California / Contra Costa County
Score 27 · moderate · #12 of 48 in CA
Crawl space moisture in Contra Costa County, CA
Contra Costa County scores moderate: no single wet-soil signal dominates its mapped acres. Where a seasonal water table is reported — 14% of acres — it averages 2.3 ft below grade.
- 1Water table 2.3 ft
- 2Wet soils 19%
- 3Moisture score 27
- 4EPA radon zone 2
Reading the Contra Costa County soil survey
The score of 27 combines four parts, each from the USDA survey: wet drainage classes contribute 22.8 points of 100 on their own scale, group D soils 59.2, a shallow water table 8.7, and flooding 5; the weights are on the methodology page. The dominant hydrologic group is C (48% of acres): slow infiltration (clay layers).
At 2.3 ft the seasonal water table is usually below a crawl-space floor, but the 14% 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.
Contra Costa County against the rest of California
Contra Costa County ranks 12 of 48 California counties, 9 points above the state median of 18. That puts it in the wettest quarter of the state. Statewide, 11% of mapped soil is in the wet drainage classes; here it is 19%, half again the state figure or more. After well drained, the next most common class is moderately well drained at 12%.
Flooding. 1.6% of acres with a flooding class are rated frequent; 14.7% 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 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 Contra Costa County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area CA013; drainage covers 85% of 510,201 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.