Frost Line Depth in Mercer County, NJ: 36 in (New Jersey rule)

State-set value

36in

91 cm · 3 ft

36 in (New Jersey rule)

Set by the state: N.J.A.C. 5:23-3.21 Table R301.2, North New Jersey zone (Mercer County).

Source and how we checked

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

Document: law.cornell.edu

Climate: 109 nights a year at or below 32 °F (NOAA 1991–2020, Trenton State College, 4.7 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

109nights a year at or below 32 °F
47%of nearby soil rated “moderate” for frost action (USDA)

How deep is the frost line in Mercer County?

New Jersey sets the frost footing depth for Mercer County at 36 in (91 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 Mercer County?

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

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

House or garage footing Deck footing Water service line Fence post

House or garage footing in Mercer County: dig at least 36 in (91 cm). Below the frost line and at least 12 in deep (IRC R403.1.4).

Minimum depths in Mercer County with a 36 in frost line
Project	Depth	Rule
House or garage footing	36 in	Below the frost line and at least 12 in deep (IRC R403.1.4)
Deck footing (attached)	36 in	Below the frost line and at least 12 in deep (IRC R507.3)
Water service line	42 in	6 in below the frost line and at least 12 in deep (IRC P2603.5)
Fence post bottom	36 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 Mercer County?

Average lows are at or below freezing from Dec to Mar. 109 nights a year reach 32 °F or colder; Jan is coldest, with an average low of 22.7 °F.

NOAA 1991–2020 climate normals, Trenton State College station, 4.7 mi away, elevation 100 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	22.7	23.7	31	40.5	49.9	59.3	65	63.2	56	44.4	34.5	26.3
High	39.9	42.9	51.2	63.2	72.6	81.6	86.5	84	77.7	66.1	55	44.5

How likely is the soil in Mercer County to heave?

Low 34%Moderate 47%High 19%

USDA rates the sampled soil around Mercer County as 34% low, 47% moderate, 19% high for frost action (main soils: Galestown, Matapeake, Udorthents).

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 Mercer County?

Call 811 at least a few working days before you dig, so buried lines are marked.
Confirm the frost line depth with the Mercer County building department.
Ask whether your project needs a permit (decks, larger sheds and footings usually do).
Dig below 36 in unless the building department tells you otherwise.
Book the footing inspection before you pour concrete.

Mercer County at a glance

Frost line depth: 36 in (New Jersey rule)
Population (2023 est.): 381,671
Mean annual temperature: 53.4 °F
ZIP codes (26): 08501 · 08512 · 08520 · 08525 · 08530 · 08534 · 08540 · 08542 · 08544 · 08550 · 08558 · 08560 · 08561 · 08608 · 08609 · 08610 · 08611 · 08618 · 08619 · 08620 · 08628 · 08629 · 08638 · 08648 · 08690 · 08691