Expansive Soil in Washington: County Ratings
None of the 35 rated counties in Washington have a dominant shrink-swell rating of High or Very High. Each rating below is the NRCS shrink-swell class covering the largest share of the county's mapped soil acres, computed from USDA SSURGO data. Open a county for the full class breakdown and what it means for a slab foundation.
| County | Dominant class | High + Very High share | Survey coverage |
|---|---|---|---|
| Adams County | Low | 0% | 99% |
| Asotin County | Low | 31% | 99% |
| Benton County | Low | 0% | 97% |
| Chelan County | Low | 0% | 89% |
| Clallam County | Low | 0% | 99% |
| Clark County | Low | 3% | 98% |
| Columbia County | Low | 0% | 96% |
| Cowlitz County | Low | 5% | 98% |
| Ferry County | Low | 0% | 96% |
| Franklin County | Low | 0% | 98% |
| Garfield County | Low | 15% | 99% |
| Grant County | Low | 0% | 96% |
| Grays Harbor County | Low | 6% | 98% |
| Jefferson County | Low | 1% | 98% |
| King County | Low | 0% | 96% |
| Kitsap County | Low | 0% | 97% |
| Kittitas County | Low | 7% | 94% |
| Klickitat County | Low | 2% | 97% |
| Lewis County | Low | 0% | 100% |
| Lincoln County | Low | 1% | 99% |
| Mason County | Low | 0% | 97% |
| Okanogan County | Low | 0% | 96% |
| Pacific County | Low | 4% | 92% |
| Pend Oreille County | Low | 0% | 97% |
| Pierce County | Low | 3% | 92% |
| Skagit County | Low | 0% | 97% |
| Skamania County | Low | 4% | 95% |
| Snohomish County | Low | 1% | 95% |
| Stevens County | Low | 1% | 98% |
| Thurston County | Low | 3% | 96% |
| Wahkiakum County | Low | 4% | 91% |
| Walla Walla County | Low | 0% | 97% |
| Whatcom County | Low | 1% | 96% |
| Whitman County | Low | 2% | 99% |
| Yakima County | Low | 2% | 97% |
How these ratings are computed
Ratings come from USDA NRCS SSURGO soil survey data: for each soil component we take the maximum linear extensibility percent (lep_r) in the top 100 cm, apply the NRCS Handbook Part 618 class limits (Low under 3 percent, Moderate 3 to 6, High 6 to 9, Very High 9 and above), assign map units by plurality of component percent, and roll acres up to the county. Full details on the methodology section of the lookup page. A county rating is not a parcel-level geotechnical assessment.