Missouri / Mercer County
Score 41 · elevated · #53 of 113 in MO
Crawl space moisture in Mercer County, MO
53% of Mercer 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 — 61% of acres — it averages 1.6 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.6 ft
- 2Wet soils 34%
- 3Moisture score 41
- 4EPA radon zone 2
Reading the Mercer County soil survey
The score of 41 combines four parts, each from the USDA survey: wet drainage classes contribute 35.4 points of 100 on their own scale, group D soils 53.8, a shallow water table 46.5, and flooding 12.7; the weights are on the methodology page. The dominant hydrologic group is C (40% 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 across much of Mercer County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Mercer County against the rest of Missouri
Mercer County ranks 53 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 34%. After well drained, the next most common class is moderately well drained at 31%.
Flooding. 4.4% of acres with a flooding class are rated frequent; 13.4% of acres with a flooding class are rated occasional; 0.8% 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 Mercer County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area MO129; drainage covers 100% of 291,488 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.