Monroe County, Illinois · Waterloo
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Monroe County averages 22 nights a year at 20°F or colder, mostly December to February, and 21% of its homes predate 1960.
Heavy rain days of an inch or more come about 9.5 times a year here, most often in July. A sump pump that hasn't run since last season is a common weak point.
Backups often follow heavy rain, which Monroe County gets about 9.5 days a year, when storm water overloads sewer mains and pushes up through the lowest drain in the house.
In Monroe County, 4.62 inches fell on the wettest day of the 2015–2024 record. Storms like that, and ice dams in snowy winters, cause most roof leaks.
23% of occupied homes in Monroe County heat mainly with wood or propane, above the Illinois county median of 16.5%.
About 80 nights a year here stay at 65°F or warmer, so basements and crawl spaces can grow mold even without a leak.
Local risk profile
Monroe County covers 385 square miles of southwest Illinois, with about 35,036 residents and 14,759 homes. Waterloo is the county seat; Columbia, Valmeyer and Hecker are the next largest communities. Much of its housing is recent, with a median build year of 1990.
37/100 Water Damage Risk Index: Moderate Monroe County ranks 93 of 102 Illinois counties. See the statewide ranking and method.The three factors where Monroe County ranks highest compared with other Illinois counties.
Winters here are on the gentler end for Illinois, and pipes still freeze. In the 2015–2024 record the county logged about 22 nights a year at or below 20°F (84th of 102) and 80 below freezing. Twenty degrees is roughly where uninsulated pipes start to freeze.
The longest cold spell was straight 12 days without the temperature rising above freezing, ending in January 2018. The coldest night reached about -5°F on December 23, 2022. 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 Oct 30 and end around Mar 30.
Rainfall is about average for Illinois. Of roughly 44.4 inches of yearly precipitation, the damage comes from the 9.5 days a year with an inch or more, ranked 37th of 102. March, May and July see the most. The single wettest day in the record: 4.62 inches, September 8, 2018. The worst three-day stretch dropped 6.53 inches, ending December 29, 2015, the kind of soaking that overwhelms sump pumps and storm sewers.
The temperature crosses 32°F in both directions on roughly 67 days a year. Each cycle melts and refreezes water at roof edges and in foundation cracks, which is how ice dams and basement seepage get started.
Housing
Roughly 41% of the county's housing was built before 1980. That puts Monroe among the newer housing stocks of Illinois's 102 counties.
Since 2000 the county has added enough housing that 32% of homes are that new. Modern PEX and copper rarely fail on their own; the leaks come from connections, appliances and storms.
27% 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
Summer nights that don't drop below 65°F, a good marker of humid air, come about 80 times a year here, above the Illinois county median of 51. 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 expectHeating fuel matters for fire risk. In Monroe County it's 46% utility gas, 30% electric, 21% propane and 2% wood. Propane heat is above the Illinois norm. Annual chimney and vent inspection, which the NFPA recommends, is one of the most effective fire-prevention habits for these homes.
Fire and smoke cleanup: first stepsCoverage
Calls connect to restoration companies that work across Monroe County. The largest communities by population:
Population from ACS 2020-2024 5-year estimates. Townships and unincorporated areas are covered too.
Nearby
St. Clair gets more pipe-freeze nights than Monroe. All of its neighbors see more heavy-rain days. Randolph County has the oldest housing nearby, with 64% of homes built before 1980.
By the numbers
| Factor | Monroe | Illinois median | Rank |
|---|---|---|---|
| Homes built before 1960 | 21% | 38% | 96 of 102 |
| Homes built before 1980 | 41% | 66% | 99 of 102 |
| Homes built 2000 or later | 32% | 15% | 4 of 102 |
| Manufactured or mobile homes | 1.9% | 5.6% | 88 of 102 |
| Vacant homes | 6.2% | 11% | 92 of 102 |
| Wood or propane heat | 23% | 17% | 24 of 102 |
| Nights at 20°F or colder | 22/yr | 35/yr | 84 of 102 |
| Days with 1 inch+ of rain | 9.5/yr | 8.4/yr | 37 of 102 |
| Freeze-thaw days | 67/yr | 81/yr | 88 of 102 |
| Muggy nights (65°F+) | 80/yr | 51/yr | 4 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 Monroe County: freezing from December to February, heavy rain in March, May 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