Frostline.info

Frost line depth in Fairfax County, Virginia

Frost reaches about 19 inches in typical soil across Fairfax County. Codes in this climate commonly set footing depth at 18 to 24 inches.

Moderate frost
Estimated frost depth
19″
typical silt/loam soil
Typical code footing depth
18–24″
rule of thumb for this climate
Design freezing index
376
°F-days, 100-year winter
ZIP codes covered
46
#50 of 131 counties in VA (1 = deepest)
19″ 0″12″24″36″ ground frozen
Est. frost (typical soil) Typical code footing depth Clay–sand range

Frost depth by soil type

Soil typeEst. frost depth
Sand / gravel (drains well)24″
Silt / loam (typical yard soil)19″
Clay (holds water)16″

County average. Methodology.

Verify before you dig. These are climate-based estimates. The legally required frost depth is the figure your building department publishes in its IRC Table R301.2 (climatic and geographic design criteria). Call the Fairfax County building or permitting office in Virginia and call 811 before digging.

Frost depth by ZIP code in Fairfax County

ZIPCityEst. frostCode range
20120Centreville20″18–30″
20121Centreville20″18–30″
20124Clifton20″18–24″
20151Chantilly20″18–24″
20170Herndon20″18–24″
20171Herndon20″18–24″
20190Reston20″18–24″
20191Reston20″18–24″
20194Reston20″18–24″
22003Annandale19″18–24″
22015Burke19″18–24″
22027Dunn Loring19″18–24″
22030Fairfax19″18–24″
22031Fairfax19″18–24″
22032Fairfax19″18–24″
22033Fairfax19″18–24″
22035Fairfax19″18–24″
22039Fairfax Station18″18–24″
22041Falls Church19″18–24″
22042Falls Church19″18–24″
22043Falls Church19″18–24″
22044Falls Church19″18–24″
22060Fort Belvoir18″18–24″
22066Great Falls20″18–24″
22067Greenway19″18–24″
22079Lorton18″18–24″
22081Merrifield19″18–24″
22101Mc Lean19″18–24″
22102Mc Lean19″18–24″
22124Oakton19″18–24″
22150Springfield19″18–24″
22151Springfield19″18–24″
22152Springfield19″18–24″
22153Springfield18″18–24″
22159Springfield19″18–24″
22180Vienna19″18–24″
22181Vienna19″18–24″
22182Vienna19″18–24″
22303Alexandria19″18–24″
22306Alexandria18″18–24″
22307Alexandria19″18–24″
22308Alexandria18″18–24″
22309Alexandria18″18–24″
22310Alexandria19″18–24″
22312Alexandria19″18–24″
22315Alexandria18″18–24″

What this means for your project in Fairfax County

Deck footings & piers

Footings must bear below the frost line and at least 12″ below undisturbed ground. For planning in this area, budget for holes around 24″ deep until your permit office confirms the exact figure.

Fence posts

Set posts at least one-third of their above-ground height. Where frost heave is a problem, carry concrete footings below the frost line (about 19″ here in typical soil) and bell or taper the bottom.

Water & irrigation lines

Bury supply lines below the frost depth. Your water utility or plumbing code sets the minimum cover; add a margin below the estimate above for unusually cold winters.

Slabs & frost-protected foundations

Frost-protected shallow foundations (IRC R403.3) use the 100-year air freezing index — 376 °F-days here — to size perimeter insulation, letting footings sit shallower than the frost line.

Weather stations used

NOAA weather stationDistance100-yr AFI
Dalecarlia Resvr D C, MD9 mi379
National Arboretum D C, MD16 mi337
College Park, MD19 mi384
Rockville 4 Ne, MD20 mi467

Nearby counties

Common questions

What is the frost line depth in Fairfax County?

About 19 inches in typical soil, based on NOAA's 100-year air freezing index. Across the county's ZIP codes the estimate runs from 18 to 20 inches.

How deep should footings be in Fairfax County, Virginia?

Codes in climates like this commonly require 18 to 24 inches. The Fairfax County building department publishes the required frost depth in its adopted code (IRC Table R301.2).

Does frost depth vary within Fairfax County?

Very little. Estimates across the county stay within 2 inches. Soil type matters more than location here.