Frost Line Depth in Fairfax County, Virginia

Fairfax County’s adopted code
Checked Oct 2026
Sources

Verified

24in

61 cm · 2 ft

Set in the building code Fairfax County has adopted.

Source and how we checked

Document: fairfaxcounty.gov

Climate: 99 nights a year at or below 32 °F (NOAA 1991–2020, Vienna, 4.4 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

2 of 3nearby verified codes use the same depth
99nights a year at or below 32 °F
68%of nearby soil rated “high” for frost action (USDA)

How deep is the frost line in Fairfax County?

"{confirmed quote}". Footings, piers and other permanent supports must reach at least this depth (IRC R403.1.4.1).

Who sets the frost line depth in Fairfax County?

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

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

House or garage footing Deck footing Water service line Fence post

House or garage footing in Fairfax County: dig at least 24 in (61 cm). Below the frost line and at least 12 in deep (IRC R403.1.4).

Minimum depths in Fairfax County with a 24 in frost line
Project	Depth	Rule
House or garage footing	24 in	Below the frost line and at least 12 in deep (IRC R403.1.4)
Deck footing (attached)	24 in	Below the frost line and at least 12 in deep (IRC R507.3)
Water service line	30 in	6 in below the frost line and at least 12 in deep (IRC P2603.5)
Fence post bottom	24 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 Fairfax County?

Average lows are at or below freezing from Dec to Mar. 99 nights a year reach 32 °F or colder; Jan is coldest, with an average low of 23.4 °F.

NOAA 1991–2020 climate normals, Vienna station, 4.4 mi away, elevation 418 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	23.4	24.3	31.6	41.2	52.1	61	66.6	64.9	57.1	45.2	34.6	28.3
High	42.3	45.3	53.3	64.8	71.6	79.5	83.8	82.4	75.9	65.4	55.8	46.2

Fairfax County has more freezing nights than 13 of the 22 other Virginia places we track. The Virginia median is 94. Coldest: Shenandoah County; mildest: Norfolk.

What frost line depth do towns near Fairfax County use?

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

Official frost line depths near Fairfax County
Place	Verified frost line	Distance
Alexandria	24 in	10 mi away
Spotsylvania County	18 in	49 mi away
Shenandoah County	24 in	70 mi away

How likely is the soil in Fairfax County to heave?

Moderate 32%High 68%

USDA rates the sampled soil around Fairfax County as 32% moderate, 68% high for frost action (main soils: Wheaton, Glenelg, Codorus).

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 Fairfax County?

Call 811 at least a few working days before you dig, so buried lines are marked.
Confirm the frost line depth with the Fairfax County building department.
Ask whether your project needs a permit (decks, larger sheds and footings usually do).
Dig below 24 in unless the building department tells you otherwise.
Book the footing inspection before you pour concrete.

Fairfax County at a glance

Frost line depth: 24 in
Population (2023 est.): 1,141,878
Mean annual temperature: 54 °F
ZIP codes (49): 20120 · 20121 · 20124 · 20151 · 20170 · 20171 · 20190 · 20191 · 20194 · 22003 · 22015 · 22027 · 22030 · 22031 · 22032 · 22033 · 22035 · 22039 · 22041 · 22042 · 22043 · 22044 · 22046 · 22060 · 22066 · 22079 · 22101 · 22102 · 22124 · 22150 · 22151 · 22152 · 22153 · 22180 · 22181 · 22182 · 22203 · 22207 · 22302 · 22303 · 22304 · 22306 · 22307 · 22308 · 22309 · 22310 · 22311 · 22312 · 22315