St. Mary's County, Maryland · Leonardtown
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St. Mary's County averages 9 nights a year at 20°F or colder, mostly December to February, and 10% of its homes predate 1960.
Basements flood here mostly in December. St. Mary's County averages 9.5 days a year with an inch of rain or more.
Backups often follow heavy rain, which St. Mary's County gets about 9.5 days a year, when storm water overloads sewer mains and pushes up through the lowest drain in the house.
Roof leaks here come from summer downpours and, with about 64 freeze-thaw days a year, from winter ice dams. Most show up as ceiling stains in the room below.
11% of occupied homes in St. Mary's County heat mainly with wood or propane, above the Maryland county median of 9.3%.
Muggy summer nights, lows of 65°F or more, come about 85 times a year in St. Mary's County. Mold after a leak spreads fastest from June through August.
Local risk profile
St. Mary's County covers 359 square miles of southeast Maryland, with about 115,126 residents and 46,385 homes. Leonardtown is the county seat; Lexington Park, California and Wildewood are the next largest communities. Much of its housing is recent, with a median build year of 1992.
37/100 Water Damage Risk Index: Moderate St. Mary's County ranks 21 of 23 Maryland counties. See the statewide ranking and method.The three factors where St. Mary's County ranks highest compared with other Maryland counties.
St. Mary's County is milder than most of Maryland, but not mild enough to skip winterizing. It averaged 68 freezing nights a year over 2015–2024, 9 of them at 20°F or colder, cold enough for pipes in outside walls and crawl spaces to freeze. Among 23 Maryland counties that's 21st.
Cold snaps are the real risk. The worst in the record lasted 7 days below freezing (ending January 2018), and the low bottomed out near 2°F on January 6, 2018.
Expect the hardest freezes in January, inside a season that runs December to February. That's when an outdoor spigot left connected to a hose, or a pipe in a crawl space, is most likely to split.
On average the first fall freeze arrives around Nov 16 and the last spring freeze around Mar 28.
Heavy rain is a bigger factor here than in most of Maryland. The county gets about 45.9 inches of rain and melted snow a year, with 9.5 days of an inch or more (6th of 23). Those downpours cluster in July, October and December, when sump pumps and gutters are tested hardest. The wettest day on record brought 4.32 inches on October 12, 2018. Multi-day storms matter most for basements; the wettest three days on record here totaled 6.6 inches (through July 24, 2018).
About 64 days a year swing from below freezing at night to above freezing by day. Those freeze-thaw cycles feed ice dams on roofs, open cracks in foundations and driveways, and stress pipes that are already marginal.
Housing
30% of homes in St. Mary's County predate 1980, below the Maryland county median of 43.9%, which ranks 21st of Maryland's 23 counties.
Since 2000 the county has added enough housing that 37% of homes are that new. Modern PEX and copper rarely fail on their own; the leaks come from connections, appliances and storms.
34% date from 1980 to 1999, the window when polybutylene, a gray flexible plastic supply pipe, was used in some homes. It's prone to leaks at the fittings, so it's worth checking for in a house from those years.
Beyond water
St. Mary's County averages 85 warm, muggy nights a year (lows of 65°F or more) and gets the most rain in July. Mold grows when indoor humidity stays above about 60%, the EPA's ceiling. A basement dehumidifier set to 50% and drying anything wet within 24 to 48 hours prevents most problems.
Mold remediation: what to expectBy main heating fuel, St. Mary's County homes break down as 12% utility gas, 62% electric, 8% propane and 3% wood. Most homes here heat with gas or electricity, so cooking and electrical faults are the likelier fire sources. Even a contained kitchen fire can send soot through the ductwork to every room.
Fire and smoke cleanup: first stepsCoverage
Calls connect to restoration companies that work across St. Mary's County. The largest communities by population:
Population from ACS 2020-2024 5-year estimates. Townships and unincorporated areas are covered too.
Nearby
Calvert, Charles and Dorchester get more pipe-freeze nights than St. Mary's. Dorchester County has the oldest housing nearby, with 57% of homes built before 1980.
By the numbers
| Factor | St. Mary's | Maryland median | Rank |
|---|---|---|---|
| Homes built before 1960 | 10% | 20% | 20 of 23 |
| Homes built before 1980 | 30% | 44% | 21 of 23 |
| Homes built 2000 or later | 37% | 23% | 1 of 23 |
| Manufactured or mobile homes | 3.4% | 2.6% | 9 of 23 |
| Vacant homes | 8.5% | 7.7% | 9 of 23 |
| Wood or propane heat | 11% | 9.3% | 10 of 23 |
| Nights at 20°F or colder | 9/yr | 12/yr | 21 of 23 |
| Days with 1 inch+ of rain | 9.5/yr | 9.3/yr | 6 of 23 |
| Freeze-thaw days | 64/yr | 70/yr | 17 of 23 |
| Muggy nights (65°F+) | 85/yr | 76/yr | 2 of 23 |
Rank 1 is the highest value. Bars run from the lowest to the highest Maryland county; the tick marks the median; orange means higher water damage risk than the median, green means lower.
Questions
Peak risk months in St. Mary's County: freezing from December to February, heavy rain in July, October and December.
Sources: U.S. Census Bureau, ACS 2024 5-year estimates (2020-2024); weather from NOAA nClimGrid-Daily county averages (EpiNOAA), 2015–2024, averaged across the county; county boundaries from Census TIGER. Reviewed 2026-10-21. Methodology