Deck Footing Depth by Zip Code

Two rules set the depth. Every footing sits at least 12 in below undisturbed ground. And a footing that holds up part of the house has to be protected from frost — which, for a deck, means the footings under a deck attached to the house. The International Residential Code has said so, in different words, since 2015:

The exact wording

• 2021 and 2024 IRC R507.3.2, Minimum depth: “Deck footings shall be placed not less than 12 inches (305 mm) below the undisturbed ground surface.” 2021 IRC · 2024 IRC
• 2021 and 2024 IRC R507.3.3, Frost protection: “Where decks are attached to a frost-protected structure, deck footings shall be protected from frost by one or more of the following methods: 1. Extending below the frost line specified in Table R301.2. 2. Erecting on solid rock. 3. Other approved methods of frost protection.”
• 2021 and 2024 IRC R507.3, exception 2: “Footings shall not be required for free-standing decks that meet all of the following criteria: 2.1. The joists bear directly on precast concrete pier blocks at grade without support by beams or posts. 2.2. The area of the deck does not exceed 200 square feet (18.6 m2). 2.3. The walking surface is not more than 20 inches (508 mm) above grade at any point within 36 inches (914 mm) measured horizontally from the edge.” R507.3 (2021)
• 2018 IRC R507.3.2, Minimum depth: “Deck footings shall extend below the frost line specified in Table R301.2(1) in accordance with Section R403.1.4.1,” with the pier-block criteria as exception 1 and, as exception 2, “Free-standing decks need not be provided with footings that extend below the frost line.” 2018 IRC
• 2015 and 2018 IRC R403.1.4.1, exception 3: “Decks not supported by a dwelling need not be provided with footings that extend below the frost line.” 2015 IRC · 2018 IRC
• AWC DCA 6 (2015 IRC edition), Footings: “All footings shall bear on undisturbed soil at least 12 inches below the undisturbed ground surface or below the frost line, whichever is deeper.” DCA 6-15

Which one applies to you depends on the edition your state or city has adopted and what it has amended — and deck provisions are among the most amended parts of the IRC. Many building departments hand out a deck guide based on AWC’s DCA 6, which asks for the frost line for every footing. So the lookup shows both figures for a free-standing deck: the model-code minimum and the frost depth, and the calculator defaults to the deeper one. Our deck permit table→ lists the adopted code in 50 cities.

The 200 sq ft / 20 in pier-block rule above is not the permit exemption. IRC R105.2 exempts decks of 200 sq ft or less that are no more than 30 in above grade, not attached to the house and not serving the exit door from a permit — a different test, with its own local variations. See deck permit requirements by city.

Attached (ledger) decks. The house carries one side of the deck through a ledger bolted to its band joist; the footings carry the outer beam. The house sits on frost-protected footings and does not move. If the deck’s footings heave an inch in February, the joists rack against a fixed ledger, fasteners work loose and the deck can pull away from the house — the failure the frost rule exists to prevent. That is why R507.3.3 ties the frost requirement to attachment.

Free-standing decks have a second beam of posts near the house instead of a ledger. The whole deck can rise and fall a little together without tearing anything, so the model code lets its footings stop at 12 in. The catch is that frost rarely lifts footings evenly: a deck on shallow footings in heaving clay can go out of level, pull its stairs away, or bind a door. Going to the frost line costs a few more bags per hole and removes the question. Either kind of deck has one more rule for footings within 5 ft of the house — see below.

Floating decks — low platforms on precast pier blocks — are the one case where the IRC (2021 onward) needs no footings at all, and only within the limits quoted above: joists straight on the blocks, no more than 200 sq ft, no more than 20 in above grade near its edges. The deck moves with the ground and is expected to.

Depth stops frost; size spreads the post’s load over enough soil. The 2021 and 2024 IRC size every deck footing from one table, by the tributary area each post carries (the deck surface it holds up), the load, and the soil’s bearing value. For a post in the middle of a beam, the tributary area is the post spacing times the depth of deck the beam carries: half the joist span, plus any overhang past the beam.

Every value in the table works out as the load on the post — design load plus 10 psf of dead load, times the area — divided by the soil’s bearing value, rounded to the inch. The one exception is marked *: at 40 psf, 20 sq ft and 2,000 psf the table prints a 9 in round footing where the arithmetic needs 9.6 in, so the calculator uses 10 in. Interpolation between rows is allowed (footnote a); going past 160 sq ft or 70 psf is not.

40 psf (live load; ground snow load 40 psf or less)

Source: 2021 and 2024 IRC Table R507.3.1; soil classes from Table R401.4.1. 2021 IRC ch. 5 · R401.4.1. The thickness applies to plain (unreinforced) concrete footings. If the building official expects soil weaker than 1,500 psf, R401.4.1 calls for a soils investigation.

What that means for a 12 in tube

The most common DIY deck footing is a 12 in cardboard tube poured full. On clay at 40 psf the table accepts a 12 in round footing for only about 20 sq ft of deck (40 sq ft on gravel). A typical interior post on a 12 ft deck with posts 8 ft apart carries about 48 sq ft and needs 18 in. Either use a bigger tube, or pour a spread pad of the table size at the bottom and run a smaller tube up from it — the calculator does both.

The first two decks are identical above ground. In Minnesota the holes are 60 in (5 ft) deep rather than 36 in (3 ft), and the heavier snow load pushes each footing from 18 in to 20 in — the pad-and-tube style keeps the concrete to 20 bags against the 31 the Ohio deck needs as full-width piers.

