Frostline.info

Frost line depth in King County, Washington

Frost reaches about 12 inches in typical soil across King County, from 10″ in the mildest ZIP code to 16″ in the coldest. Codes in this climate commonly set footing depth at 12 to 18 inches.

Shallow frost
Estimated frost depth
12″
typical silt/loam soil
Typical code footing depth
12–18″
rule of thumb for this climate
Design freezing index
140
°F-days, 100-year winter
ZIP codes covered
83
#35 of 39 counties in WA (1 = deepest)
12″ 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)15″
Silt / loam (typical yard soil)12″
Clay (holds water)10″

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 King County building or permitting office in Washington and call 811 before digging.

Frost depth by ZIP code in King County

ZIPCityEst. frostCode range
98001Auburn12″12–18″
98002Auburn12″12–18″
98003Federal Way12″12–18″
98004Bellevue11″12–18″
98005Bellevue11″12–18″
98006Bellevue11″12–18″
98007Bellevue11″12–18″
98008Bellevue11″12–18″
98010Black Diamond13″12–18″
98011Bothell11″12–18″
98014Carnation13″12–18″
98019Duvall13″12–18″
98022Enumclaw13″12–18″
98023Federal Way11″12–18″
98024Fall City13″12–18″
98027Issaquah13″12–18″
98028Kenmore11″12–18″
98029Issaquah12″12–18″
98030Kent12″12–18″
98031Kent12″12–18″
98032Kent12″12–18″
98033Kirkland11″12–18″
98034Kirkland11″12–18″
98038Maple Valley13″12–18″
98039Medina11″12–18″
98040Mercer Island11″12–18″
98042Kent12″12–18″
98045North Bend16″12–24″
98047Pacific12″12–18″
98051Ravensdale15″12–18″
98052Redmond11″12–18″
98053Redmond12″12–18″
98055Renton12″12–18″
98056Renton12″12–18″
98057Renton12″12–18″
98058Renton12″12–18″
98059Renton12″12–18″
98065Snoqualmie13″12–18″
98070Vashon12″12–18″
98072Woodinville12″12–18″
98074Sammamish12″12–18″
98075Sammamish12″12–18″
98077Woodinville12″12–18″
98092Auburn12″12–18″
98101Seattle11″12–18″
98102Seattle11″12–18″
98103Seattle11″12–18″
98104Seattle11″12–18″
98105Seattle10″12–18″
98106Seattle12″12–18″
98107Seattle11″12–18″
98108Seattle11″12–18″
98109Seattle11″12–18″
98112Seattle11″12–18″
98115Seattle11″12–18″
98116Seattle11″12–18″
98117Seattle11″12–18″
98118Seattle11″12–18″
98119Seattle11″12–18″
98121Seattle11″12–18″
98122Seattle11″12–18″
98125Seattle11″12–18″
98126Seattle11″12–18″
98131Seattle11″12–18″
98133Seattle11″12–18″
98134Seattle11″12–18″
98136Seattle11″12–18″
98144Seattle11″12–18″
98146Seattle12″12–18″
98148Seattle12″12–18″
98154Seattle11″12–18″
98155Seattle11″12–18″
98158Seattle12″12–18″
98161Seattle11″12–18″
98164Seattle11″12–18″
98166Seattle12″12–18″
98168Seattle12″12–18″
98174Seattle11″12–18″
98177Seattle11″12–18″
98178Seattle12″12–18″
98188Seattle12″12–18″
98198Seattle12″12–18″
98199Seattle11″12–18″

What this means for your project in King 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 18″ 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 12″ 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 — 140 °F-days here — to size perimeter insulation, letting footings sit shallower than the frost line.

Weather stations used

NOAA weather stationDistance100-yr AFI
Seattle U Of W, WA7 mi113
Seattle-Tacoma, WA8 mi148
Kent, WA12 mi148
Landsburg, WA17 mi167

Nearby counties

Common questions

What is the frost line depth in King County?

About 12 inches in typical soil, based on NOAA's 100-year air freezing index. Across the county's ZIP codes the estimate runs from 10 to 16 inches.

How deep should footings be in King County, Washington?

Codes in climates like this commonly require 12 to 18 inches. The King County building department publishes the required frost depth in its adopted code (IRC Table R301.2).

Does frost depth vary within King County?

Yes. Estimates differ by 6 inches between the coldest and mildest ZIP codes, mostly from latitude and elevation. Check your own ZIP below.