Frost Line Depth in Hillsboro, OR: 12 in (Oregon rule)

State-set value

12in

30 cm · 1 ft

12 in (Oregon rule)

Set by the state: Oregon Residential Specialty Code Table R301.2(1), Washington County.

Source and how we checked

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

Document: codes.iccsafe.org

Climate: 64 nights a year at or below 32 °F (NOAA 1991–2020, Portland-Hillsboro Airport, 1.1 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

2 of 2nearby verified codes use the same depth
64nights a year at or below 32 °F
100%of nearby soil rated “low” for frost action (USDA)

How deep is the frost line in Hillsboro?

Oregon sets the frost footing depth for Hillsboro 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 Hillsboro?

Oregon writes this depth into its statewide residential code, so Hillsboro’s building department enforces the state’s number rather than setting its own. Washington County uses the same 12 in, so the depth does not change at the city limits. See how Oregon sets frost line depth.

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

House or garage footing Deck footing Water service line Fence post

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

Minimum depths in Hillsboro 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 Hillsboro?

Hillsboro never averages a low below freezing. 64 nights a year reach 32 °F or colder; Dec is coldest, with an average low of 33.3 °F.

NOAA 1991–2020 climate normals, Portland-Hillsboro Airport station, 1.1 mi away, elevation 204 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	33.6	33.5	36.5	39.2	44.7	48.6	52.2	51.8	47.5	40.9	36.5	33.3
High	47.3	51	55.9	60.9	68.7	73.5	82.1	82.4	76.4	63.7	52.6	46.2

Hillsboro has more freezing nights than 13 of the 22 other Oregon places we track. The Oregon median is 54. Coldest: Deschutes County; mildest: Lincoln County.

What frost line depth do towns near Hillsboro use?

2 official codes near Hillsboro are verified; each links to its document.

Official frost line depths near Hillsboro
Place	Verified frost line	Distance
Marion County	12 in	47 mi away
Corvallis	12 in	68 mi away

How likely is the soil in Hillsboro to heave?

Low 100%

USDA rates the sampled soil around Hillsboro as 0% none, 100% low for frost action (main soils: Quatama, Willamette, Amity).

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 Hillsboro?

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

Hillsboro at a glance

Frost line depth: 12 in (Oregon rule)
County: Washington County
Population (2023 est.): 107,730
Mean annual temperature: 52.5 °F
ZIP codes (6): 97003 · 97006 · 97078 · 97123 · 97124 · 97229