Johnson County, Iowa · Iowa City
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With 17% of homes built before 1960 and about 58 hard-freeze nights a year, frozen and split pipes are a leading winter risk in the county.
June brings the most downpours to Johnson County, part of 6.7 heavy-rain days a year. Check the sump first.
Sewage backups in Johnson County spike on heavy-rain days, about 6.7 a year, when sewer mains fill up and water finds the lowest drain.
Johnson County's wettest day in the 2015–2024 record brought 3.48 inches, and about 98 freeze-thaw days a year mean ice dams in some winters.
7% of occupied homes in Johnson County heat mainly with wood or propane, below the Iowa county median of 22.1%.
About 32 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
Johnson County covers 613 square miles of eastern Iowa, with about 156,639 residents and 68,217 homes. Besides Iowa City, the county seat, the biggest communities are Coralville, North Liberty and Tiffin. Much of its housing is recent, with a median build year of 1992.
30/100 Water Damage Risk Index: Moderate Johnson County ranks 95 of 99 Iowa counties. See the statewide ranking and method.The three factors where Johnson County ranks highest compared with other Iowa counties.
For frozen pipes, Johnson County is a typical Iowa county, which is still plenty. It averaged 140 freezing nights a year over 2015–2024, 58 of them at 20°F or colder, cold enough for pipes in outside walls and crawl spaces to freeze. Among 99 Iowa counties that's 57th.
Cold snaps are the real risk. The worst in the record lasted 17 days below freezing (ending February 2019), and the low bottomed out near -32°F on January 31, 2019.
The freeze season here runs November to March, 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 16 and the last spring freeze around Apr 25.
Rainfall is about average for Iowa. Of roughly 35.8 inches of yearly precipitation, the damage comes from the 6.7 days a year with an inch or more, ranked 65th of 99. June, September and October see the most. The single wettest day in the record: 3.48 inches, August 12, 2016. Multi-day storms matter most for basements; the wettest three days on record here totaled 3.62 inches (through August 13, 2016).
The temperature crosses 32°F in both directions on roughly 98 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
37% of homes in Johnson County predate 1980, below the Iowa county median of 72.8%, which ranks 98th of Iowa's 99 counties.
37% of homes were built in 2000 or later, more than in most of Iowa. Newer plumbing rarely corrodes, so damage here tends to come from appliance supply lines, water heaters, sump pumps and storms.
The 1980s and 1990s account for 26% 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.
Renters occupy 41% 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
Summer nights that don't drop below 65°F, a good marker of humid air, come about 32 times a year here, above the Iowa county median of 29. 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 expectBy main heating fuel, Johnson County homes break down as 63% utility gas, 28% electric, 7% 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 Johnson County. The largest communities by population:
Population from ACS 2020-2024 5-year estimates. Townships and unincorporated areas are covered too.
Nearby
Linn gets more pipe-freeze nights than Johnson. Louisa County has the oldest housing nearby, with 70% of homes built before 1980.
By the numbers
| Factor | Johnson | Iowa median | Rank |
|---|---|---|---|
| Homes built before 1960 | 17% | 48% | 98 of 99 |
| Homes built before 1980 | 37% | 73% | 98 of 99 |
| Homes built 2000 or later | 37% | 13% | 2 of 99 |
| Manufactured or mobile homes | 3.8% | 3.2% | 36 of 99 |
| Vacant homes | 7.1% | 9.5% | 82 of 99 |
| Wood or propane heat | 7% | 22% | 93 of 99 |
| Nights at 20°F or colder | 58/yr | 62/yr | 57 of 99 |
| Days with 1 inch+ of rain | 6.7/yr | 7.1/yr | 65 of 99 |
| Freeze-thaw days | 98/yr | 94/yr | 3 of 99 |
| Muggy nights (65°F+) | 32/yr | 29/yr | 37 of 99 |
Rank 1 is the highest value. Bars run from the lowest to the highest Iowa county; the tick marks the median; orange means higher water damage risk than the median, green means lower.
Questions
Peak risk months in Johnson County: freezing from November to March, heavy rain in June, September and October.
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