Texas / Brazoria County
Score 56 · elevated · #7 of 209 in TX
Crawl space moisture in Brazoria County, TX
91% of Brazoria County's soil falls in hydrologic group D or a dual D group: clays and high water tables that shed water slowly. Where a seasonal water table is reported — 43% of acres — it averages 0.5 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 0.5 ft
- 2Wet soils 44%
- 3Moisture score 56
- 4EPA radon zone 3
Reading the Brazoria County soil survey
The score of 56 combines four parts, each from the USDA survey: wet drainage classes contribute 52.2 points of 100 on their own scale, group D soils 90.4, a shallow water table 39.5, and flooding 21.6; the weights are on the methodology page. The dominant hydrologic group is D (88% 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 on the soils that report a water table — about 43% of Brazoria County's acres. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
Brazoria County against the rest of Texas
Brazoria County ranks 7 of 209 Texas counties, 38 points above the state median of 18. That puts it in the wettest quarter of the state. Statewide, 6% of mapped soil is in the wet drainage classes; here it is 44%, half again the state figure or more. After moderately well drained, the next most common class is somewhat poorly drained at 23%.
Flooding. 3.3% of acres with a flooding class are rated frequent; 6.3% of acres with a flooding class are rated occasional; 33.2% 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 24.7% of acres.
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 Brazoria County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area TX039; drainage covers 96% of 951,468 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.