Washington: basement moisture and foundation risk by county
39 counties, ranked by our combined index. Highest combined risk is Pierce County (51), lowest is Mason County (21). The worst soil for foundation cracks specifically is in Clallam County.
| County | Index | Moisture | Cracks | Water table | Region |
|---|---|---|---|---|---|
| Pierce County | 51 | 47 | 33 | 82 | Puget Sound and Willamette Lowland |
| Clallam County | 49 | 36 | 92 | 16 | Pacific Coast Ranges |
| Grays Harbor County | 49 | 35 | 83 | 30 | Pacific Coast Ranges |
| Island County | 48 | 47 | 35 | 66 | Puget Sound and Willamette Lowland |
| Skagit County | 47 | 46 | 24 | 82 | Puget Sound and Willamette Lowland |
| Wahkiakum County | 47 | 38 | 82 | 16 | Pacific Coast Ranges |
| Skamania County | 46 | 47 | 29 | 66 | Puget Sound and Willamette Lowland |
| Whatcom County | 46 | 47 | 19 | 82 | Puget Sound and Willamette Lowland |
| Pacific County | 45 | 34 | 82 | 16 | Pacific Coast Ranges |
| Jefferson County | 44 | 44 | 28 | 66 | Puget Sound and Willamette Lowland |
| Thurston County | 44 | 44 | 27 | 66 | Puget Sound and Willamette Lowland |
| Cowlitz County | 41 | 43 | 20 | 66 | Puget Sound and Willamette Lowland |
| King County | 41 | 46 | 31 | 46 | Puget Sound and Willamette Lowland |
| Klickitat County | 41 | 44 | 32 | 46 | Puget Sound and Willamette Lowland |
| Lewis County | 39 | 48 | 21 | 46 | Puget Sound and Willamette Lowland |
| Clark County | 38 | 47 | 20 | 46 | Puget Sound and Willamette Lowland |
| Kitsap County | 37 | 43 | 23 | 46 | Puget Sound and Willamette Lowland |
| Adams County | 35 | 33 | 41 | 30 | Columbia Plateau and Palouse |
| Asotin County | 35 | 34 | 40 | 30 | Columbia Plateau and Palouse |
| Lincoln County | 35 | 33 | 41 | 30 | Columbia Plateau and Palouse |
| Snohomish County | 35 | 43 | 17 | 46 | Puget Sound and Willamette Lowland |
| Walla Walla County | 34 | 30 | 42 | 30 | Columbia Plateau and Palouse |
| Douglas County | 33 | 32 | 36 | 30 | Columbia Plateau and Palouse |
| Ferry County | 33 | 32 | 36 | 30 | Columbia Plateau and Palouse |
| Garfield County | 32 | 32 | 43 | 16 | Columbia Plateau and Palouse |
| Stevens County | 31 | 32 | 29 | 30 | Columbia Plateau and Palouse |
| Whitman County | 31 | 31 | 43 | 16 | Columbia Plateau and Palouse |
| Benton County | 30 | 34 | 23 | 30 | Columbia Plateau and Palouse |
| Pend Oreille County | 30 | 35 | 22 | 30 | Columbia Plateau and Palouse |
| Columbia County | 29 | 34 | 20 | 30 | Columbia Plateau and Palouse |
| Chelan County | 28 | 29 | 26 | 30 | Sierra Nevada and Cascades |
| Grant County | 28 | 34 | 17 | 30 | Columbia Plateau and Palouse |
| Spokane County | 28 | 35 | 18 | 30 | Columbia Plateau and Palouse |
| Kittitas County | 27 | 28 | 32 | 16 | Sierra Nevada and Cascades |
| Franklin County | 26 | 35 | 22 | 16 | Columbia Plateau and Palouse |
| Okanogan County | 26 | 30 | 17 | 30 | Columbia Plateau and Palouse |
| Yakima County | 26 | 27 | 21 | 30 | Sierra Nevada and Cascades |
| San Juan County | 24 | 27 | 25 | 16 | Sierra Nevada and Cascades |
| Mason County | 21 | 28 | 16 | 16 | Sierra Nevada and Cascades |
Values from USDA NRCS SSURGO where available, otherwise our regional model — each county page states which. Methodology.
The physiographic regions inside Washington
State lines are political; soil follows geology. These are the distinct ground types inside Washington and why each behaves the way it does.
Columbia Plateau and Palouse
Deep permeable loess on rolling hills, with basalt below. Water table deep, shrink-swell low. Loess is erodible and can settle under load when saturated, so a broken downspout does more damage here than the climate does.
16 counties: Adams County, Asotin County, Benton County, Columbia County, Douglas County, Ferry County, Franklin County, Garfield County, Grant County, Lincoln County, Okanogan County, Pend Oreille County, Spokane County, Stevens County, Walla Walla County, Whitman County
Puget Sound and Willamette Lowland
The one part of the West where basements are genuinely common — and the reason they leak is glacial: dense Vashon till under a permeable cap perches water just above it through the wet season. Volume change is low. Drainage, and hillside seepage, decide everything.
14 counties: Clark County, Cowlitz County, Island County, Jefferson County, King County, Kitsap County, Klickitat County, Lewis County, Pierce County, Skagit County, Skamania County, Snohomish County, Thurston County, Whatcom County
Sierra Nevada and Cascades
Coarse material over rock. Volume change is minimal and drainage is good; the seasonal problem is snowmelt arriving at foundations faster than it can be led away, not a water table.
5 counties: Chelan County, Kittitas County, Mason County, San Juan County, Yakima County
Pacific Coast Ranges
Some of the most expansive soils in North America sit here, on some of the least stable slopes. Winter rain saturates clay-rich colluvium and it moves — which is why hillside foundation work on the California and Oregon coast is engineered rather than standardised.
4 counties: Clallam County, Grays Harbor County, Pacific County, Wahkiakum County
Page data updated