Williamson County, Illinois · Marion
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Williamson County averages 20 nights a year at 20°F or colder, mostly December to February, and 27% of its homes predate 1960.
Heavy rain days of an inch or more come about 11.3 times a year here, most often in March. A sump pump that hasn't run since last season is a common weak point.
Backups often follow heavy rain, which Williamson County gets about 11.3 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 66 freeze-thaw days a year, from winter ice dams. Most show up as ceiling stains in the room below.
6% of occupied homes in Williamson County heat mainly with wood or propane, below the Illinois county median of 16.5%.
About 75 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
Williamson County covers 420 square miles of southern Illinois, with about 66,876 residents and 32,084 homes. Marion is the county seat; Herrin, Carterville and Johnston City are the next largest communities. The county's median home was built in 1981.
40/100 Water Damage Risk Index: Moderate Williamson County ranks 86 of 102 Illinois counties. See the statewide ranking and method.The three factors where Williamson County ranks highest compared with other Illinois counties.
Williamson County is milder than most of Illinois, but not mild enough to skip winterizing. It averaged 79 freezing nights a year over 2015–2024, 20 of them at 20°F or colder, cold enough for pipes in outside walls and crawl spaces to freeze. Among 102 Illinois counties that's 91st.
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 -7°F on February 19, 2015.
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.
On average the first fall freeze arrives around Oct 28 and the last spring freeze around Apr 1.
Heavy rain is a bigger factor here than in most of Illinois. The county gets about 49.5 inches of rain and melted snow a year, with 11.3 days of an inch or more (20th of 102). Those downpours cluster in March, April and July, when sump pumps and gutters are tested hardest. The wettest day on record brought 3.38 inches on April 30, 2017. Multi-day storms matter most for basements; the wettest three days on record here totaled 7.65 inches (through May 1, 2017).
About 66 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
49% of homes in Williamson County predate 1980, below the Illinois county median of 65.9%, which ranks 90th of Illinois's 102 counties.
Growth since 2000 added 28% of today's homes. In houses that new, the weak points are washing machine hoses, ice maker lines, water heaters and sump pumps rather than the pipes in the walls.
24% 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.
Renters occupy 29% 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 75 nights a year in Williamson County stay at 65°F or warmer, a sign of muggy summer air (13th of 102 counties). April 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 expectMain heating fuel in Williamson County's occupied homes: 29% utility gas, 64% electric, 5% 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 Williamson County. The largest communities by population:
Population from ACS 2020-2024 5-year estimates. Townships and unincorporated areas are covered too.
Nearby
Franklin and Jackson get more pipe-freeze nights than Williamson. All of its neighbors see more heavy-rain days. Franklin County has the oldest housing nearby, with 70% of homes built before 1980.
By the numbers
| Factor | Williamson | Illinois median | Rank |
|---|---|---|---|
| Homes built before 1960 | 27% | 38% | 88 of 102 |
| Homes built before 1980 | 49% | 66% | 90 of 102 |
| Homes built 2000 or later | 28% | 15% | 9 of 102 |
| Manufactured or mobile homes | 6.8% | 5.6% | 43 of 102 |
| Vacant homes | 9.7% | 11% | 63 of 102 |
| Wood or propane heat | 6% | 17% | 86 of 102 |
| Nights at 20°F or colder | 20/yr | 35/yr | 91 of 102 |
| Days with 1 inch+ of rain | 11/yr | 8.4/yr | 20 of 102 |
| Freeze-thaw days | 66/yr | 81/yr | 93 of 102 |
| Muggy nights (65°F+) | 75/yr | 51/yr | 13 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 Williamson 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