Frost Line Depth in Williamson County, IL: Illinois Codes Range 36

No official local value

No official frost line depth is published for Williamson County.

No verified code nearby; verified Illinois codes range 36–42 in.

Source and how we checked

We searched this place’s own code and website and found no published frost line depth.

Climate: 103 nights a year at or below 32 °F (NOAA 1991–2020, Carbondale Sewage Plt, 12.9 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

36–42 inrange of verified Illinois codes
103nights a year at or below 32 °F
89%of nearby soil rated “high” for frost action (USDA)

Does Williamson County have an official frost line depth?

No adopted frost line depth found for Williamson County in its official documents (checked Oct 2026). No verified code lies near Williamson County; the Illinois codes we verified range from 36 to 42 in. Ask the building department for Williamson County’s own number before you dig.

What frost line depth does Williamson County use for footings under Table R301.2(1)?

Call 811 (JULIE) at least a few working days before you dig, so buried lines are marked.
Confirm the frost line depth with the Williamson County building department.
Ask whether your project needs a permit (decks, larger sheds and footings usually do).
Dig below 42 in (the deepest verified state value) unless the building department tells you otherwise.
Book the footing inspection before you pour concrete.

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

Drawn with the deepest Illinois official value (42 in). Confirm Williamson County’s own depth before you dig.

House or garage footing Deck footing Water service line Fence post

House or garage footing in Williamson County: dig at least 42 in (107 cm). Below the frost line and at least 12 in deep (IRC R403.1.4).

Using the nearby official value of 42 in until Williamson County confirms its own
Project	Depth	Rule
House or garage footing	42 in	Below the frost line and at least 12 in deep (IRC R403.1.4)
Deck footing (attached)	42 in	Below the frost line and at least 12 in deep (IRC R507.3)
Water service line	48 in	6 in below the frost line and at least 12 in deep (IRC P2603.5)
Fence post bottom	42 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 Williamson County?

Average lows are at or below freezing from Dec to Feb. 103 nights a year reach 32 °F or colder; Jan is coldest, with an average low of 23.4 °F.

NOAA 1991–2020 climate normals, Carbondale Sewage Plt station, 12.9 mi away, elevation 390 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	23.4	26.4	34.5	44.3	55.1	63.9	67.6	64.6	56	44.1	34.8	27.2
High	41.3	46.3	56.1	67.7	76.6	84.9	88.1	87.4	81.2	69.9	56.2	45.4

Williamson County has fewer freezing nights than 65 of the 71 other Illinois places we track. The Illinois median is 127. Coldest: Hoffman Estates; mildest: Madison County.

How likely is the soil in Williamson County to heave?

High 89%

USDA rates the sampled soil around Williamson County as 11% moderate, 89% high for frost action (main soils: Bluford, Ava, Rend).

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

Williamson County at a glance

Frost line depth: No official local value
Population (2023 est.): 66,706
Mean annual temperature: 56 °F
ZIP codes (22): 62841 · 62890 · 62896 · 62901 · 62902 · 62915 · 62917 · 62918 · 62921 · 62922 · 62924 · 62933 · 62935 · 62939 · 62948 · 62949 · 62951 · 62958 · 62959 · 62967 · 62974 · 62987