Score 36 · moderate · #36 of 100 in NC

Crawl space moisture in Johnston County, NC

Johnston County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 80% of acres — it averages 2.2 ft below grade.

Size venting or encapsulation

1Water table 2.2 ft
2Wet soils 31%
3Moisture score 36
4EPA radon zone 3

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

Dominant drainage

Well drained

49% of acres

Group D soils

31%

slow infiltration

Flood-prone acres

19%

any flooding frequency

Ponding

1.7%

acres where water ponds

Reading the Johnston County soil survey

The score of 36 combines four parts, each from the USDA survey: wet drainage classes contribute 36 points of 100 on their own scale, group D soils 25.5, a shallow water table 53.1, and flooding 16.4; the weights are on the methodology page. The dominant hydrologic group is B (43% of acres): moderate infiltration.

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

At 2.2 ft the seasonal water table is usually below a crawl-space floor, but the 80% of acres that report one are by definition the wettest parts of the county; on those lots, a wet spring can bring water close to the footings.

Johnston County against the rest of North Carolina

Johnston County ranks 36 of 100 North Carolina counties, 19 points above the state median of 17. That places it in the middle range, nationally and for the state. Statewide, 30% of mapped soil is in the wet drainage classes; here it is 31%. After well drained, the next most common class is poorly drained at 25%.

Flooding. 13.4% of acres with a flooding class are rated frequent; 4.5% of acres with a flooding class are rated occasional; 1.4% 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. Ponding — water standing at the surface with nowhere to go — affects 1.7% of acres.

What it means for a crawl space here

In a moderate county the soil record is mixed rather than uniformly wet, so conditions vary from lot to lot. Where the lot drains, the model-code default — a vented crawl space with 1 sq ft of net vent per 150 sq ft of floor, or 1 per 1500 with a Class I vapor retarder on the ground — is a reasonable starting point; where the sections above point to a shallow water table or flood-prone soil on your lot, plan as for an elevated county. Either way, check the site first: grading that falls away from the foundation, gutters and downspouts that discharge well clear of it, and a ground cover over bare earth. A wet crawl space in a moderate county points to one of those 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 Johnston County by score

Durham County 37 Greene County 37 Cumberland County 34 Halifax County 34

All 100 North Carolina counties

Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area NC101; drainage covers 99% of 510,137 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.