St. Louis County, Missouri
A shallow water table limits basements on 37% of St. Louis County's mapped land, a higher share than 56% of US counties. Shrink-swell clay limits houses without basements on 5%.
9% of the county is in drought on the October 13, 2026 Drought Monitor map, up from 0.6% a week earlier. FEMA's flood data list 1,291 flood losses here since 2016, 55% of them in buildings with basements. 69% of homes were built before 1980, against 50% nationally; the median home dates from 1969.
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 St. Louis County
| Expansive clay (limits slab homes) | 5.5% |
| High water table (limits basements) | 37.1% |
| Septic fields very limited | 60.1% |
| Clay swell (LEP, by area) | 4.4% |
| High-swell land (LEP 6%+) | 6.1% |
| Urban land, not rated | 32.9% |
| Mapped soil | 377,747 acres |
How we count · survey areas MO189 (2025-09-02)
What the numbers mean here
On 37% of St. Louis 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: 37.98 in, 4.34 in below the 1991-2020 average year of 42.32 in.
USDA rates 60% of the land very limited for septic absorption fields.
Drought
9% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 20 of the last 52 weeks; the most was 100% of its area, in 9 weeks, most recently on the February 10, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 9% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
1,291 flood losses since 2016, $82.1 million paid. The worst year was 2022, with 592. Foundations of the buildings, where recorded: 55% basements, 36% slabs, 2% crawl spaces, 7% elevated. Median water depth outside the building: 25 in.
| Year | Flood losses |
|---|---|
| 2016 | 63 |
| 2017 | 207 |
| 2018 | 9 |
| 2019 | 108 |
| 2020 | 112 |
| 2021 | 6 |
| 2022 | 592 |
| 2023 | 17 |
| 2024 | 152 |
| 2025 | 19 |
| 2026 to date | 6 |
Rain
| Average year, 1991-2020 | 42.32 in |
| 12 months to August 2026 | 37.98 in (-4.34) |
| 2025 | 37.61 in (-4.71) |
| Wettest year since 1895 | 2015: 57.01 in |
| Driest year since 1895 | 1953: 23.65 in |
| Days with 1 inch or more, normal at ST CHARLES 7 SSW (5 mi away) | 11.8 |
Homes
| Housing units | 446,192 |
| Median year built | 1969 |
| Built before 1980 | 69% |
| Built 2000 or later | 11% |
| Owner-occupied | 69% |
| Median value, owner-occupied | $276,800 |
| Single-family detached | 72% |
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 |
|---|---|---|---|
| St. Louis city, MO | 11 mi | 1% | 32% |
| St. Charles County, MO | 16 mi | 41% | 62% |
| Jefferson County, MO | 27 mi | 37% | 34% |
| Monroe County, IL | 29 mi | 20% | 59% |
| St. Clair County, IL | 30 mi | 28% | 73% |
| Jersey County, IL | 31 mi | 22% | 53% |
| Madison County, IL | 32 mi | 33% | 73% |
| Franklin County, MO | 37 mi | 14% | 61% |
Sources: USDA SSURGO (September 2, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status