How deep should deck footings be? Find your frost depth

Deck footings go below your local frost line, and at least 12 inches below the undisturbed ground surface, whichever is deeper. Your city or county sets the frost number, and in the places we checked it ran from 24 inches in northern Virginia to 60 inches in northern Minnesota. One more rule can send a footing far deeper: under the American Wood Council’s deck guide, a footing within 5 feet of a basement wall has to sit as low as the house’s own footing.

The frost line is roughly how far down the ground freezes in a hard winter. Wet soil swells as it freezes, and it can push up anything sitting in it. Footings included. A footing whose bottom is below that line rests on soil that shouldn’t freeze, so the post stays where you set it.

Six local numbers, from 12 to 60 inches

Six local sources, each read on October 12, 2026:

Minnesota’s rule is the clearest example of how local this gets. It sets 5 feet in Zone I, 34 counties across the north and center of the state, and 3 1/2 feet in Zone II, everywhere else. Two decks in the same state can differ by 18 inches of digging. Phoenix’s sheet is about soil bearing and never mentions frost, but it does set a 12 inch minimum for shallow foundations.

Measure to the bottom of the footing, starting from ground nobody has dug. Rochester’s handout puts it plainly: “The minimum depth to the base of the footing is 48”.” If your town’s website doesn’t list a frost depth, ask the building department.

Table R301.2(1) of the International Residential Code is a blank that each town fills in with its frost depth when it adopts the code (the fence post guide links one filled-in copy). Section R507.3.2 then sends deck footings below “the frost line specified in Table R301.2(1),” in the 2021 text Rochester prints.

The 12 inch floor is a separate rule for every exterior footing. Section R403.1.4 puts every one “not less than 12 inches (305 mm) below the undisturbed ground surface,” in the wording of Ohio’s adopted version. The American Wood Council’s prescriptive deck guide, DCA 6, puts both in one line: footings bear on undisturbed soil “at least 12 inches below the undisturbed ground surface” or below the frost line, “whichever is deeper.” The edition AWC links is built on the 2015 code, but on these two depth rules it matches the 2021 text.

Quikrete wants a little more. Its post-setting guide, the one our fence post guide quotes, says the hole for a post that holds up decking must go “at least 6” (150mm) below the frost line.” That’s Quikrete’s margin, not the code’s. We’d dig the extra 6 inches anyway. It’s cheap insurance against a frost number you measured from the wrong spot.

Wherever the hole goes, DCA 6 says to “Contact local utilities (call 811) before digging,” and not to build footings over septic systems or leach fields, utility lines, or enclosed meters.

Near a basement, plan on a much deeper hole

DCA 6 has a rule the model code text doesn’t print. Deck footings “closer than 5’-0” to an exterior house foundation wall must bear at the same elevation” as the house’s footing. If the house has a basement, that’s the footing under the basement wall, so you’d be digging to the bottom of it. DCA 6 doesn’t say why. It does recommend a cylindrical footing (a caisson) for houses with basements, to limit how much you dig out next to the wall.

At least one city prints the rule in its own handout. Alexandria’s Figure 16 says it directly: “when less than 5’, footings must be at same elevation.” Rochester’s 2021 IRC handout doesn’t mention it. Whether it applies to you comes down to whether your town builds by DCA 6 or writes the rule in itself.

Most ledger decks stay clear of it. The ledger board carries the house end, and the posts stand out under the far beam. On any deck deeper than 5 feet with the beam at its outer edge, those posts land outside the zone. A free-standing deck has it harder. It needs a beam near the house, and DCA 6’s section on non-ledger decks applies the same rule there: footings that close to the house go to “the same elevation as the house footing.” If yours is free-standing and runs along a basement wall, price in those deep holes before you settle on the design.

When a deck can skip the frost line

Both frost exceptions in the 2021 code are for free-standing decks, and Rochester’s 2021 IRC handout prints them word for word. The first covers small, low decks on precast concrete pier blocks: joists sitting right on the blocks with no beams or posts, 200 square feet or less, and a walking surface no more than 20 inches above grade within 36 inches of the edge. Exception 2 is much wider: “Free-standing decks need not be provided with footings that extend below the frost line.” Read literally, it swallows the first, so ask your building department how it applies them before you skip the deep holes.

