Score 65 · high · #2 of 46 in SC

Crawl space moisture in Colleton County, SC

78% of Colleton 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 — 98% of acres — it averages 1.1 ft below grade, shallow enough to reach a typical crawl-space footing.

Size venting or encapsulation

1Water table 1.1 ft
2Wet soils 78%
3Moisture score 65
4EPA radon zone 3

Share of mapped soil acres in Colleton County, SC by NRCS drainage class (dominant condition), bars to scale; wetter classes highlighted. Covers 99% of mapped acres.

Dominant drainage

Poorly drained

30% of acres

Group D soils

76%

slow infiltration

Flood-prone acres

18%

any flooding frequency

Ponding

0.1%

acres where water ponds

Reading the Colleton County soil survey

The score of 65 combines four parts, each from the USDA survey: wet drainage classes contribute 75.4 points of 100 on their own scale, group D soils 50.7, a shallow water table 81.5, and flooding 14.2; the weights are on the methodology page. The dominant hydrologic group is B/D (36% of acres): D when undrained, B when drained.

Hydrologic soil groups in Colleton County by share of acres, columns to scale. Group D and dual groups (x/D) shed water slowly and hold it near the surface.

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 Colleton County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.

Colleton County against the rest of South Carolina

Colleton County ranks 2 of 46 South Carolina counties, 40 points above the state median of 25. Only 1 other county in the state score higher. Statewide, 34% of mapped soil is in the wet drainage classes; here it is 78%, half again the state figure or more. After poorly drained, the next most common class is very poorly drained at 25%.

Flooding. 7.3% of acres with a flooding class are rated frequent; 4.5% 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 Colleton County by score

Charleston County 64 Jasper County 64 Beaufort County 67 Dorchester County 63

All 46 South Carolina counties

Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area SC029; drainage covers 99% of 671,000 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.