In all locations pummeled by Ian, the percentage of homes covered by flood policies is down from five years ago. 

The widespread lack of flood insurance will force many people to seek federal disaster assistance in the form of grants and loans. This will slow efforts to rebuild as people patch together funds. That is what happened after extensive flooding in Houston from Hurricane Harvey in 2017.  

Flood coverage isn’t part of a standard homeowner’s policy. Flooding has emerged in recent years as a more serious risk after storms such as Harvey and Sandy, which dumped record amounts of rain on the Northeast in 2012. Rising temperatures appear to be a factor. Warmer air holds more moisture and warmer oceans give storms more energy. 

The 15 inches of rain that fell at the Sanford Orlando International Airport in Seminole was 50% higher than the previous record of 10 inches in 24 hours set in 1992, according to the National Weather Service. Florida Gov. Ron DeSantis called Ian “basically, a 500-year flood event.”

When Amber Thorne woke up Thursday in Sanford in Seminole County, she was stunned to find that a creek overflowed and flooded her yard. She doesn’t have flood insurance for her two-bedroom house. “I’m regretting not having it,” she said.  

About 97% of residences in Seminole County and 98% in Orange County, home to Orlando, don’t have flood insurance, according to Neptune. Neptune Chief Executive Trevor Burgess said that based on policies his company sells statewide, in general about a third of the residential properties in Florida’s inland counties are identified on government maps as at high risk of flooding. That compares with 85% for many coastal counties. 

Risk-modeler Karen Clark & Co. on Friday estimated that the privately insured loss from Hurricane Ian would be close to $63 billion, including wind, storm surge and inland flood damages, while total economic damage will be well over $100 billion, including uninsured properties, damage to infrastructure and cleanup costs. 

Ian’s landfall was outside Fort Myers in Lee County, some 150 miles from Ms. Thorne’s property. Some of the worst storm surge damage occurred in Lee, where about 28% of housing units are covered by flood policies, Neptune’s analysis shows. 

Residential flood-insurance policies relative to housing units

Received 10 inches or more

of rainfall on Sept. 29

Residential flood-insurance policies relative to housing units

Received 10 inches or more

of rainfall on Sept. 29

Residential flood-insurance policies relative to housing units

Received 10 inches or more

of rainfall on Sept. 29

Residential flood-insurance policies relative to housing units

Received 10 inches or more

of rainfall on Sept. 29

Residential flood-insurance policies relative to housing units

Received 10 inches or more

of rainfall on Sept. 29

In nearby Charlotte County and Collier County, home to Naples, 31% and 41% of homes, respectively, have flood insurance. Monroe County, at the bottom of Florida, has one of the state’s highest take-up rates, at 53%.

The U.S. government’s National Flood Insurance Program, managed by the Federal Emergency Management Agency, is by far the biggest seller of the policies. Private-sector options are becoming more available. 

Across Florida over the past five years, the portion of homes covered by flood policies has declined, to 15.4% in August from 17.8% in 2017.   

Insurance agents and executives attribute some of the decline over the past year to rising inflation generally and double-digit increases in Florida homeowners policies. Another factor is price increases for federal flood insurance for many homes, to more accurately reflect the true flooding risk. Some rates will jump from hundreds of dollars annually to thousands of dollars, though most increases are capped at 18% a year.

Lenders require flood coverage for borrowers who live in designated high-risk zones. 

Another reason for declining uptake in recent years in Florida is that a large number of people moving to the state have paid cash for their homes. “No mortgage, no requirement,” said Neptune’s Mr. Burgess. 

“Homeowners think if they are away from the ocean or a river they can’t flood,”​ which is wrong, Mr. Burgess said. In fact, about a fifth of Neptune’s claims over the past five years have been in supposedly lower-risk zones, he said. 

In its analysis, Neptune used government residential flood-insurance data along with its own issuance figures, and contrasted them against U.S. Census housing data. Other private-sector insurers, with generally small totals, aren’t reflected in the tallies.

Harrison Froid, an agent in Pinellas County in the Tampa area for

Goosehead Insurance,

said most clients buying homes decline the purchase of flood coverage. If not required by their lender, “they say, ‘I’ll circle back to you after closing,’ ” Mr. Froid said, and generally don’t.  

Michael Bristol, an engineer who lives with his wife in a double-wide mobile home on 10 acres with a horse stable in Seminole County, spent Thursday afternoon cleaning up branches and other debris from their sodden yard. A creek had overflowed, and the road was impassable near the couple’s property. Ditches churned with water.

Mr. Bristol says the property escaped serious damage. He had skipped flood insurance as “just too cost prohibitive,” and he doesn’t see that changing. He said his plan is to “keep a decent amount in savings and hope for the best…You know, fingers crossed.”  

Write to Leslie Scism at [email protected] and Cameron McWhirter at [email protected]