Louisiana / St. Tammany Parish
Score 58 · elevated · #38 of 64 in LA
Crawl space moisture in St. Tammany Parish, LA
67% of St. Tammany Parish'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 — 71% of acres — it averages 0.9 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 0.9 ft
- 2Wet soils 67%
- 3Moisture score 58
- 4EPA radon zone 3
Reading the St. Tammany Parish soil survey
The score of 58 combines four parts, each from the USDA survey: wet drainage classes contribute 64.8 points of 100 on their own scale, group D soils 52.2, a shallow water table 60.7, and flooding 37.6; the weights are on the methodology page. The dominant hydrologic group is C/D (45% of acres): D when undrained, C 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 St. Tammany Parish. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
St. Tammany Parish against the rest of Louisiana
St. Tammany Parish ranks 38 of 64 Louisiana counties, 4 points below the state median of 62. Wetter counties exist in Louisiana, but the soil here still sits well above the national middle. Statewide, 64% of mapped soil is in the wet drainage classes; here it is 67%. After somewhat poorly drained, the next most common class is poorly drained at 26%.
Flooding. 30.3% of acres with a flooding class are rated frequent; 2% of acres with a flooding class are rated occasional; 5.4% of acres with a flooding class are rated rare; 1.4% 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 St. Tammany Parish by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area LA103; drainage covers 75% of 721,800 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.