Deck Footings: Depth, Size and the Frost Line, Read From DCA6 Table 4

Deck footings carry the whole deck into the ground, and they are also the part most often built from a rumor: “12 inches deep is fine” or “just use a big block.” This guide gives you the deck footings rules from the free AWC¹ DCA6 deck guide: how deep they go, how wide they must be for your spans, and what to do about the frost line, which is where most advice online goes wrong. It also covers low and floating decks, and the footing under deck stairs.

We are not your building department or your engineer. Where a number is our own design choice, we say so.

Deck Footing Types: Poured, Precast and Deck Blocks

There are three families of deck footing in common use. The code guide sizes only one of them, and that matters when you choose.

Poured concrete footings

A poured footing is a hole dug to depth and filled with concrete, usually round in a form tube or square in a box form. The DCA6¹ table sizes this type, for concrete at 2,500 psi on soil rated at 1,500 psf. It is the type we assume in the deck designer too. The post sits on it with a post base, or is carried by a pier cast on top.

Precast piers and deck blocks

Precast concrete piers are made off site and set in the hole. Deck blocks are smaller precast blocks that sit on the surface. A county guide using the 2021 IRC says a post on a footing needs lateral restraint, either by a connector or by being embedded at least 12 in in the concrete pier. A block that sits on the surface does not reach the 12 in depth rule in the next section, so where a building department allows blocks for a low deck, that is a local decision we have not checked.

Footings without concrete or without digging

People also ask for footings with no concrete or no digging. Screw piles and manufactured systems exist, but every row in this guide assumes concrete. A different footing needs its own tested capacity from the maker, and we have not checked any, so we do not recommend one. If you want to use one, ask your building department whether it will accept it with the maker’s data.

How Deep Should Deck Footings Be?

The rule has two parts, and the deeper one wins.

The 12 in minimum

DCA6¹ asks for the bottom of a footing to sit at least 12 in below the undisturbed ground surface or below the frost line, whichever is deeper. Undisturbed means the soil has not been dug and put back, so remove topsoil and loose fill before you measure. In a warm area with no frost the 12 in is the answer, and in a cold one the frost line is.

The 5 ft from the house rule

A footing less than 5 ft from the house wall must bear at the same level as the house footing. The house foundation is usually deeper, and a shallow deck footing next to it sits on soil that was disturbed when the foundation was dug. The deck designer flags any footing near the house for this reason.

When the soil is weak

The depth and the sizes below assume soil that carries 1,500 psf. If you know the soil is softer than that, DCA6¹ asks for a soils investigation. The guide’s commentary gives tables for stronger soils at 2,000, 2,500 and 3,000 psf, and our designer does not use them. It keeps every footing at 1,500 psf, which is the conservative side.

Deck Footing Frost Depth by ZIP Code: What Is Real

Pages that promise a frost depth by ZIP code are everywhere, and none of the ones we found gives a number you can build to.

Where the legal number comes from

The frost line for a deck is set by the town or county. It is filled in on the climate table of the local building code, a table in the IRC called R301.2(1), where each jurisdiction states its frost depth. We found examples printed in York County, South Carolina, Rockingham County, Virginia, the Town of Eagle, Colorado, and Topeka, Kansas, and the numbers were not the same. The inspector reads that table, so you should too.

Why a ZIP code map does not work

We found no public dataset that maps a ZIP code to a legal frost depth. The pages that claim one are search filler. A ZIP code often crosses several jurisdictions, and a map that blends them cannot be defended at an inspection. Observed soil frost data from weather services is real data, but it describes one winter in one region and is not a design value.

The closest honest hint

The closest public dataset we found is the NOAA air freezing index, which has 3,110 stations and was built to size frost protected shallow foundations. It is a climate estimate, so it is a hint at the order of magnitude and not your legal depth. Use it to check that an answer from the building department is plausible, not to replace it.

What our designer does

Our deck designer asks you to enter the frost depth. It starts at 36 in, which is our default and not a rule, and it shows an amber check until you confirm the depth with your building department. The deck footing calculator takes the same input and sizes the concrete.

Deck Footing Size by Beam and Joist Span

Footing size depends on how much deck each post carries. DCA6¹ Table 4 sets it by beam span and joist span, for a post with the table’s load on 1,500 psf soil.

