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.

How do frost-line depths vary 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.

Which plumbing has to sit below the frost line?

Anything holding water year-round: the water service from the meter or well, the sewer lateral, irrigation mains, and the supply to any yard hydrant. Hose bibs are the common exception — a frost-proof sillcock keeps its shutoff inside the heated envelope and drains the barrel when closed, which is why it survives at wall height while a standard bib in the same wall splits.

How is frost depth set for a permit?

The published figure is a design depth adopted by the local jurisdiction, not a measurement taken on the day. Inspectors work from the code-adopted number for the county, which is generally conservative relative to a typical winter, and most jurisdictions add a cover requirement on top of it. Because the adopted depth can differ between neighbouring counties, the number that governs a job is the one in the permitting authority's own code, not a regional map.

What should homeowners do about frost-line depth?

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).