• Frost heave from below. Water drawn up to the freezing front forms ice lenses that lift anything bearing in the frozen layer. A footing whose bottom is above the frost line gets jacked up a little each winter and rarely settles all the way back. The frost line page→ explains the physics.
• Adfreeze on the sides. Frozen soil also grips the sides of a pier and can lift it even when its bottom is below the frost line, especially a rough-sided pier in wet clay. A smooth form tube left in place, a footing wider at the bottom than the pier, and backfill with gravel rather than clay all reduce the grip.
• The backfill next to the house. The soil around a foundation was dug out and put back, so it is not the undisturbed ground footings must bear on. AWC DCA 6 puts it plainly: deck footings closer than 5 ft to an exterior house foundation wall “must bear at the same elevation as the footing of the house foundation” — the bottom of a basement wall, if there is one. Put the beam at least 5 ft out, or plan for deep footings there.
• Undersized footings. A footing that is deep enough but too small sinks instead of heaving — slowly, and usually at the most heavily loaded interior post. The size chart above is the fix.

Local rules vary. DCA 6 also says to keep footings off septic systems, leach fields, utility lines and enclosed meters. Call 811 before you dig so buried lines are marked.

“How deep to dig deck post holes” and “how deep should deck footings be” have the same answer: the hole goes to the bottom of the footing, which is the depth in the lookup. What differs is where the post sits. The usual method is a concrete pier to just above grade with a galvanized post base on top, which keeps the wood out of the soil. The 2021 IRC (R507.4.1) accepts either “manufactured connectors or a minimum post embedment of 12 inches (305 mm) in surrounding soils or concrete piers” for lateral restraint at the bottom of the post.

A post set straight into the hole and concreted in place rots at the soil line first, even when rated for ground contact, and it is the hardest part of a deck to replace. That is the main difference from a fence post→, which is normally buried: a fence post depth follows the one-third rule or the frost line, a deck footing follows the frost line and the size table.

R507.3 allows “concrete footings or other approved structural systems designed to accommodate all loads.” The common alternatives:

• Helical piles — steel shafts with screw plates, driven by machine to below the frost line with no digging or concrete. They are an engineered product, so the building department will want the manufacturer’s evaluation report or an engineer’s design for the load, and the installer’s torque log. Useful on sites where digging to 60 in (5 ft) is impractical, near the house backfill, or in rocky or wet ground.
• Precast pier blocks sit on the surface. They qualify as no-footing construction only under the pier-block exception above; used under posts and beams, they are just shallow footings and have to meet the depth rule.
• Plastic footing forms (bell-shaped or flat pads that go at the bottom of the hole with a tube above) produce the spread-pad-and-tube footing in the calculator with less concrete. Check the pad’s rated diameter against the size chart.
• Frost-protected shallow footings with rigid insulation are allowed for houses (IRC R403.3) and fall under “other approved methods” for decks, but need the building official’s agreement.

For an attached deck, the deep end of each state’s adopted frost depth, never under 12 in. Free-standing decks need 12 in under the model IRC in every state; the recommended figure is the same as the attached one. The climate column is the median of the 100-year bare-ground frost estimates across the state’s counties, from our frost line dataset.

Planning figures only. Frost depth is set by your local building department, and it is the legal number — confirm before you dig.

Alaska’s adopted depths run 60–120 in, and parts of the state sit on permafrost, where footing design is a job for an engineer.

Depth. The frost depths are the site’s single state table, the same one behind the frost line lookup and the fence post calculator, with the code or ordinances each figure comes from. Where a state’s depth varies we use the deep end. Zip codes resolve to counties through the Census Bureau’s 2020 ZCTA-to-county file, and each county shows its NOAA 1981–2010 climate estimate as a cross-check; the method is on the frost line page. We do not have every city’s adopted number, so a town that adopts a deeper figure than its state summary will show too shallow here.

Code. Code sections were read on the International Code Council’s free online views of the 2015, 2018, 2021 and 2024 IRC, and AWC’s DCA 6 (2015 IRC edition), on September 30, 2026. The quotes above are verbatim. Local amendments are common and override the model text.

Size and layout. The calculator assumes a rectangular deck with evenly spaced posts along straight beams, posts at the beam ends, and every footing on a beam sized for its most heavily loaded (interior) post. A free-standing deck gets a second beam at the house side with no overhang. It does not size joists, beams or posts — IRC Tables R507.6, R507.5 and R507.4 do that — and it does not cover hot tubs, roofs, stair landings or multi-level decks, which put concentrated loads on particular footings.

Concrete. A poured pier uses the next common tube size at or above the table diameter, full depth plus the height above grade. A pad-and-tube footing is a round pad of the table diameter and thickness with the chosen tube above it. Totals add 10% waste; bag yields are the printed 0.375, 0.45 and 0.6 cu ft for 50, 60 and 80 lb bags.

Local rules vary. These are planning figures, not an engineered design. Your building department’s frost depth, snow load and deck rules decide what is inspected — confirm them before you dig.

The state table — code frost depth, attached and free-standing footing depths and the county climate range — is free to reuse under CC BY 4.0: credit FenceLevel with a link to this page.

Download the CSV (51 rows)

Linking to one place? Add ?zip= and a zip code to this page’s address, and &type=freestanding for a free-standing deck.