Local Rules
How deep is the frost line in Minnesota?
Per Minnesota DNR frost depth records, the median maximum frost depth is about 20 inches, the deepest on record is 35.2 inches from the winter of 2007–08, and 2024–25 reached 25 inches. Frost typically begins forming around November 23 and is gone by roughly April 10.
What the code actually requires
Minnesota Rules 4714.0609 requires buried water service piping to be installed at least 12 inches below finish grade and at least 12 inches below the maximum local frost depth.
Note how that is written: the requirement is relative, not a fixed number. You will see "six feet" quoted on plenty of websites. That is not what the code says, and municipal water main standards vary from city to city. The depth that is correct for your address depends on your jurisdiction's frost depth figure, not on a number somebody repeated online.
Why is the frost depth different every winter?
Frost depth is not really about how cold it gets. It is about how long the cold lasts, how much snow is on the ground, and what the soil is made of.
Snow is the variable most people miss. Roughly four inches of loose snow is a genuine insulating layer. A bare, windswept winter with moderate temperatures will drive frost deeper than a brutally cold winter with two feet of snow cover. That is why the worst frozen-service calls we see come after a cold snap that follows a thaw, when the snow is gone and the ground is exposed.
Which parts of the house freeze first?
Not the buried service line, usually. Interior lines in exterior walls, rim joist runs, unheated crawl spaces and hose bibs freeze long before anything at code depth does.
Buried lines fail when something has changed at the surface: a driveway that got plowed down to bare pavement, a section of yard where the snow blew clear, a line that was buried shallow during a remodel.
Related questions
- How deep are water lines buried in Minnesota?
- Code requires at least 12 inches below finish grade and at least 12 inches below maximum local frost depth. Because frost depth varies by location, the actual required depth is set by your municipality rather than by a single statewide number.
- What was the deepest frost depth ever recorded in Minnesota?
- 35.2 inches, during the winter of 2007–08, according to Minnesota DNR frost depth records. The median maximum is closer to 20 inches.
- When does frost leave the ground in Minnesota?
- Typically around April 10, per DNR records, though it varies year to year. Frost generally starts forming around November 23.
Keep reading
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