How deep do deck footings need to be?

The short answer

Deck footings must reach below the frost line: the depth to which the ground freezes in an average winter where you live. Your local building department publishes the figure, it is written into the code you are permitted under, and inspectors will not accept less.

Typical figures run from around 12 inches in the warmest parts of the south to 42 or 48 inches across much of the northern US, and deeper again in the coldest states. Some southern jurisdictions have no frost depth requirement at all and simply require bearing on undisturbed soil.

Why the frost line matters

When saturated soil freezes it expands, and the expansion has to go somewhere. The force involved is enormous — enough to lift a house, let alone a deck. A footing whose base sits inside the freezing zone gets pushed upward each winter and settles back unevenly in spring. That is frost heave.

On a deck the symptoms are unmistakable once you know them: the outer edge rises relative to the house over a couple of winters, the deck racks out of square, gaps open at the ledger, doors onto the deck start to bind, and the railing pulls away from the house. By the time it is obvious, the fix is digging new footings under a standing deck.

A footing below the frost line sits in ground that never freezes, so it does not move. That is the entire reason for the depth.

Depth is not the only requirement

• Bearing. The base has to sit on undisturbed soil or properly compacted fill. Digging past the depth and backfilling loose soil under the footing achieves nothing.
• Diameter. Twelve inches suits most residential decks on firm soil. Soft or clay soils, taller decks, roofs and hot tubs need 16 or 18 inches, or a flared base.
• Drainage. Water sitting at the base of a footing is what makes frost heave severe. A few inches of compacted gravel below the tube helps.
• Post separation. Keep the post itself off the concrete and out of the soil on a standoff post base. An inch of air under the end grain is the cheapest lifespan extension on the whole deck.

Work out the concrete you need with the footing concrete calculator, and the number and spacing of footings with the deck footing calculator.

Where deck blocks are and are not acceptable

Precast deck blocks sit on the surface by definition, so they are above the frost line everywhere. That is not a loophole; it is a different design. A floating deck on blocks is expected to ride seasonal ground movement rather than resist it, which is acceptable on a low, freestanding, unpermitted deck and unacceptable on anything attached to a house.

The moment a deck is attached to the building, the two have to move together — and the house does not move. That is why an attached deck always needs footings below the frost line, whatever its size. See the floating deck cost calculator for what the block-based approach saves and gives up.

Common mistakes

1. Measuring from the bottom of the tube form rather than from finished grade. The depth requirement is to the underside of the footing, below final ground level.
2. Digging to depth, then dropping a short tube in and backfilling under it.
3. Pouring on a rainy day into a hole with standing water, which weakens the concrete exactly where the load arrives.
4. Skipping the footing inspection. It is the one inspection that cannot be done retrospectively, and it is the one most often forgotten.
5. Assuming a neighbour's deck is a guide. Plenty of standing decks were built wrong; they simply have not heaved badly enough to be noticed yet.