Missouri / St. Charles County
Score 47 · elevated · #42 of 113 in MO
Crawl space moisture in St. Charles County, MO
53% of St. Charles County's soil falls in hydrologic group D or a dual D group: clays and high water tables that shed water slowly. Where a seasonal water table is reported — 73% of acres — it averages 2 ft below grade.
- 1Water table 2 ft
- 2Wet soils 41%
- 3Moisture score 47
- 4EPA radon zone 2
Reading the St. Charles County soil survey
The score of 47 combines four parts, each from the USDA survey: wet drainage classes contribute 46.1 points of 100 on their own scale, group D soils 53.9, a shallow water table 50.3, and flooding 28.6; the weights are on the methodology page. The dominant hydrologic group is D (32% of acres): very slow infiltration, high runoff (clays, high water table).
At 2 ft the seasonal water table is usually below a crawl-space floor, but the 73% 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.
St. Charles County against the rest of Missouri
St. Charles County ranks 42 of 113 Missouri counties, 8 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 41%. After moderately well drained, the next most common class is well drained at 27%.
Flooding. 10.8% of acres with a flooding class are rated frequent; 28.2% of acres with a flooding class are rated occasional; 4.8% 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. 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 St. Charles County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area MO183; drainage covers 98% of 380,020 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.