Carroll County, Illinois · Mount Carroll
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Carroll County averages 52 nights a year at 20°F or colder, mostly November to March, and 53% of its homes predate 1960.
Heavy rain days of an inch or more come about 8.4 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 Carroll County gets about 8.4 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 91 freeze-thaw days a year, from winter ice dams. Most show up as ceiling stains in the room below.
20% of occupied homes in Carroll County heat mainly with wood or propane, above the Illinois county median of 16.5%.
About 29 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
About 15,576 people live in Carroll County, 445 square miles in northern Illinois, in roughly 8,204 homes. The county seat is Mount Carroll, and Savanna, Thomson and Lanark round out the largest towns. It has some of the oldest housing in the state: the median home went up in 1957.
71/100 Water Damage Risk Index: High Carroll County ranks 2 of 102 Illinois counties. See the statewide ranking and method.The three factors where Carroll County ranks highest compared with other Illinois counties.
Carroll is on the colder side of Illinois for pipes. It averaged 131 freezing nights a year over 2015–2024, 52 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 3rd.
On January 31, 2019 the low fell to about -31°F, and in the stretch ending February 2021 the county went 16 days in a row below freezing. Leaks from split pipes typically appear the day the thaw arrives.
Most freezing nights fall from November to March, 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 20 and the last spring freeze around Apr 23.
Rainfall is about average for Illinois. About 8.4 days a year bring an inch or more (51st of 102), most often in July, out of 39.6 inches of total precipitation. On July 22, 2017, 4.41 inches fell in a single day. Multi-day storms matter most for basements; the wettest three days on record here totaled 6.8 inches (through July 22, 2017).
Freeze-thaw days, below freezing overnight and above it by afternoon, come about 91 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
Roughly 71% of the county's housing was built before 1980. That puts Carroll near the middle of Illinois's 102 counties.
In Carroll County 53% of homes are older than 1960. If yours is one of them and the plumbing has never been replaced, expect galvanized steel supply pipe and cast iron drain lines, the two materials most likely to fail quietly.
The Census counted 20% of homes as vacant, above the Illinois county median of 10.8%. Empty houses are where a small leak turns into a large claim, because nobody is there to hear it.
Beyond water
Carroll County averages 29 warm, muggy nights a year (lows of 65°F or more) and gets the most rain in June. 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 Carroll County it's 68% utility gas, 12% electric, 18% propane and 2% wood. Most homes here heat with gas or electricity, so cooking and electrical faults are the likelier fire sources. Even a contained kitchen fire can send soot through the ductwork to every room.
Fire and smoke cleanup: first stepsCoverage
Calls connect to restoration companies that work across Carroll County. The largest communities by population:
Population from ACS 2020-2024 5-year estimates. Townships and unincorporated areas are covered too.
Nearby
Jo Daviess and Stephenson get more pipe-freeze nights than Carroll. Whiteside County has the oldest housing nearby, with 78% of homes built before 1980.
By the numbers
| Factor | Carroll | Illinois median | Rank |
|---|---|---|---|
| Homes built before 1960 | 53% | 38% | 5 of 102 |
| Homes built before 1980 | 71% | 66% | 35 of 102 |
| Homes built 2000 or later | 11% | 15% | 72 of 102 |
| Manufactured or mobile homes | 3.3% | 5.6% | 71 of 102 |
| Vacant homes | 20% | 11% | 12 of 102 |
| Wood or propane heat | 20% | 17% | 39 of 102 |
| Nights at 20°F or colder | 52/yr | 35/yr | 3 of 102 |
| Days with 1 inch+ of rain | 8.4/yr | 8.4/yr | 51 of 102 |
| Freeze-thaw days | 91/yr | 81/yr | 3 of 102 |
| Muggy nights (65°F+) | 29/yr | 51/yr | 94 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 Carroll County: freezing from November to March, heavy rain in June, July and September.
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