Frost Line Depth in Washtenaw County, MI: 42 in (Michigan rule)

State-set value

42in

107 cm · 3 ft 6 in

42 in (Michigan rule)

Set by the state: Michigan Residential Code R403.1.4 (R 408.30522); the building official may modify.

Source and how we checked

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

Document: codes.iccsafe.org

Climate: 150 nights a year at or below 32 °F (NOAA 1991–2020, Ann Arbor Municipal Airport, 5.5 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

150nights a year at or below 32 °F
52%of nearby soil rated “high” for frost action (USDA)

How deep is the frost line in Washtenaw County?

Michigan sets the frost footing depth for Washtenaw County 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 Washtenaw County?

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

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

House or garage footing Deck footing Water service line Fence post

House or garage footing in Washtenaw 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 Washtenaw 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 Washtenaw 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 15.2 °F.

NOAA 1991–2020 climate normals, Ann Arbor Municipal Airport station, 5.5 mi away, elevation 839 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	15.2	16	25.3	35.3	45.6	53.7	57.2	56.9	48.4	38.3	29.5	21.5
High	31.8	34.6	45.4	58.7	70.1	79.2	83.8	81.4	74.2	61.5	47.9	36.4

Washtenaw County has more freezing nights than 53 of the 62 other Michigan places we track. The Michigan median is 143. Coldest: Marquette County; mildest: Allegan County.

How likely is the soil in Washtenaw County to heave?

Moderate 37%High 52%

USDA rates the sampled soil around Washtenaw County as 11% low, 37% moderate, 52% high for frost action (main soils: Houghton, Pewamo, Fox).

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 Washtenaw County?

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

Washtenaw County at a glance

Frost line depth: 42 in (Michigan rule)
Population (2023 est.): 365,536
Mean annual temperature: 47.8 °F
ZIP codes (27): 48103 · 48104 · 48105 · 48108 · 48109 · 48111 · 48118 · 48130 · 48137 · 48158 · 48160 · 48167 · 48168 · 48169 · 48170 · 48176 · 48178 · 48189 · 48190 · 48191 · 48197 · 48198 · 49229 · 49236 · 49240 · 49285 · 49287