How deep should deck footings be?
Below the frost line, and at least 12 in into undisturbed soil.
Across the northern states and Canada that is 42 to 60 in. Where the ground does not freeze, 12 to 18 in is normal — the requirement there is bearing, not frost.
A deck heaves differently from a fence
A fence that lifts an inch looks wrong. A deck that lifts an inch at one footing and not the others racks the frame, opens the ledger connection and puts the stairs out of level — and the ledger is the connection that matters, because it is bolted to the house. The house does not move. If the deck does, something has to give at that joint.
That is why deck footings are inspected and fence posts are not, and why "deep enough" is not a judgement call here.
Undisturbed soil, not just depth
Depth is the headline, but the requirement is to bear on ground that has never been dug. Backfill against a foundation, or the spoil from your own hole tipped back in to correct an over-dig, compacts for years under load and the footing follows it down. If you go too deep, fill with compacted gravel rather than the soil you removed, or pour the extra depth in concrete.
How many footings
Footing count is set by the beam, not by the deck's area. Beams span a limited distance between posts — commonly 6 to 8 ft for a doubled 2x8, more for a 2x10 — and each post lands on a footing. So a wider deck adds footings along the beam line, while a deeper deck may instead need a second beam.
Our size pages give a computed footing count for each standard deck, and it is worth checking against your actual beam: the same 12x16 deck can want six footings or eight depending on whether the beam is a doubled 2x8 or a doubled 2x10.
Common questions
- Can I use deck blocks instead?
- Only where nothing freezes and nothing is attached to the house. A precast block sits on the surface, so it moves with the ground every winter. Freestanding ground-level decks in mild climates are the honest use case.
- How deep in a frost-free area?
- 12 to 18 in, into undisturbed soil, sized for bearing rather than frost. The footing still has to be wide enough to spread the load — depth stops being the constraint, diameter does not.
- Does a ground-level deck still need footings?
- If it is attached to the house, yes, and to full frost depth — the ledger makes it part of the building. Freestanding and low, the rules relax, which is also why detaching a small deck can simplify the whole job.
- What size footing for a deck post?
- 12 in diameter is the common answer for a residential deck on average soil. Heavier loads, a roof over the deck, or soft ground push it to 16 in or more.
- Do footings need rebar?
- A plain cylinder in compression usually does not, but vertical rebar tying the footing to a pier above it is common and cheap. Where inspectors ask for it, they ask specifically — worth a call before you pour rather than after.