States / California
48 counties · median score 18
Crawl space moisture in California
Most of California drains well: only 0 of 48 counties score high or elevated, and the median is 18. Sacramento County is the exception at 38; crawl-space moisture problems here usually come from grading, gutters and plumbing rather than the soil itself.
- 1Wettest county Sacramento County
- 2High or elevated 0 of 48
- 3Median score 18
- 4Driest county Trinity County
California's soil, taken as a whole
Across the 44,125,501 acres mapped in these 48 counties, 11% of soil with a drainage class is somewhat poorly, poorly or very poorly drained, and 13% of acres report a seasonal water table within about two metres. That is a dry figure; wet soils here are confined to river bottoms and wetlands. 45 of the 48 counties have mapped soils subject to occasional or frequent flooding.
| Drainage class | Share of mapped acres |
|---|---|
| Excessively drained | 2.2% |
| Somewhat excessively drained | 12.2% |
| Well drained | 65.7% |
| Moderately well drained | 9.4% |
| Somewhat poorly drained | 5.4% |
| Poorly drained | 3.9% |
| Very poorly drained | 1.2% |
Radon. By EPA zone, 2 California counties here are zone 1 (highest potential), 22 zone 2 and 24 zone 3. EPA says to test regardless of zone; a sealed crawl space is the cheapest moment to add radon venting if a test calls for it.
Every California county
Sorted wettest first. High and elevated counties link to their own page with the full soil breakdown.
What drives the California numbers
The wettest county, Sacramento County, is 43% moderately well drained by dominant condition, with a seasonal water table averaging 0.9 ft below grade across the 32% of acres that report one, and sits in EPA radon zone 3. The driest, Trinity County, is 82% well drained. No California county reaches the high tier.
The score is a guide to the soil, not to a house: a well-graded lot with working gutters in a high county can have a dry crawl space, and a bad downspout can flood one in a low county. The R408 calculator sizes venting or encapsulation for any footprint.
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted; EPA Map of Radon Zones, county table (June 2024 JSON). Checked 2026-10-23.