Work out how many deck footings a deck needs, the minimum footing diameter for the load each post carries, how deep each footing goes and how much concrete the holes take.
Footing diameter comes from the load each post carries, how much deck area it supports, and depth comes from the frost line where you live. Both are code numbers, not preferences.
A deck footing has one job: spread the load of the posts over enough soil that the soil does not move. So the two numbers that matter are diameter, sized to the load each post carries, and depth, set by the frost line beneath it.
The load is expressed as tributary area: the patch of deck whose weight ends up on that one post. On a deck attached to the house with a single row of posts, each post carries half the joist span multiplied by the post spacing. On a freestanding deck with two rows, each post carries nearer a quarter of the deck.
Deck design loads are the standard residential ones: 40 psf live load plus 10 psf dead load, per the IRC's residential loading table. If there is a hot tub, a planter wall or a roof over the deck, none of this applies, that is a designed structure, not a prescriptive one.
| Tributary area | 1,500 psf soil | 2,000 psf | 3,000 psf |
|---|---|---|---|
| 20 sq ft | 12 in | 11 in | 9 in |
| 40 sq ft | 17 in | 15 in | 13 in |
| 60 sq ft | 21 in | 18 in | 15 in |
| 80 sq ft | 24 in | 21 in | 17 in |
| 100 sq ft | 27 in | 23 in | 19 in |
| 120 sq ft | 29 in | 25 in | 21 in |
| 160 sq ft | 34 in | 29 in | 24 in |
These are minimum diameters for round footings cast against undisturbed soil or properly compacted fill, from the prescriptive deck footing table the IRC references (IRC R507.3.1, AWC DCA-6 Table 4), on the 40 psf live + 10 psf dead residential deck load. For a square footing, the side length is about 0.886 × the round diameter for the same bearing area.
Rule 1: at least 12 in below grade. The bottom of the footing has to sit in undisturbed soil 12 in down, even where there is no frost (IRC R403.1.4).
Rule 2: below the local frost line (IRC R403.1.4.1). Soil above the frost line expands and contracts as it freezes and thaws, and a footing bearing in that zone gets lifted, frost heave, which racks the deck out of square and tears the ledger off the house.
| Region | Typical frost depth |
|---|---|
| Gulf Coast, southern Florida | 0–12 in (12 in minimum still applies) |
| Southeast, Mid-Atlantic | 12–24 in |
| Midwest (Ohio, Illinois, Indiana) | 30–36 in |
| Northeast (New York, New England) | 36–48 in |
| Upper Midwest, northern New England | 48–60 in |
Frost-protected shallow foundations exist, but they rely on a heated building to keep the soil warm, which an open, unheated deck is not. Frost depth varies by county and is often amended locally, so confirm it with your building department.
Extend the pier above grade. Poured footings and piers are usually brought 4–6 in above the soil so the wood post never sits in dirt; a post-base anchor connects them. Pressure-treated lumber tolerates ground contact, but it lasts several times longer with air around it.
Reinforce the pier if it is structural. A short, fully confined pier in compression is fine as plain concrete, but tall slender piers, hillside footings and anything resisting bending need vertical bars with ties. Rebar also gives you something to tie the post anchor into.
Do not bell or flare the base by accident. Over-digging makes the hole wider than the tube and the concrete simply fills the gap, extra cost, no extra capacity. Add waste rather than over-digging on purpose.
Pour concrete for one footing at a time and place the anchor while the concrete is still workable. A footing poured and then left with no anchor means drilling and epoxying later, or a footing that cannot be used.
Footing diameters by tributary area and soil bearing capacity are the prescriptive values referenced by IRC R507.3.1 and tabulated in AWC DCA-6 (the prescriptive residential wood deck guide the IRC points to), based on 40 psf live plus 10 psf dead load. The 12 in minimum bearing depth comes from IRC R403.1.4 and the frost-line requirement from IRC R403.1.4.1, with regional frost depths compiled from IRC-adopting jurisdictions as general guidance only, the legal depth is the one your local building department enforces. Concrete volumes use the published bag yields from the Quikrete Concrete Mix data sheet. Any deck with a hot tub, a roof or unusual geometry needs an engineered design rather than any prescriptive table.
The diameter is sized to the deck area each post carries. On 1,500 psf soil, the code default when no soil test exists, a 20 sq ft tributary area needs a 12 in footing, 40 sq ft needs 17 in, 60 sq ft needs 21 in and 80 sq ft needs 24 in. Tighter post spacing reduces both the diameter and the load.
At least 12 inches below grade in every climate, and below the local frost line wherever frost occurs, 30–36 in across much of the Midwest and 42–48 in in the Northeast. Use whichever rule is deeper. Frost depth is set locally and varies by county.
The patch of deck whose load lands on one post. For a deck attached to the house with a single row of posts, it is the post spacing multiplied by half the joist span: each post carries the area halfway to each neighbour. Divide the whole deck area by the number of footings to get it.
A 12 in diameter footing 42 in deep holds about 2.75 cu ft, which is 5 bags of 80 lb or 7 of 60 lb. Add about 10% for over-digging and spillage. Six such piers work out around 30 bags of 80 lb.
A short, fully confined pier in pure compression generally does not, and plain concrete handles the load. Rebar matters where there is bending or lateral load, tall slender piers above about 4 ft, hillside footings, or piers resisting a cantilever moment. Rebar also gives you something to tie the post anchor to.
Because soil above the frost line expands when it freezes and settles when it thaws. A footing bearing in that zone gets lifted annually, a process called frost heave, which racks the deck, loosens fasteners and can tear the ledger away from the house. Below the frost line the soil stays above freezing year-round and the bearing surface is stable.