California / Alameda County
Score 26 · moderate · #13 of 48 in CA
Crawl space moisture in Alameda County, CA
54% of Alameda 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 — 15% of acres — it averages 2.9 ft below grade.
- 1Water table 2.9 ft
- 2Wet soils 16%
- 3Moisture score 26
- 4EPA radon zone 2
Reading the Alameda County soil survey
The score of 26 combines four parts, each from the USDA survey: wet drainage classes contribute 16.6 points of 100 on their own scale, group D soils 65.1, a shallow water table 8.4, and flooding 7.8; the weights are on the methodology page. The dominant hydrologic group is D (53% of acres): very slow infiltration, high runoff (clays, high water table).
At 2.9 ft the seasonal water table is usually below a crawl-space floor, but the 15% 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.
Alameda County against the rest of California
Alameda County ranks 13 of 48 California counties, 8 points above the state median of 18. That places it in the middle range, nationally and for the state. Statewide, 11% of mapped soil is in the wet drainage classes; here it is 16%. After well drained, the next most common class is poorly drained at 9%.
Flooding. 6.1% of acres with a flooding class are rated frequent; 0.8% of acres with a flooding class are rated occasional; 6.6% of acres with a flooding class are rated rare. Flood-prone soils are a minority here, concentrated along streams; the score mostly reflects what happens between floods.
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 2. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near Alameda County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey areas CA609, CA610; drainage covers 80% of 531,530 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.