South Carolina / Charleston County
Score 64 · high · #3 of 46 in SC
Crawl space moisture in Charleston County, SC
77% of Charleston 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 — 88% of acres — it averages 1 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1 ft
- 2Wet soils 77%
- 3Moisture score 64
- 4EPA radon zone 3
Reading the Charleston County soil survey
The score of 64 combines four parts, each from the USDA survey: wet drainage classes contribute 78.7 points of 100 on their own scale, group D soils 43.5, a shallow water table 75, and flooding 31; the weights are on the methodology page. The dominant hydrologic group is A/D (37% of acres): D when undrained, A when drained.
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 Charleston County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Charleston County against the rest of South Carolina
Charleston County ranks 3 of 46 South Carolina counties, 39 points above the state median of 25. Only 2 other counties in the state score higher. Statewide, 34% of mapped soil is in the wet drainage classes; here it is 77%, half again the state figure or more. After very poorly drained, the next most common class is poorly drained at 21%.
Flooding. 5.7% of acres with a flooding class are rated frequent; 1.3% of acres with a flooding class are rated occasional; 8.1% 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 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 Charleston County by score
All 46 South Carolina counties
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area SC690; drainage covers 86% of 669,300 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.