Washington / King County
Score 30 · moderate · #5 of 36 in WA
Crawl space moisture in King County, WA
King County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 75% of acres — it averages 1.9 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.9 ft
- 2Wet soils 13%
- 3Moisture score 30
- 4EPA radon zone 3
Reading the King County soil survey
The score of 30 combines four parts, each from the USDA survey: wet drainage classes contribute 33.1 points of 100 on their own scale, group D soils 12.9, a shallow water table 53, and flooding 7; the weights are on the methodology page. The dominant hydrologic group is B (53% of acres): moderate infiltration.
A water table under two feet is the single most important number here. A crawl-space floor is usually 18 to 36 inches below the joists and often below outside grade, so in a wet season groundwater can stand at or above the crawl-space floor across much of King County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
King County against the rest of Washington
King County ranks 5 of 36 Washington counties, 18 points above the state median of 12. That puts it in the wettest quarter of the state. Statewide, 6% of mapped soil is in the wet drainage classes; here it is 13%, half again the state figure or more. After moderately well drained, the next most common class is somewhat excessively drained at 16%.
Flooding. 2.6% of acres with a flooding class are rated frequent; 6.6% of acres with a flooding class are rated occasional. Flood-prone soils are a minority here, concentrated along streams; the score mostly reflects what happens between floods. Ponding — water standing at the surface with nowhere to go — affects 1.3% of acres.
What it means for a crawl space here
In a moderate county the soil record is mixed rather than uniformly wet, so conditions vary from lot to lot. Where the lot drains, the model-code default — a vented crawl space with 1 sq ft of net vent per 150 sq ft of floor, or 1 per 1500 with a Class I vapor retarder on the ground — is a reasonable starting point; where the sections above point to a shallow water table or flood-prone soil on your lot, plan as for an elevated county. Either way, check the site first: grading that falls away from the foundation, gutters and downspouts that discharge well clear of it, and a ground cover over bare earth. A wet crawl space in a moderate county points to one of those first.
Radon. EPA radon zone 3. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near King County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area WA633; drainage covers 93% of 414,219 mapped acres); EPA Map of Radon Zones, county table (June 2024 JSON); IRC R408.3 Unvented crawl space, as adopted in S.C. Code Regs. 8-1221 (LII). Checked 2026-10-23.