South Carolina / Sumter County
Score 42 · elevated · #16 of 46 in SC
Crawl space moisture in Sumter County, SC
53% of Sumter 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 — 78% of acres — it averages 1.5 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 1.5 ft
- 2Wet soils 46%
- 3Moisture score 42
- 4EPA radon zone 3
Reading the Sumter County soil survey
The score of 42 combines four parts, each from the USDA survey: wet drainage classes contribute 41.1 points of 100 on their own scale, group D soils 33.8, a shallow water table 59.7, and flooding 19.7; the weights are on the methodology page. The dominant hydrologic group is B/D (30% of acres): D when undrained, B 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 Sumter County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Sumter County against the rest of South Carolina
Sumter County ranks 16 of 46 South Carolina counties, 17 points above the state median of 25. Wetter counties exist in South Carolina, but the soil here still sits well above the national middle. Statewide, 34% of mapped soil is in the wet drainage classes; here it is 46%. After well drained, the next most common class is somewhat poorly drained at 20%.
Flooding. 19.4% of acres with a flooding class are rated frequent; 1% of acres with a flooding class are rated rare; 0.2% 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 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 3. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near Sumter 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 SC085; drainage covers 96% of 436,800 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.