Frost heave is the most patient fence destroyer: each winter, freezing soil expands and lifts posts a fraction of an inch; each spring they settle imperfectly; a decade later the fence line reads like a seismograph. The fix is not stronger concrete — it is deeper geometry.
The physics
Freezing soil expands ~9% as pore water turns to ice, and frozen ground grips whatever is in it. A post with its base above the frost line sits inside the freezing zone — the ground grips the concrete collar and lifts the whole assembly. A post whose footing extends below the frost line is anchored in ground that never freezes; the grip above slides on the smooth concrete side instead of lifting it.
The three rules
- Foot below frost line: 36–48 inches in most cold climates (your building department publishes the number). The one-third-above rule sets the MINIMUM; the frost line sets the real depth — use whichever is deeper.
- Bell the bottom: widen the hole's base (or use a tube form) so the footing is wider below than above — mechanical uplift resistance even where frost is shallow.
- Drain the collar: gravel at the base, concrete crowned above grade — water pooling at the collar freezes, expands, and jacks the post from the top down.
The regional notes
| Climate | Frost depth | Typical post detail |
|---|---|---|
| Southern US | 0–6 in | 24 in min depth, bell, crowned collar |
| Mid-Atlantic / Midwest | 30–42 in | 42 in footing, bell mandatory |
| Upper Midwest / Northeast | 48–60 in | Below-frost footing — dig or accept the march |
| Mountain West | 36–60 in | Below frost; dry-set concrete works in gravelly soils |
When heave has already won
Lifted posts can sometimes be re-seated: cut the fence free, pull or dig the post, re-dig below frost, and reset with a belled footing. More than a few lifted posts means the whole line was set shallow — the economics usually favor re-setting corner and gate posts properly and living with minor movement elsewhere until panels age out.
Size your holes honestly with the post depth calculator, respect the frost line as a floor not a suggestion, and the fence will outlive your patience for maintaining it — which is the goal.