Frost line and swales

Written 5 Sep 2026. Frost depth across the US runs from zero along the Gulf and Pacific coasts to six feet in northern Minnesota. The number matters to drainage in two places: buried pipe must sit below the frost depth or freeze-thaw heaves it, cracks it, and ungrades it; and a swale's inlet and outlet structures cross the frozen zone, so they freeze first, spill onto frozen ground, and teach the water a new path.

Written 2026-09-05 · re-checked 2026-09-10

Frost decides burial depth. The pipe above the line fails upward, one thaw at a time. (Desk drawing, not to scale.)

What frost does, in order

Heaves the pipe. Water in surrounding soil freezes and lifts. A pipe bedded in gravel drains the water away and heaves far less; a pipe bedded in clay sits in the heave's favorite material.
Un-grades the run. A 1% pipe that heaves at one joint becomes a 0% pipe with a puddle at the bellies. Sags are frost's calling card.
Closes the outlet. The daylight end and the catch basin freeze first because they meet the air. A frozen outlet turns the whole system into storage, and the storage finds its own exit.

What to do about it

Item	Rule
Buried pipe depth	Below your local frost depth	Your building department publishes the number — read the code
Pipe bedding	Clean gravel, not clay	Bedding that drains can't heave hard
Outlets and inlets	Check after first thaw	Frost moves them; the spring walk finds it
Planting window	6–8 weeks before frost	The planting job runs on this clock

The disagreement sentence

Southern-franchise drainage advice treats frost as an optional upgrade. North of the line, frost is the design load — every spring thaw is the annual proof test, and the failures surface exactly when the ground can't be worked yet.

Where to go next

French drain — the pipe that must get below the line.
Plant a swale — the other clock frost sets.
Read your code — where the local depth is published.

Sources

USGS Water Science School, groundwater and frost — usgs.gov
ICC International Plumbing Code — iccsafe.org
Minnesota Stormwater Manual — stormwater.pca.state.mn.us