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San Joaquin County, California
Shrink-swell clay limits houses without basements on 30% of San Joaquin County's mapped land, a higher share than 80% of US counties and the 10th highest of 54 California counties. A shallow water table limits basements on 23%.
None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 109 flood losses here since 2016, 8% of them in buildings with basements. 44% of homes were built before 1980, against 50% nationally; the median home dates from 1984.
Data as of October 13, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, October 13, 2026
of homes built before 1980
Soil under San Joaquin County
| Expansive clay (limits slab homes) | 30.3% |
| High water table (limits basements) | 23.3% |
| Septic fields very limited | 76.8% |
| Clay swell (LEP, by area) | 5.1% |
| High-swell land (LEP 6%+) | 34.3% |
| Urban land, not rated | 1.8% |
| Mapped soil | 911,788 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Capay | 4% | n/a |
| Rindge | 4% | n/a |
| San Joaquin | 3% | n/a |
| Tokay | 3% | n/a |
| Redding | 3% | n/a |
| Jacktone | 3% | n/a |
How we count · survey areas CA077 (2025-09-09)
What the numbers mean here
Clay swell averages a linear extensibility of 5.1% by land area here, and 34% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 30% of the mapped land.
On 23% of San Joaquin County's mapped land, soil stays saturated within about 6 feet of the surface for part of the year, which is basement depth.
Rain over the 12 months to August 2026: 16.37 in, 1.78 in above the 1991-2020 average year of 14.59 in.
USDA rates 77% of the land very limited for septic absorption fields.
Drought
0% of the county is in drought (D1 or worse) on the October 13, 2026 map. No part of the county was in drought in the last 52 weeks.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 13.6% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
109 flood losses since 2016, $2.8 million paid. The worst year was 2023, with 47. Foundations of the buildings, where recorded: 8% basements, 48% slabs, 38% crawl spaces, 6% elevated. Median water depth outside the building: 9.5 in.
| Year | Flood losses |
|---|---|
| 2016 | 4 |
| 2017 | 30 |
| 2018 | 0 |
| 2019 | 5 |
| 2020 | 0 |
| 2021 | 3 |
| 2022 | 15 |
| 2023 | 47 |
| 2024 | 3 |
| 2025 | 1 |
| 2026 to date | 1 |
Rain
| Average year, 1991-2020 | 14.59 in |
| 12 months to August 2026 | 16.37 in (+1.78) |
| 2025 | 13.26 in (-1.33) |
| Wettest year since 1895 | 1983: 29.05 in |
| Driest year since 1895 | 2013: 5.06 in |
| Days with 1 inch or more, normal at STOCKTON METRO AP (4 mi away) | 1.4 |
Homes
| Housing units | 260,151 |
| Median year built | 1984 |
| Built before 1980 | 44% |
| Built 2000 or later | 29% |
| Owner-occupied | 62% |
| Median value, owner-occupied | $530,700 |
| Single-family detached | 75% |
What repairs cost
Published ranges from cost guides, not quotes. More sources, and where they disagree.
Nearby counties
| Place | Distance | Expansive clay | High water table |
|---|---|---|---|
| Stanislaus County, CA | 30 mi | 24% | 7% |
| Sacramento County, CA | 36 mi | 16% | 26% |
| Contra Costa County, CA | 37 mi | 48% | 11% |
| Alameda County, CA | 40 mi | 38% | 8% |
| Calaveras County, CA | 43 mi | 2% | 2% |
| Solano County, CA | 43 mi | 50% | 21% |
| Amador County, CA | 49 mi | 1% | 3% |
| Santa Clara County, CA | 54 mi | 24% | 3% |
Sources: USDA SSURGO (September 9, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status