Frost Line
The frost line (also called frost depth or freezing depth) is the maximum depth at which soil reaches 32°F (0°C) during winter in a given climate. Plumbing buried above the frost line is at risk of freezing, expanding, and bursting.
Frost-line depths vary dramatically by climate:
- Phoenix, AZ — 0 inches (no freeze risk)
- Atlanta, GA — 5 inches
- St. Louis, MO — 36 inches
- Boston, MA — 48 inches
- Minneapolis, MN — 60 inches
- Fairbanks, AK — 100+ inches
Local building codes require water and sewer pipes to be buried at minimum 6 inches below the published frost line for the jurisdiction. NOAA NCEI maintains the authoritative frost-depth dataset.
For homeowners: if you live in a climate with 30+ inches of frost line and are renovating, ask your plumber to confirm pipe burial depth and add insulation to any pipes near the frost-line boundary or in unheated spaces (rim joists, crawl spaces, attics).