Lewis County, WA Frost Line Depth: 12 in

Verified

12in

30 cm · 1 ft

Set in the building code Lewis County has adopted.

Source and how we checked

Document: lewiscountywa.gov

Climate: 66 nights a year at or below 32 °F (NOAA 1991–2020, Mayfield Pwr Plt, 11.5 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

3 of 4nearby verified codes use the same depth
66nights a year at or below 32 °F
33%of nearby soil rated “high” for frost action (USDA)

How deep is the frost line in Lewis County?

Lewis County's building authority sets the frost line depth at 12 in (30 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 Lewis County?

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

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

House or garage footing Deck footing Water service line Fence post

House or garage footing in Lewis County: dig at least 12 in (30 cm). Below the frost line and at least 12 in deep (IRC R403.1.4).

Minimum depths in Lewis County with a 12 in frost line
Project	Depth	Rule
House or garage footing	12 in	Below the frost line and at least 12 in deep (IRC R403.1.4)
Deck footing (attached)	12 in	Below the frost line and at least 12 in deep (IRC R507.3)
Water service line	18 in	6 in below the frost line and at least 12 in deep (IRC P2603.5)
Fence post bottom	12 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 Lewis County?

Lewis County never averages a low below freezing. 66 nights a year reach 32 °F or colder; Jan is coldest, with an average low of 33.1 °F.

NOAA 1991–2020 climate normals, Mayfield Pwr Plt station, 11.5 mi away, elevation 280 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	33.1	33.5	35.2	38.7	43.2	47.8	51.4	51.9	48.1	42.7	37.5	33.5
High	45	49.2	54.1	60	69.1	72.3	81.1	80.9	74.3	61.7	51	44.4

Lewis County has more freezing nights than 26 of the 47 other Washington places we track. The Washington median is 54. Coldest: Chelan County; mildest: Lakewood.

What frost line depth do towns near Lewis County use?

4 official codes near Lewis County are verified; each links to its document.

Official frost line depths near Lewis County
Place	Verified frost line	Distance
Cowlitz County	12 in	30 mi away
Pierce County	18 in	34 mi away
Mason County	12 in	65 mi away
Bremerton	12 in	68 mi away

How likely is the soil in Lewis County to heave?

None 18%Low 19%Moderate 29%High 33%

USDA rates the sampled soil around Lewis County as 18% none, 19% low, 29% moderate, 33% high for frost action (main soils: Cinebar, Newberg, Schneider).

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 Lewis County?

Call 811 at least a few working days before you dig, so buried lines are marked.
Confirm the frost line depth with the Lewis County building department.
Ask whether your project needs a permit (decks, larger sheds and footings usually do).
Dig below 12 in unless the building department tells you otherwise.
Book the footing inspection before you pour concrete.

Lewis County at a glance

Frost line depth: 12 in
Population (2023 est.): 86,154
Mean annual temperature: 51.6 °F
ZIP codes (24): 98304 · 98336 · 98355 · 98356 · 98361 · 98377 · 98531 · 98532 · 98533 · 98538 · 98539 · 98542 · 98544 · 98564 · 98565 · 98568 · 98570 · 98572 · 98579 · 98582 · 98585 · 98591 · 98593 · 98596