Mississippi / Coahoma County
Score 68 · high · #9 of 82 in MS
Crawl space moisture in Coahoma County, MS
88% of Coahoma County's mapped soil is somewhat poorly to very poorly drained, which means ground that stays wet near the surface for significant periods. Where a seasonal water table is reported — 81% 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 88%
- 3Moisture score 68
- 4EPA radon zone 3
Reading the Coahoma County soil survey
The score of 68 combines four parts, each from the USDA survey: wet drainage classes contribute 75.4 points of 100 on their own scale, group D soils 71.3, a shallow water table 64, and flooding 38.2; the weights are on the methodology page. The dominant hydrologic group is D (64% of acres): very slow infiltration, high runoff (clays, high water table).
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 Coahoma County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Coahoma County against the rest of Mississippi
Coahoma County ranks 9 of 82 Mississippi counties, 33 points above the state median of 35. That puts it in the wettest quarter of the state. Statewide, 35% of mapped soil is in the wet drainage classes; here it is 88%, half again the state figure or more. After somewhat poorly drained, the next most common class is poorly drained at 35%.
Flooding. 22.5% of acres with a flooding class are rated frequent; 5.2% of acres with a flooding class are rated occasional; 62.9% of acres with a flooding class are rated rare; 0.3% of acres with a flooding class are rated very 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 a high county, the model-code default of a vented crawl space over bare earth works against the soil: outside air brings humidity in all summer, and the ground underneath keeps supplying moisture from below. The R408 alternative — an unvented crawl space with a sealed vapor retarder lapped 6 inches and turned 6 inches up the stem wall, plus conditioned or exhaust air at 1 cfm per 50 sq ft or a dehumidifier — is built for exactly this. If the water table is shallow, drainage comes 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 Coahoma County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area MS027; drainage covers 91% of 373,400 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.