Frost Line Depth in Onslow County, NC: 12 in (North Carolina rule)

State-set value

12in

30 cm · 1 ft

12 in (North Carolina rule)

Set by the state: North Carolina Residential Code Table R301.2(1) (statewide).

Source and how we checked

Onslow County does not publish its own value; the state rule applies.

Document: codes.iccsafe.org

Climate: 63 nights a year at or below 32 °F (NOAA 1991–2020, Jacksonville Eoc, 6 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

63nights a year at or below 32 °F
100%of nearby soil rated “none” for frost action (USDA)

How deep is the frost line in Onslow County?

North Carolina sets the frost footing depth for Onslow County 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 Onslow County?

North Carolina writes this depth into its statewide residential code, so Onslow County’s building department enforces the state’s number rather than setting its own. See how North Carolina sets frost line depth.

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

House or garage footing Deck footing Water service line Fence post

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

Minimum depths in Onslow County 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 Onslow County?

Onslow County never averages a low below freezing. 63 nights a year reach 32 °F or colder; Jan is coldest, with an average low of 32.4 °F.

NOAA 1991–2020 climate normals, Jacksonville Eoc station, 6 mi away, elevation 17 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	32.4	34.3	39.6	48.8	58.5	66.8	71.3	69.5	63.7	51.4	41	35.7
High	56.1	58.8	65.7	74.5	80.6	87	89.8	88.4	84	75.5	66.7	59.5

Onslow County has fewer freezing nights than 50 of the 82 other North Carolina places we track. The North Carolina median is 68. Coldest: Watauga County; mildest: Carteret County.

How likely is the soil in Onslow County to heave?

None 100%

USDA rates the sampled soil around Onslow County as 100% none for frost action (main soils: Stallings, Grifton, Baymeade).

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 Onslow County?

Call 811 at least a few working days before you dig, so buried lines are marked.
Confirm the frost line depth with the Onslow County 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.

Onslow County at a glance

Frost line depth: 12 in (North Carolina rule)
Population (2023 est.): 213,676
Mean annual temperature: 62.5 °F
ZIP codes (17): 28445 · 28454 · 28460 · 28518 · 28521 · 28539 · 28540 · 28542 · 28543 · 28544 · 28546 · 28547 · 28555 · 28572 · 28574 · 28582 · 28584