St. Louis County, Missouri · Clayton
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With 34% of homes built before 1960 and about 24 hard-freeze nights a year, frozen and split pipes are a leading winter risk in the county.
Heavy rain days of an inch or more come about 9.9 times a year here, most often in April. A sump pump that hasn't run since last season is a common weak point.
Backups often follow heavy rain, which St. Louis County gets about 9.9 days a year, when storm water overloads sewer mains and pushes up through the lowest drain in the house.
In St. Louis County, 4.73 inches fell on the wettest day of the 2015–2024 record. Storms like that, and ice dams in snowy winters, cause most roof leaks.
2% of occupied homes in St. Louis County heat mainly with wood or propane, below the Missouri county median of 24.5%.
With 82 muggy nights a year, mold in St. Louis County can appear within days of a leak. Dry first, then clean.
Local risk profile
With 446,192 homes and around 995,569 residents, St. Louis County is 508 square miles of eastern Missouri. Clayton is the county seat; Florissant, Chesterfield and Oakville are the next largest communities. Half the homes here were built before 1969, half after.
59/100 Water Damage Risk Index: Elevated St. Louis County ranks 27 of 114 Missouri counties. See the statewide ranking and method.The three factors where St. Louis County ranks highest compared with other Missouri counties.
Pipe-freeze risk in St. Louis County sits near the middle of the state. From 2015–2024 it averaged 24 nights a year at 20°F or colder, the range where pipes in exterior walls, crawl spaces and unheated garages freeze, and 82 nights at or below 32°F. That ranks 70th of 114 counties.
The longest cold spell was straight 14 days without the temperature rising above freezing, ending in January 2018. The coldest night reached about -6°F on January 15, 2024. Burst pipes usually show up when a spell like that breaks and the ice thaws.
The freeze season here runs December to February, with January the coldest. If you travel then, leave the thermostat at 55°F or above and open sink cabinets on outside walls.
Freezing nights typically start around Nov 3 and end around Apr 4.
Rainfall is about average for Missouri. The county gets about 43.9 inches of rain and melted snow a year, with 9.9 days of an inch or more (63rd of 114). Those downpours cluster in April, July and August, when sump pumps and gutters are tested hardest. The wettest day on record brought 4.73 inches on December 27, 2015. The worst three-day stretch dropped 8.4 inches, ending December 29, 2015, the kind of soaking that overwhelms sump pumps and storm sewers.
About 67 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
Missouri's 114 counties range widely in housing age; St. Louis ranks 11th, with 69% of its homes built before 1980.
Before 1960 is when 34% of these homes went up. Galvanized supply lines and cast iron drains were standard then, and after 60-plus years both corrode and crack, often out of sight inside a wall or under a slab.
Renters occupy 31% of homes. If you rent, the building is usually the landlord's to repair, but your belongings are only covered by renters insurance.
Beyond water
Summer nights that don't drop below 65°F, a good marker of humid air, come about 82 times a year here, above the Missouri county median of 64.5. Keep indoor humidity under 60%, as the EPA advises, and dry any leak within 24 to 48 hours; in a humid month a wet basement left alone won't dry by itself.
Mold remediation: what to expectMain heating fuel in St. Louis County's occupied homes: 75% utility gas, 23% electric, 1% propane and under 1% wood. Fires here are likelier to start in the kitchen or the wiring than in the furnace. After any fire, keep the HVAC off until ducts are checked, or soot will travel through the house.
Fire and smoke cleanup: first stepsCoverage
Calls connect to restoration companies that work across St. Louis County. The largest communities by population:
Population from ACS 2020-2024 5-year estimates. Townships and unincorporated areas are covered too.
Nearby
Franklin and St. Charles get more pipe-freeze nights than St. Louis. All of its neighbors see more heavy-rain days. Its housing is the oldest in the area by share of homes built before 1980.
By the numbers
| Factor | St. Louis | Missouri median | Rank |
|---|---|---|---|
| Homes built before 1960 | 34% | 25% | 27 of 114 |
| Homes built before 1980 | 69% | 53% | 11 of 114 |
| Homes built 2000 or later | 11% | 21% | 110 of 114 |
| Manufactured or mobile homes | 0.3% | 9.8% | 114 of 114 |
| Vacant homes | 7.2% | 15% | 102 of 114 |
| Wood or propane heat | 1.5% | 25% | 114 of 114 |
| Nights at 20°F or colder | 24/yr | 28/yr | 70 of 114 |
| Days with 1 inch+ of rain | 9.9/yr | 10/yr | 63 of 114 |
| Freeze-thaw days | 67/yr | 77/yr | 106 of 114 |
| Muggy nights (65°F+) | 82/yr | 65/yr | 8 of 114 |
Rank 1 is the highest value. Bars run from the lowest to the highest Missouri county; the tick marks the median; orange means higher water damage risk than the median, green means lower.
Questions
Peak risk months in St. Louis County: freezing from December to February, heavy rain in April, July and August.
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