Flagstaff, AZ Frost Line Depth: 30 in

Verified

30in

76 cm · 2 ft 6 in

Set in the building code Flagstaff has adopted.

Source and how we checked

Document: flagstaff.az.gov

Climate: 202 nights a year at or below 32 °F (NOAA 1991–2020, Flagstaff Pulliam Airport, 3.8 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

202nights a year at or below 32 °F
56%of nearby soil rated “low” for frost action (USDA)

How deep is the frost line in Flagstaff?

Flagstaff's building authority sets the frost line depth at 30 in (76 cm). Footings, piers and other permanent supports must reach at least this depth (IRC R403.1.4.1).

Who sets the frost line depth in Flagstaff?

Arizona has no statewide residential code, so Flagstaff adopts its own and fills in the frost line depth. The value above comes from Flagstaff’s own document on flagstaff.az.gov. See how Arizona sets frost line depth.

How deep do footings, decks and water lines need to be in Flagstaff?

House or garage footing Deck footing Water service line Fence post

House or garage footing in Flagstaff: dig at least 30 in (76 cm). Below the frost line and at least 12 in deep (IRC R403.1.4).

Minimum depths in Flagstaff with a 30 in frost line
Project	Depth	Rule
House or garage footing	30 in	Below the frost line and at least 12 in deep (IRC R403.1.4)
Deck footing (attached)	30 in	Below the frost line and at least 12 in deep (IRC R507.3)
Water service line	36 in	6 in below the frost line and at least 12 in deep (IRC P2603.5)
Fence post bottom	30 in	No code rule; common practice is below the frost line

Work out posts, concrete and footing sizes with the fence post depth calculator or the deck footing calculator.

How cold does it get in Flagstaff?

Average lows are at or below freezing from Oct to Apr. 202 nights a year reach 32 °F or colder; Dec is coldest, with an average low of 16.9 °F.

NOAA 1991–2020 climate normals, Flagstaff Pulliam Airport station, 3.8 mi away, elevation 7,003 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	17.6	19.6	23.8	28.2	34.6	42.5	51.4	50.6	42.5	31.5	23	16.9
High	43.4	45.7	52.2	59.2	68.1	79.1	82	79.3	74.1	63.6	52.1	43

Flagstaff has more freezing nights than 35 of the 35 other Arizona places we track. The Arizona median is 12. Coldest: Flagstaff; mildest: Yuma.

How likely is the soil in Flagstaff to heave?

Low 56%Moderate 22%High 22%

USDA rates the sampled soil around Flagstaff as 56% low, 22% moderate, 22% high for frost action (main soils: Tortugas, Daze, Baldy).

USDA NRCS soil survey, 9 sample points within 2 mi. Higher frost action means more risk of heave; see frost heave.

How do you confirm the frost line before you dig in Flagstaff?

Call 811 at least a few working days before you dig, so buried lines are marked.
Confirm the frost line depth with the Flagstaff building department.
Ask whether your project needs a permit (decks, larger sheds and footings usually do).
Dig below 30 in unless the building department tells you otherwise.
Book the footing inspection before you pour concrete.

Flagstaff at a glance

Frost line depth: 30 in
County: Coconino County
Population (2023 est.): 76,586
Mean annual temperature: 46.8 °F
ZIP codes (4): 86001 · 86004 · 86005 · 86011