Texas / Williamson County
Score 28 · moderate · #44 of 209 in TX
Crawl space moisture in Williamson County, TX
90% of Williamson 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 — 1% of acres — it averages 2.5 ft below grade.
- 1Water table 2.5 ft
- 2Wet soils 0%
- 3Moisture score 28
- 4EPA radon zone 3
Reading the Williamson County soil survey
The score of 28 combines four parts, each from the USDA survey: wet drainage classes contribute 11.5 points of 100 on their own scale, group D soils 91.5, a shallow water table 0.9, and flooding 5.7; the weights are on the methodology page. The dominant hydrologic group is D (90% of acres): very slow infiltration, high runoff (clays, high water table).
At 2.5 ft the seasonal water table is usually below a crawl-space floor, but the 1% 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.
Williamson County against the rest of Texas
Williamson County ranks 44 of 209 Texas counties, 10 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 0%, below the state figure — the score here comes from heavy group D soils rather than drainage class alone. After well drained, the next most common class is moderately well drained at 35%.
Flooding. 4.3% of acres with a flooding class are rated frequent; 2.3% of acres with a flooding class are rated occasional. Flood-prone soils are a minority here, concentrated along streams; the score mostly reflects what happens between floods. Ponding — water standing at the surface with nowhere to go — affects 2.4% 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 Williamson County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area TX491; drainage covers 99% of 727,520 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.