The R507 text that New Prague, Minnesota publishes goes one step further. Its section R507.3 drops the footing requirement entirely for “Free-standing decks consisting of joists directly supported on grade over their entire length.”

Cities still add conditions. St. Paul’s handout requires frost footings for any deck “attached to a dwelling or a garage that has frost footings.” And if your town builds by DCA 6, the 5 foot basement rule above still covers a free-standing deck’s footings near the house, exceptions or not.

How wide the footing has to be

Depth gets you below the frost. Width spreads the deck’s weight over enough soil to carry it, and that depends on two numbers: how much deck each post holds up (its tributary area) and how much weight your soil can bear.

Soil changes the answer a lot. The code’s Table R401.4.1, as Ohio adopts it, gives clay and silt 1,500 pounds per square foot. Sand, and gravel with silt or clay in it, get 2,000. Sandy gravel and plain gravel get 3,000. If you don’t know your soil, size for 1,500. DCA 6 assumes that number in its footing tables, and Phoenix’s sheet uses it too.

Take a 16 by 12 foot deck: ledger on the house, joists running 12 feet out to a beam, and three posts under that beam, 8 feet apart. No overhangs. That’s the same setup the deck calculator assumes for its joist span check, and the joist span table shows which joists reach 12 feet.

The middle post carries half the joist span (6 feet) times the 8 feet of beam it holds up, so 48 square feet. If the beam is one 16 foot piece running over that post, DCA 6 says to multiply by 1.25, which makes it 60. Each end post carries half of the middle post’s 48: 24 square feet.

DCA 6’s Table B3 sizes them like this. It assumes a 40 psf live load, 10 psf of deck weight and the weight of the concrete. Its rows go up in steps of 10 square feet, so where our area fell between two rows we used the bigger footing.

DCA 6’s simpler Table 4 skips the tributary math. Its appendix says that table “assumes full cantilevers on both the joist and the beam,” and for an 8 foot beam span with joists up to 14 feet it calls for a 24 inch round footing, 10 inches thick. That’s safe for our deck, but it’s a lot more digging. The tributary math takes a few minutes and can shrink every hole.

Your city may ask for more. Des Moines’ handout says “Minimum thickness is 12” for all footings,” double the 6 inches DCA 6 starts at.

What depth does to your concrete order

The footing pad barely changes as you go deeper. The column above it does. DCA 6 allows a 12 inch round concrete stem on top of the footing; its commentary says that option is there “to reduce the amount of concrete required.” We worked out the clay middle post from the example (20 inch round, 8 inches thick) at four of the depths in the first table, both ways: pad plus a 12 inch stem up to grade, or the whole 20 inch hole filled. Bags are 80 lb Quikrete Concrete Mix at 0.60 cubic feet each, the yield on its data sheet, rounded up.

At 42 inches, filling the whole hole takes 13 bags instead of 7. That’s 480 pounds of extra concrete for one post. At 60 inches the gap is 10 bags a post. The stem needs a form tube and some backfill around it, and we’d still build a deep footing that way.

With Des Moines’ 12 inch minimum, the pad gets thicker and the stem shorter. At its 42 inch depth that comes to 4.15 cubic feet, which is still 7 bags.

Quikrete rates that mix at 4,000 psi at 28 days. DCA 6’s tables assume 2,500 psi, so the bag clears it with room to spare.

To count bags for your own deck, open the concrete calculator and pick “Round post or column.” Do the pads as separate runs, because they aren’t the same size: in the clay example, one run for the middle post at 20 inches and one for the two end posts at 14 inches, with “How many?” set to match. Then run the stems once, with “How many?” set to all three posts. It adds 10% for waste by default, so its counts can come out higher than our table. If you’re planning the whole deck, the deck calculator counts boards, joists and screws, and if it needs steps down to the yard, the stair calculator works out the risers.