St. Mary's County, Maryland
A shallow water table limits basements on 47% of St. Mary's County's mapped land, a higher share than 64% of US counties. Shrink-swell clay limits houses without basements on 1%.
None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 82 flood losses here since 2016, 16% of them in buildings with basements. 30% of homes were built before 1980, against 50% nationally; the median home dates from 1992.
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. Mary's County
| Expansive clay (limits slab homes) | 0.7% |
| High water table (limits basements) | 47.1% |
| Septic fields very limited | 80.2% |
| Clay swell (LEP, by area) | 1.7% |
| High-swell land (LEP 6%+) | 0.0% |
| Mapped soil | 262,600 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Beltsville | 13% | n/a |
| Othello | 7% | n/a |
| Sassafras | 7% | n/a |
| Evesboro | 6% | n/a |
| Mattapex | 5% | n/a |
| Croom | 4% | n/a |
How we count · survey areas MD037 (2025-09-09)
What the numbers mean here
On 47% of St. Mary's 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: 34.38 in, 10.88 in below the 1991-2020 average year of 45.26 in.
USDA rates 80% 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. Part of the county was in drought in 34 of the last 52 weeks; the most was 100% of its area, in 30 weeks, most recently on the July 28, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 3.3% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
82 flood losses since 2016, $1.1 million paid. The worst year was 2021, with 27. Foundations of the buildings, where recorded: 16% basements, 13% slabs, 42% crawl spaces, 29% elevated. Median water depth outside the building: 11 in.
| Year | Flood losses |
|---|---|
| 2016 | 2 |
| 2017 | 3 |
| 2018 | 11 |
| 2019 | 2 |
| 2020 | 27 |
| 2021 | 27 |
| 2022 | 3 |
| 2023 | 3 |
| 2024 | 3 |
| 2025 | 1 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 45.26 in |
| 12 months to August 2026 | 34.38 in (-10.88) |
| 2025 | 41.35 in (-3.91) |
| Wettest year since 1895 | 2018: 68.16 in |
| Driest year since 1895 | 1930: 23.31 in |
| Days with 1 inch or more, normal at PATUXENT RIVER NAS (8 mi away) | 12 |
Homes
| Housing units | 46,385 |
| Median year built | 1992 |
| Built before 1980 | 30% |
| Built 2000 or later | 37% |
| Owner-occupied | 73% |
| Median value, owner-occupied | $407,600 |
| Single-family detached | 73% |
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 |
|---|---|---|---|
| Westmoreland County, VA | 17 mi | 8% | 38% |
| Calvert County, MD | 20 mi | under 1% | 56% |
| Richmond County, VA | 22 mi | 6% | 42% |
| Northumberland County, VA | 26 mi | 1% | 23% |
| Essex County, VA | 29 mi | 1% | 45% |
| Dorchester County, MD | 30 mi | 0% | 69% |
| Charles County, MD | 31 mi | 4% | 61% |
| King George County, VA | 34 mi | 0% | 37% |
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