Reading Table 4

At the small end, a 6 ft beam span with joists up to 10 ft calls for an 18 in round footing, or a 16 in by 16 in square one, 7 in thick. At the large end, an 18 ft beam span with joists up to 18 ft calls for a 42 in round footing, or a 37 in by 37 in square one, 19 in thick. Most decks sit between those. The guide allows the footing at a corner post to be 0.9 times the listed size, and our designer does not apply that. It uses the full size.

Posts and post height

Footing size is only one row of the same table. DCA6¹ sets 6x6 as the minimum post. An 8x8 can replace a 6x6, up to 14 ft high. The table also limits post height by species. For a 12 ft beam span and joists up to 14 ft, a 6x6 in Southern Pine can be 13 ft high, in Douglas fir-larch 12 ft, and in the weakest species group listed only 5 ft. The height is measured from the ground or the top of the footing to the underside of the beam. A three-ply beam needs a post cap, and a beam bolted to the side of a post without a notch is prohibited.

The 2021 table, for reference

County guides that use the 2021 IRC print footing sizes by tributary area, for soil at 2,000 psf. A post carrying 20 sq ft gets an 11 in round footing, and one carrying 160 sq ft gets 30 in. Those figures are for reference only, because we use the DCA6¹ table. If your county prints the 2021 version, show it on your plan.

How to Build Deck Footings, in Order

The sequence matters, because a footing that is out of place by two inches cannot be moved once the concrete sets.

Layout and digging

Mark the post centers from your plan with string lines, and check the corners square before you dig. A simple way is to measure 3 ft along one side and 4 ft along the other, and the diagonal should be 5 ft. Use multiples of those numbers for a deck. Have underground utilities marked first. Dig to the depth you set from the 12 in rule or your frost depth, with the hole at least as wide as the footing size you read from the table, and remove loose soil from the bottom so the concrete bears on undisturbed ground.

Forming and pouring

Use a form tube for a round footing or a box for a square one. DCA6¹ sizes the footing for concrete at 2,500 psi, so ask the supplier for that strength or mix a bag mix that states it. Fill the hole, work out the air, and set the post base or the pier on a template so it ends up in the right place. Check the height of every base against the others before the concrete begins to set, because the beam must sit level across all of them.

Cure, check and inspect

Let the concrete cure before the posts take load, and follow the time on the bag. Many offices inspect footings before they are covered or before the posts go up, so ask when your inspection is due. Pouring before the inspector has seen the hole can mean digging it up again.

Common Deck Footing Mistakes

Most failed footing inspections come from a short list of mistakes.

Stopping at 12 in in frost country

The 12 in minimum is for warm ground. Where the frost line is deeper, 12 in is a footing that heaves. Find your legal frost depth first.

Digging into backfill

Soil that was dug and put back, as next to a new foundation, is not undisturbed. A footing set on it can settle. That is why the guide asks for footings near the house to bear at the house footing elevation.

Using one size for every post

Posts carry different loads. The table gives a size for each combination of spans, and a corner post may be smaller than a middle one. Use the calculator for each post line, or let the designer do it.

Footings for Ground-Level and Floating Decks

A low deck is easier to build but not exempt from the footing question. Two different cases get confused here.

Freestanding is not the same as floating

A freestanding deck has its own beams and posts and does not attach to the house. DCA6¹ does not cover it, because the guide assumes the house resists the sideways load. Our designer allows a freestanding deck up to 8 ft high with knee braces both ways at the corner posts, and blocks anything higher. That limit is our choice. Our ground-level deck plans and freestanding deck plans show both in the designer.

Floating decks on blocks

A floating deck sits on blocks with no footings in the ground. We found no code row that accepts blocks for a deck, so whether your office does is a local question. Our ground-level plan stands on footings below the frost depth, not on loose blocks, because a deck that must pass inspection needs them. Blocks on soil can shift and heave with frost, so they suit only a small, light deck that your rules treat as exempt.

Footings near the house

Even a low deck that sits next to the house comes back to the 5 ft rule above. Dig those footings to the house footing elevation.

Footings for the Deck Stairs

Stairs have their own footing rule. The DCA6¹ guide shows a 4x4 post that shortens the stringer span standing on a footing 12 in round or 10 in square, 6 in thick, and wants stair footings below the frost line. Stringers must not rest on a slab that is not founded below frost. The full stair rules, including the riser and tread limits, are in our deck stairs guide, and the footing for the post in the stair is the one place the low-deck shortcuts do not work.