Polk County, IA Frost Line Depth: 42 in

Verified

42in

107 cm · 3 ft 6 in

Set in the building code Polk County has adopted.

Source and how we checked

Document: polkcountyiowa.gov

Climate: 150 nights a year at or below 32 °F (NOAA 1991–2020, Ankeny, 5.3 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

3 of 3nearby verified codes use the same depth
150nights a year at or below 32 °F
61%of nearby soil rated “moderate” for frost action (USDA)

How deep is the frost line in Polk County?

Polk County's building authority sets the frost line depth at 42 in (107 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 Polk County?

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

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

House or garage footing Deck footing Water service line Fence post

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

Minimum depths in Polk County with a 42 in frost line
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 Polk County?

Average lows are at or below freezing from Nov to Mar. 150 nights a year reach 32 °F or colder; Jan is coldest, with an average low of 11.7 °F.

NOAA 1991–2020 climate normals, Ankeny station, 5.3 mi away, elevation 975 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	11.7	16	27.4	38.1	50.3	60.8	64.7	61.7	52.9	40.5	28.9	18
High	29.5	34.2	47.4	61	71.9	81.8	85.7	83.7	77.1	63.6	47.8	34.7

Polk County has more freezing nights than 17 of the 28 other Iowa places we track. The Iowa median is 144. Coldest: Cerro Gordo County; mildest: Davenport.

What frost line depth do towns near Polk County use?

3 official codes near Polk County are verified; each links to its document.

Official frost line depths near Polk County
Place	Verified frost line	Distance
Ankeny	42 in	4 mi away
West Des Moines	42 in	14 mi away
Dallas County	42 in	24 mi away

How likely is the soil in Polk County to heave?

Moderate 61%High 39%

USDA rates the sampled soil around Polk County as 61% moderate, 39% high for frost action (main soils: Webster, Clarion, Nicollet).

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 Polk County?

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

Polk County at a glance

Frost line depth: 42 in
Population (2023 est.): 505,255
Mean annual temperature: 49.6 °F
ZIP codes (42): 50007 · 50009 · 50021 · 50023 · 50032 · 50035 · 50046 · 50047 · 50061 · 50073 · 50109 · 50111 · 50124 · 50131 · 50156 · 50161 · 50168 · 50169 · 50226 · 50228 · 50237 · 50243 · 50244 · 50265 · 50266 · 50309 · 50310 · 50311 · 50312 · 50313 · 50314 · 50315 · 50316 · 50317 · 50319 · 50320 · 50321 · 50322 · 50323 · 50324 · 50325 · 50327