along beam: 10 ft
  • - Joist span between beams: 10 ft
  • - Tributary area per post: 10 × 10 / 2 = 50 sq ft
  • - On 2,000 psf soil, the chart calls for ~17-inch diameter (between 40 and 60 sq ft rows; round up).

All six posts are sized identically. Run the numbers for your deck with the footing calculator or the post hole calculator.

Pier vs Spread Footing

Two common deck-footing geometries solve the same problem in different ways.

Pier footing (Sonotube)

A continuous round concrete pier extending from below the frost line to grade. Pour into a cardboard form tube (Sonotube). Bottom must equal the required footing diameter. Add a bell-shaped flare at the base if soil is marginal, or pour a spread footing below the pier. This is the simplest method and works for most residential decks. Use a post anchor or J-bolt embedded in the top to attach the wood post.

Spread footing with pier

A square or round concrete pad poured at the bottom of the hole, with a smaller pier rising from it to grade. The pad provides the bearing area; the pier connects to the post above. This uses less concrete than a full-diameter pier and is more efficient for deep frost-line footings (e.g., 48-inch holes with a 12-inch diameter at grade and a 24-inch square pad below).

Helical pier

Steel helical piles driven into the ground with hydraulic equipment. No excavation, no concrete. A common alternative when soil is poor (organic, fill, or expansive clay) or when the deck location can't accommodate a Sonotube hole. Helical piers are rated by the installer's torque measurement at depth; the installer provides a certification matching the loads. Cost runs $200–$400 per pier installed, comparable to a deep frost-line Sonotube pour.

Common Sizing Mistakes

  • - Sizing footings by post size, not load. A 6×6 post does not require a 14-inch footing just because the post is large. The footing diameter comes from the tributary load and soil bearing, not from the post dimension.
  • - Ignoring snow load on decks in northern climates. The 40 psf live load assumption covers most U.S. regions, but ground snow load above 50 psf (much of New England, upper Midwest, mountain west) requires a heavier design. Check IRC Table R301.2(5) for local snow load.
  • - Forgetting the frost line. A 16-inch-diameter footing only 24 inches deep in a 42-inch frost zone will heave every winter. Depth is as critical as diameter.
  • - Assuming 1,500 psf is conservative. Organic or fill soils can rate below 1,000 psf. If the hole walls slough, the soil is loose. A geotechnical test costs $300–$500 and is cheap insurance for a large deck.
  • - Bell-bottoming Sonotubes in clay. A flared base looks like extra bearing area but is essentially impossible to compact properly. Pour a separate spread footing below the pier instead.

Sizing a Deck Footing in 5 Steps

  1. Calculate tributary area for the most-loaded post: post spacing × joist span / 2 (ledger-attached) or × joist span / 2 for freestanding both rows.
  2. Determine soil bearing capacity. Default to 1,500 psf if no soil report exists.
  3. Look up footing diameter in the IRC R507.3.1 table above. Round up to the next size if your tributary area is between rows.
  4. Determine footing depth: 12 inches minimum, or below the local frost line. Use the deeper number. Look up your frost line.
  5. Use the same dimensions for every post on the deck. Sizing each post individually saves a few dollars in concrete but introduces error.
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