Score 41 · elevated · #17 of 46 in SC

Crawl space moisture in Marlboro County, SC

Marlboro County scores elevated because several moderate signals stack up rather than one dominant one. Where a seasonal water table is reported — 83% of acres — it averages 1.8 ft below grade, shallow enough to reach a typical crawl-space footing.

Size venting or encapsulation

1Water table 1.8 ft
2Wet soils 38%
3Moisture score 41
4EPA radon zone 3

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

Dominant drainage

Well drained

38% of acres

Group D soils

43%

slow infiltration

Flood-prone acres

20%

any flooding frequency

Ponding

acres where water ponds

Reading the Marlboro County soil survey

The score of 41 combines four parts, each from the USDA survey: wet drainage classes contribute 40.2 points of 100 on their own scale, group D soils 33, a shallow water table 60.6, and flooding 17.8; the weights are on the methodology page. The dominant hydrologic group is C/D (27% of acres): D when undrained, C when drained.

Hydrologic soil groups in Marlboro 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 Marlboro County. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.

Marlboro County against the rest of South Carolina

Marlboro County ranks 17 of 46 South Carolina counties, 16 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 38%. After well drained, the next most common class is poorly drained at 25%.

Flooding. 14.8% of acres with a flooding class are rated frequent; 4.9% of acres with a flooding class are rated occasional; 0.6% 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 3. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.

Near Marlboro County by score

Orangeburg County 41 Sumter County 42 Bamberg County 43 Allendale County 38

All 46 South Carolina counties

Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area SC069; drainage covers 99% of 310,400 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.