What is the frost line?
The frost line (frost depth) is the maximum depth the ground freezes in a given area. Footings, water lines, and anything that can't tolerate movement or freezing must be placed below it; local code sets the number, roughly 36 to 48 inches in the Northeast.
Wet soil expands when it freezes. Ground above the frost line lifts and drops with the seasons, and anything bearing on it, a footing, a slab edge, a deck pier, moves with it. That movement is frost heave, and it cracks foundations, tilts posts, and shears shallow pipes.
What has to be below it
- Footings and piers: code requires bearing below frost depth so the structure sits on ground that never freezes.
- Water services: below frost or they freeze solid in January. Sewer usually flows enough to survive shallower, but code and common sense both prefer depth.
- Frost-protected exceptions: insulated shallow foundations and sleeved lines exist, but they're engineered exceptions, not defaults.
Know your number
Frost depth is local: the building department publishes it, and it changes surprisingly fast with geography. Bid work in a new county, check the number, because six extra inches of trench depth across a long water service is real money.