Frost depth and deck footings
A footing has to sit below the depth to which the ground freezes, or the frost will lift it. That depth ranges from zero in the Deep South to more than six feet in northern Minnesota, and it is set locally rather than by a national number.
What frost heave does
Water in the soil expands about 9% when it freezes. In a soil that holds moisture and conducts cold — silt and clay are the worst — that expansion lifts whatever is sitting in it. A footing at 30 inches in a region with a 48-inch frost depth will rise, and it will rise unevenly, because the soil around it does not freeze uniformly.
The result is a deck that is level in October and out of level in March, and a ledger that is being pulled in two directions because the house is not moving and the deck is.
Typical frost depths
| Region | Typical frost depth | Footing depth |
|---|---|---|
| Gulf Coast, Florida, south Texas | 0–6″ | 12–18″ for bearing, not frost |
| Mid-South (Tennessee, Arkansas) | 12–18″ | 18–24″ |
| Mid-Atlantic, lower Midwest | 24–36″ | 30–42″ |
| Upper Midwest, New England | 42–60″ | 48–66″ |
| Northern Minnesota, Montana, Maine | 60–96″ | 72–96″ |
These are ranges for orientation only. The number that governs is in your local building code, usually in a table or a map, and it often varies within a state because it depends on soil type and elevation as much as latitude.
Two details that matter as much as depth
- Flared footing. A pier that is straight-sided relies on friction; a footing with a flared base at the bottom resists uplift by bearing on the soil above it. In cold climates most jurisdictions require the flare, and it is why a tube form with a bell base exists.
- Slip sleeve. A cardboard tube left in place gives a smooth shaft that frost can grip and lift. Some builders wrap the pier with a slip sleeve or use a form that tapers, so the frost grips the soil rather than the concrete.
A third detail that is cheaper than digging deeper: keep water away. Frost heave needs moisture, and a footing in waterlogged soil heaves far more than one in drained soil. Grade the ground away from the deck and do not let a downspout discharge beside a post.
The deck block problem
Precast deck blocks sit on the surface, which means they are entirely within the frost zone. In a cold climate they will move every winter. They work in the South, on grade that drains, for low decks — and they are the single most common cause of a deck that racks out of square after two winters. See deck blocks for where they are acceptable.
What a depth rule cannot account for
- It does not give your frost depth. That is a local code question, and the ranges above are not a substitute.
- It does not size the footing. Diameter and bearing area depend on the load and the soil's bearing capacity.
- It does not calculate concrete volume — that is the footing calculator.
- It does not cover permafrost or expansive clay, both of which need an engineered design.
Questions people actually ask
How deep should deck footings be?
Below the local frost depth, which ranges from nothing in the Deep South to over six feet in northern Minnesota. Most of the US falls between 30 and 48 inches. The governing number is in your local code.
What happens if a footing is above the frost line?
Frost heave lifts it, unevenly. The deck goes out of level over winter, the ledger is stressed because the house does not move with it, and the frame can rack out of square over a few seasons.
Do deck blocks work in cold climates?
Generally no. They sit on the surface, entirely within the frost zone, and will move every winter. They are acceptable in warm climates for low decks on well-drained grade.
Does a footing need a flared base?
In cold climates, usually yes. A flare at the bottom resists uplift by bearing on the soil above it, and many jurisdictions require it for frost protection.