Jefferson County, Illinois · Mount Vernon
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With 28% of homes built before 1960 and about 23 hard-freeze nights a year, frozen and split pipes are a leading winter risk in the county.
March brings the most downpours to Jefferson County, part of 11.2 heavy-rain days a year. Check the sump first.
28% of homes here predate 1960, when clay and cast iron sewer laterals were standard. Roots and cracks in those lines are a common cause of backups.
Jefferson County's wettest day in the 2015–2024 record brought 3.22 inches, and about 69 freeze-thaw days a year mean ice dams in some winters.
24% of occupied homes in Jefferson County heat mainly with wood or propane, above the Illinois county median of 16.5%.
With 69 muggy nights a year, mold in Jefferson County can appear within days of a leak. Dry first, then clean.
Local risk profile
With 16,629 homes and around 36,550 residents, Jefferson County is 571 square miles of southern Illinois. Besides Mount Vernon, the county seat, the biggest communities are Ina, Woodlawn and Dix. Half the homes here were built before 1977, half after.
43/100 Water Damage Risk Index: Moderate Jefferson County ranks 80 of 102 Illinois counties. See the statewide ranking and method.The three factors where Jefferson County ranks highest compared with other Illinois counties.
Winters here are on the gentler end for Illinois, and pipes still freeze. It averaged 83 freezing nights a year over 2015–2024, 23 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 81st.
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 December 23, 2022. Burst pipes usually show up when a spell like that breaks and the ice thaws.
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.
Freezing nights typically start around Oct 28 and end around Apr 10.
Heavy rain is a bigger factor here than in most of Illinois. The county gets about 46.7 inches of rain and melted snow a year, with 11.2 days of an inch or more (21st of 102). Those downpours cluster in March, April and July, when sump pumps and gutters are tested hardest. The wettest day on record brought 3.22 inches on August 15, 2016. The worst three-day stretch dropped 7.63 inches, ending August 16, 2016, the kind of soaking that overwhelms sump pumps and storm sewers.
About 69 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
55% of homes in Jefferson County predate 1980, below the Illinois county median of 65.9%, which ranks 84th of Illinois's 102 counties.
Housing age is mixed: 28% of homes are pre-1960 and 17% are 2000 or newer. Ask any plumber which era your house is from and you'll know which failures to watch for.
The 1980s and 1990s account for 28% 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.
Manufactured and mobile homes make up 17% of the housing here. Water lines in the underbelly depend on intact skirting and working heat tape, and they're usually the first to freeze in a cold snap.
Beyond water
Jefferson County averages 69 warm, muggy nights a year (lows of 65°F or more) and gets the most rain in July. 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 Jefferson County it's 34% utility gas, 40% electric, 21% propane and 4% wood. Wood and propane heat are 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 Jefferson County. The largest communities by population:
Population from ACS 2020-2024 5-year estimates. Townships and unincorporated areas are covered too.
Nearby
Marion and Washington get more pipe-freeze nights than Jefferson. Franklin County has the oldest housing nearby, with 70% of homes built before 1980.
By the numbers
| Factor | Jefferson | Illinois median | Rank |
|---|---|---|---|
| Homes built before 1960 | 28% | 38% | 84 of 102 |
| Homes built before 1980 | 55% | 66% | 84 of 102 |
| Homes built 2000 or later | 17% | 15% | 34 of 102 |
| Manufactured or mobile homes | 17% | 5.6% | 10 of 102 |
| Vacant homes | 11% | 11% | 49 of 102 |
| Wood or propane heat | 24% | 17% | 21 of 102 |
| Nights at 20°F or colder | 23/yr | 35/yr | 81 of 102 |
| Days with 1 inch+ of rain | 11/yr | 8.4/yr | 21 of 102 |
| Freeze-thaw days | 69/yr | 81/yr | 80 of 102 |
| Muggy nights (65°F+) | 69/yr | 51/yr | 24 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 Jefferson 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