Louisiana / St. Mary Parish
Score 85 · high · #2 of 64 in LA
Crawl space moisture in St. Mary Parish, LA
100% of St. Mary 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 — 84% of acres — it averages 0.4 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 0.4 ft
- 2Wet soils 100%
- 3Moisture score 85
- 4EPA radon zone 3
Reading the St. Mary Parish soil survey
The score of 85 combines four parts, each from the USDA survey: wet drainage classes contribute 96.6 points of 100 on their own scale, group D soils 79.3, a shallow water table 79, and flooding 70.2; the weights are on the methodology page. The dominant hydrologic group is D (57% of acres): very slow infiltration, high runoff (clays, high water table).
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. Mary Parish. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
St. Mary Parish against the rest of Louisiana
St. Mary Parish ranks 2 of 64 Louisiana counties, 23 points above the state median of 62. Only 1 other county in the state score higher. Statewide, 64% of mapped soil is in the wet drainage classes; here it is 100%, half again the state figure or more. After very poorly drained, the next most common class is poorly drained at 30%.
Flooding. 32.6% of acres with a flooding class are rated frequent; 5.3% of acres with a flooding class are rated occasional; 27.6% 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 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 St. Mary Parish by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area LA101; drainage covers 90% of 442,400 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.