Missouri / Clay County
Score 41 · elevated · #51 of 113 in MO
Crawl space moisture in Clay County, MO
Clay County scores elevated because several moderate signals stack up rather than one dominant one. Where a seasonal water table is reported — 79% of acres — it averages 2.7 ft below grade.
- 1Water table 2.7 ft
- 2Wet soils 31%
- 3Moisture score 41
- 4EPA radon zone 1
Reading the Clay County soil survey
The score of 41 combines four parts, each from the USDA survey: wet drainage classes contribute 40.6 points of 100 on their own scale, group D soils 48.2, a shallow water table 46.8, and flooding 12.3; the weights are on the methodology page. The dominant hydrologic group is C (38% of acres): slow infiltration (clay layers).
At 2.7 ft the seasonal water table is usually below a crawl-space floor, but the 79% of acres that report one are by definition the wettest parts of the county; on those lots, a wet spring can bring water close to the footings.
Clay County against the rest of Missouri
Clay County ranks 51 of 113 Missouri counties, 2 points above the state median of 39. Wetter counties exist in Missouri, but the soil here still sits well above the national middle. Statewide, 32% of mapped soil is in the wet drainage classes; here it is 31%, below the state figure — the score here comes from a shallow water table rather than drainage class alone. After moderately well drained, the next most common class is somewhat poorly drained at 24%.
Flooding. 1.4% of acres with a flooding class are rated frequent; 17.8% of acres with a flooding class are rated occasional; 1.1% of acres with a flooding class are rated rare. That is a large flood-prone share; a crawl space in the floodplain needs flood vents under most local floodplain ordinances, which is a separate rule from R408 and can rule out a fully sealed design.
What it means for a crawl space here
In an elevated county the soil supplies more moisture than average, but a vented crawl space can work if the ground is covered: the code lets vent area drop from 1 sq ft per 150 to 1 per 1500 with a Class I vapor retarder on the ground, which is an admission of how much moisture comes from below. Grading, gutters and a ground cover usually decide whether a crawl space here stays dry before any decision about sealing it.
Radon. Clay County is in EPA radon zone 1, the highest predicted potential. A sealed crawl space and a radon system share most of their parts — the liner, the sealed seams, a fan — so if you are encapsulating, test for radon first and design both together. EPA itself says the zone map is not a reason to skip testing in zones 2 and 3.
Near Clay County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area MO047; drainage covers 99% of 261,491 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.