Mobile, AL Frost Line Depth: 12 in

Verified

12in

30 cm · 1 ft

Set in the building code Mobile has adopted.

Source and how we checked

Document: library.municode.com

Climate: 15 nights a year at or below 32 °F (NOAA 1991–2020, Mobile Dwtn Airport, 3.5 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

15nights a year at or below 32 °F
100%of nearby soil rated “none” for frost action (USDA)

How deep is the frost line in Mobile?

Mobile's building authority sets the frost line depth at 12 in (30 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 Mobile?

Alabama leaves the frost line depth to each city or county, and Mobile fills it in when it adopts the residential code. The value above comes from Mobile’s own document on library.municode.com. See how Alabama sets frost line depth.

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

House or garage footing Deck footing Water service line Fence post

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

Minimum depths in Mobile with a 12 in frost line
Project	Depth	Rule
House or garage footing	12 in	Below the frost line and at least 12 in deep (IRC R403.1.4)
Deck footing (attached)	12 in	Below the frost line and at least 12 in deep (IRC R507.3)
Water service line	18 in	6 in below the frost line and at least 12 in deep (IRC P2603.5)
Fence post bottom	12 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 Mobile?

Mobile never averages a low below freezing. 15 nights a year reach 32 °F or colder; Jan is coldest, with an average low of 42.5 °F.

NOAA 1991–2020 climate normals, Mobile Dwtn Airport station, 3.5 mi away, elevation 26 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	42.5	46.1	51.8	58.6	66.3	73.1	75.3	75.3	71.8	61	49.9	44.9
High	62.1	65.8	71.8	77.9	85	90	91.7	91.9	88.8	81.3	71.6	64.3

Mobile has fewer freezing nights than 39 of the 39 other Alabama places we track. The Alabama median is 52. Coldest: Tallapoosa County; mildest: Mobile.

How likely is the soil in Mobile to heave?

None 100%

USDA rates the sampled soil around Mobile as 100% none for frost action (main soils: Harleston, Wadley, Escambia).

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 Mobile?

Call 811 at least a few working days before you dig, so buried lines are marked.
Confirm the frost line depth with the Mobile building department.
Ask whether your project needs a permit (decks, larger sheds and footings usually do).
Dig below 12 in unless the building department tells you otherwise.
Book the footing inspection before you pour concrete.

Mobile at a glance

Frost line depth: 12 in
County: Mobile County
Population (2023 est.): 182,595
Mean annual temperature: 69.1 °F
ZIP codes (23): 36571 · 36575 · 36582 · 36602 · 36603 · 36604 · 36605 · 36606 · 36607 · 36608 · 36609 · 36610 · 36611 · 36612 · 36613 · 36615 · 36617 · 36618 · 36619 · 36652 · 36688 · 36693 · 36695