Oregon / Yamhill County
Score 23 · moderate · #8 of 34 in OR
Crawl space moisture in Yamhill County, OR
Yamhill County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 37% of acres — it averages 1.4 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.4 ft
- 2Wet soils 17%
- 3Moisture score 23
- 4EPA radon zone 2
Reading the Yamhill County soil survey
The score of 23 combines four parts, each from the USDA survey: wet drainage classes contribute 19.3 points of 100 on their own scale, group D soils 30.4, a shallow water table 28.9, and flooding 5.4; the weights are on the methodology page. The dominant hydrologic group is C (48% of acres): slow infiltration (clay layers).
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 on the soils that report a water table — about 37% of Yamhill County's acres. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Yamhill County against the rest of Oregon
Yamhill County ranks 8 of 34 Oregon counties, 5 points above the state median of 18. That puts it in the wettest quarter of the state. Statewide, 7% of mapped soil is in the wet drainage classes; here it is 17%, half again the state figure or more. After well drained, the next most common class is moderately well drained at 17%.
Flooding. 0.6% of acres with a flooding class are rated frequent; 8% of acres with a flooding class are rated occasional; 0% of acres with a flooding class are rated rare. 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 2.8% 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 2. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near Yamhill County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area OR071; drainage covers 99% of 459,840 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.