St. Clair County, Illinois · Belleville
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Pipes freeze in St. Clair County on about 24 nights a year, mostly December to February. Older homes, 32% of the stock here, are most at risk.
March brings the most downpours to St. Clair County, part of 10.6 heavy-rain days a year. Check the sump first.
Sewage backups in St. Clair County spike on heavy-rain days, about 10.6 a year, when sewer mains fill up and water finds the lowest drain.
St. Clair County's wettest day in the 2015–2024 record brought 3.86 inches, and about 69 freeze-thaw days a year mean ice dams in some winters.
5% of occupied homes in St. Clair County heat mainly with wood or propane, below the Illinois county median of 16.5%.
With 79 muggy nights a year, mold in St. Clair County can appear within days of a leak. Dry first, then clean.
Local risk profile
About 253,694 people live in St. Clair County, 658 square miles in southern Illinois, in roughly 115,630 homes. The county seat is Belleville, and O'Fallon, East St. Louis and Cahokia Heights round out the largest towns. The county's median home was built in 1975.
47/100 Water Damage Risk Index: Elevated St. Clair County ranks 70 of 102 Illinois counties. See the statewide ranking and method.The three factors where St. Clair County ranks highest compared with other Illinois counties.
St. Clair County is milder than most of Illinois, but not mild enough to skip winterizing. In the 2015–2024 record the county logged about 24 nights a year at or below 20°F (76th of 102) and 83 below freezing. Twenty degrees is roughly where uninsulated pipes start to freeze.
Cold snaps are the real risk. The worst in the record lasted 14 days below freezing (ending January 2018), and the low bottomed out near -5°F on December 23, 2022.
Most freezing nights fall from December to February, peaking in January. Disconnect garden hoses and shut off outdoor spigots before the first hard frost, and keep the heat at 55°F or higher if you'll be away.
On average the first fall freeze arrives around Oct 25 and the last spring freeze around Apr 3.
Heavy rain is a bigger factor here than in most of Illinois. Of roughly 43.3 inches of yearly precipitation, the damage comes from the 10.6 days a year with an inch or more, ranked 26th of 102. March, April and July see the most. The single wettest day in the record: 3.86 inches, September 8, 2018. Multi-day storms matter most for basements; the wettest three days on record here totaled 6.7 inches (through December 29, 2015).
Freeze-thaw days, below freezing overnight and above it by afternoon, come about 69 times a year here. They're the main driver of ice dams and of water working its way through foundation cracks in late winter.
Housing
Illinois's 102 counties range widely in housing age; St. Clair ranks 84th, with 55% of its homes built before 1980.
Since 2000 the county has added enough housing that 23% of homes are that new. Modern PEX and copper rarely fail on their own; the leaks come from connections, appliances and storms.
The 1980s and 1990s account for 23% of the county's homes. If yours is from those decades, look under the sinks and at the water heater for gray plastic pipe stamped PB, which is polybutylene and leak-prone.
With 13% of homes empty at survey time, landlords and seasonal owners should shut off the main or schedule checks in winter. An unheated vacant house is also where many policies limit freeze coverage.
Renters occupy 32% 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
About 79 nights a year in St. Clair County stay at 65°F or warmer, a sign of muggy summer air (5th of 102 counties). May is the wettest month. 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 expectHeating fuel matters for fire risk. In St. Clair County it's 61% utility gas, 33% electric, 5% propane and under 1% wood. Heating equipment, cooking and electrical failures cause most home fires nationally. Even a small kitchen fire can leave soot through the whole house, because smoke follows air currents into closets, ducts and attic space.
Fire and smoke cleanup: first stepsCoverage
Calls connect to restoration companies that work across St. Clair County. The largest communities by population:
Population from ACS 2020-2024 5-year estimates. Townships and unincorporated areas are covered too.
Nearby
Madison gets more pipe-freeze nights than St. Clair. Randolph County has the oldest housing nearby, with 64% of homes built before 1980.
By the numbers
| Factor | St. Clair | Illinois median | Rank |
|---|---|---|---|
| Homes built before 1960 | 32% | 38% | 79 of 102 |
| Homes built before 1980 | 55% | 66% | 84 of 102 |
| Homes built 2000 or later | 23% | 15% | 15 of 102 |
| Manufactured or mobile homes | 4.4% | 5.6% | 58 of 102 |
| Vacant homes | 13% | 11% | 28 of 102 |
| Wood or propane heat | 5% | 17% | 88 of 102 |
| Nights at 20°F or colder | 24/yr | 35/yr | 76 of 102 |
| Days with 1 inch+ of rain | 11/yr | 8.4/yr | 26 of 102 |
| Freeze-thaw days | 69/yr | 81/yr | 80 of 102 |
| Muggy nights (65°F+) | 79/yr | 51/yr | 5 of 102 |
Rank 1 is the highest value. Bars run from the lowest to the highest Illinois county; the tick marks the median; orange means higher water damage risk than the median, green means lower.
Questions
Peak risk months in St. Clair County: freezing from December to February, heavy rain in March, April and July.
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