Decatur, IL Frost Line Depth: 36 in

Verified

36in

91 cm · 3 ft

Set in the building code Decatur has adopted.

Source and how we checked

Document: decaturil.gov

Climate: 110 nights a year at or below 32 °F (NOAA 1991–2020, Decatur Wtp, 2.1 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

0 of 1nearby verified codes use the same depth
110nights a year at or below 32 °F
52%of nearby soil rated “moderate” for frost action (USDA)

How deep is the frost line in Decatur?

Decatur's building authority sets the frost line depth 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 Decatur?

Illinois leaves the frost line depth to each city or county, and Decatur fills it in when it adopts the residential code. The value above comes from Decatur’s own document on decaturil.gov. See how Illinois sets frost line depth.

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

House or garage footing Deck footing Water service line Fence post

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

Minimum depths in Decatur 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 Decatur?

Average lows are at or below freezing from Dec to Feb. 110 nights a year reach 32 °F or colder; Jan is coldest, with an average low of 19.7 °F.

NOAA 1991–2020 climate normals, Decatur Wtp station, 2.1 mi away, elevation 639 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	19.7	23.5	32.8	43.3	53.8	63.1	66.5	64.9	57	45.7	34.3	25
High	34	39.1	50.8	63.4	73.5	82.2	84.7	83.5	77.7	65.3	50.3	38.6

Decatur has fewer freezing nights than 57 of the 71 other Illinois places we track. The Illinois median is 127. Coldest: Hoffman Estates; mildest: Madison County.

What frost line depth do towns near Decatur use?

One official code near Decatur is verified.

Official frost line depths near Decatur
Place	Verified frost line	Distance
Bloomington	40 in	43 mi away

How likely is the soil in Decatur to heave?

Moderate 52%High 48%

USDA rates the sampled soil around Decatur as 52% moderate, 48% high for frost action (main soils: Flanagan, Drummer, Catlin).

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 Decatur?

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

Decatur at a glance

Frost line depth: 36 in
County: Macon County
Population (2023 est.): 68,670
Mean annual temperature: 53 °F
ZIP codes (7): 62501 · 62521 · 62522 · 62523 · 62526 · 62535 · 62554