Frost Line Depth in Arizona

How deep is the frost line in Arizona?

Arizona has no single statewide frost line depth. Arizona has no statewide residential code; each city or county adopts its own and sets the frost line depth there. The 12 verified local values we have read range from 0 to 30 inches, so check your own city or county.

Who sets the frost line depth in Arizona?

Under IRC Table R301.2, the jurisdiction fills in the frost line depth as the minimum depth of footing below finish grade. In Arizona that job falls to each city or county, so the number lives in local codes rather than in one state table, and two neighbouring places can differ.

Why do Arizona frost depths range from 0 to 30 inches?

Arizona has no statewide residential code, so each city and county adopts the IRC and fills in its own frost line depth. In the low desert the number is zero: Phoenix, Casa Grande and Bullhead City all list 0 inches. Elevation changes that. Cochise County lists 0 or 12 inches depending on elevation, and Flagstaff, high on the plateau, lists 30 inches.

How deep do footings need to be in Arizona?

Exterior footings go at least 12 inches below undisturbed ground and below the depth your city or county sets, measured to the bottom of the footing. The residential code (IRC R403.1.4.1) also accepts a frost-protected shallow foundation, a design to ASCE 32 or solid rock in place of digging to the frost line, and it exempts small free-standing accessory buildings: light-frame ones up to 600 square feet with eaves 10 feet high or lower. Arizona may amend these rules, so confirm them with your building department. More detail: footing depth.

How deep should deck footings and fence posts be in Arizona?

A deck attached to the house needs footings below the depth your city or county sets. Under the 2021 and 2024 IRC a free-standing deck needs footings only 12 inches deep, and the 2018 edition exempts free-standing decks from the frost rule, so the answer depends on the edition Arizona adopted. See deck footing depth.

The residential code sets no fence-post depth, and fences up to 7 feet tall are exempt from a building permit (IRC R105.2), though zoning still applies. A common practice is to bury about half the above-ground height, and to reach the frost line when posts are set in concrete. See fence post depth.

How deep are water lines buried in Arizona?

Under the IRC plumbing rule (P2603.5), a water service line goes at least 12 inches deep and at least 6 inches below the frost line. States that use the Uniform Plumbing Code or their own plumbing rules word this differently, so ask your plumbing inspector which code applies in Arizona. See water line depth.

How cold do Arizona winters get?

Across the 36 Arizona places we track, nights at or below 32 °F range from 0 a year in Yuma to 202 in Flagstaff. The average low in the coldest month runs from 16.7 °F in Navajo County to 46.8 °F in Yuma (NOAA 1991–2020 normals, nearest station). Colder winters drive frost deeper in a hard year, but the code depth does not change with the weather. See frost penetration depth.

How do Arizona soils affect frost heave?

The USDA soil survey rates how prone soil is to frost action. Around the 36 Arizona places we sampled, the main rating is none at 28 places, moderate at 5 places and low at 3 places. Fine, wet soils rated moderate or high heave the most, so drainage and clean gravel backfill matter more there. The rating does not change the code depth. See frost heave.

How do you confirm the frost line with your Arizona building department?

1. Find your city or county in the table above, or enter your ZIP code; a ZIP can cross into a second county.
2. Ask the building department what frost line depth it uses for Table R301.2, measured to the bottom of the footing, and whether it has local rules for decks, sheds or fences.
3. Ask the plumbing inspector how deep water and sewer lines must go.
4. Call 811 a few days before you dig so buried utilities are marked.