Guide · Deck footings
There is no “N footings per deck size.” The count comes from beam span — a row of posts every 6 to 8 feet under the beam — and how much deck each post carries. Here is the mechanism, so you can size it for your deck.
A ledger-attached deck is held up on two lines: the house ledger carries the inner half of the load, and a single beam on posts carries the outer half. So an attached deck usually needs just one row of footings, under that beam. How many posts go in that row is set by the maximum beam span — the distance the beam can bridge between posts — which comes from the beam’s size, the wood species, and the joist span it carries (AWC DCA6 Table 3A).
Number of footings ≈ the beam length divided by its maximum span, plus one, minus a little where the beam cantilevers past the end posts. A freestanding deck has two beams instead of a ledger, so it needs roughly double the footings.
These are worked from a common case — a ledger-attached deck with a Southern Pine double 2×10 beam — to show the mechanism. Change the beam size, species, joist span, or soil and the count changes. Your plan or local deck code is the authority, not this table.
| Deck (ft) | Max beam span | Footings (1 beam) | Note |
|---|---|---|---|
| 12 × 12 | ~7 ft | 2 | One beam, ends cantilevered |
| 12 × 16 | ~7 ft | 2–3 | 3 if no end cantilever |
| 14 × 20 | ~6.5 ft | 3–4 | Longer joist span shortens the beam span |
| 16 × 24 | ~6 ft | 4–5 | Wider deck, more posts |
Illustrative, from AWC DCA6 Table 3A beam spans (Southern Pine, 2-ply 2×10). A bigger 3-ply 2×12 beam spans past 10 feet and needs fewer posts; weaker species or longer joist spans need more. Freestanding decks roughly double these.
Each footing is sized for the deck area it carries (its tributary area) and the soil’s bearing value. Round-footing diameters from IRC Table R507.3.1 / DCA6 Table B3, minimum 6 inches thick:
| Tributary area (ft²) | Round footing, 1,500 psf soil | 2,000 psf soil |
|---|---|---|
| 10 | 8″ ⌀ | 7″ ⌀ |
| 20 | 12″ ⌀ | 10″ ⌀ |
| 40 | 16″ ⌀ | 14″ ⌀ |
| 60 | 20″ ⌀ | 17″ ⌀ |
| 80 | 23″ ⌀ | 20″ ⌀ |
| 100 | 26″ ⌀ | 23″ ⌀ |
| 120 | 29″ ⌀ | 25″ ⌀ |
Assumes 40 psf live + 10 psf dead load, 2,500 psi concrete. Where soil is untested, 1,500 psf is the common default. Center posts on a continuous beam carry more — DCA6 multiplies their tributary area by 1.25.
Every pier must reach below the frost line (or 12 inches, whichever is deeper) — see how deep footings need to be. Once you know diameter and depth, get the concrete per tube on the sonotube size chart and the whole-deck total on the deck footing calculator.
FootingTakeoff reads a footing plan and returns total linear feet, pad count, and an invoice at your rates — automatically.
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