Original data
Most basement flooding does not happen where the flood maps say it will
We pulled every basement flood-insurance claim FEMA has released for the 156 counties containing our 200 cities: 131,361 claims on buildings with a finished or unfinished basement. Then we sorted them by whether the property sat inside the mapped 100-year floodplain.
29% of basement flood claims were filed outside the mapped floodplain.
37,074 of 126,429 claims with a rated flood zone. Narrow it to claims caused by rainfall or snowmelt accumulation, which is seepage rather than a river coming up, and it is 33% (19,096 of 57,969).
And that is a floor, not a rate. Read the next paragraph before you quote it.
Own a home with a basement?The usual reasoning, "I'm not in a flood zone, so I don't need to worry," is the wrong test. Most basement water is rain against the wall, a high water table, or a failed pump, and none of that follows a floodplain line. The map answers a different question than the one you are asking.
Covering flood risk or housing? This is claims data on real buildings, cut by basement and flood zone in a way FEMA does not publish, and it undercuts the assumption baked into a lot of coverage: that the flood map is where the water damage is.
Why this number understates the problem
These are National Flood Insurance Program claims, so every one of them is a property that had flood insurance. Inside the floodplain, a federally backed mortgage effectively requires it. Outside, almost nobody buys it. So the pool these claims are drawn from is heavily weighted toward flood-zone properties, and out-of-zone basements are badly under-represented before we start counting.
That cuts one way: 29% is the share we can prove, not the share that happens. The real proportion of basement flooding outside mapped zones is almost certainly higher. This is not a rate of flooding, and nobody should use it as one.
Standard NFIP policies also limit what they pay on basement contents, which shapes what gets claimed at all.
Methodology
Source is FEMA's NFIP redacted claims dataset (openFEMA FimaNfipClaims v2), filtered to buildings recorded with a finished or unfinished basement. Crawlspaces are excluded. "Inside the floodplain" means a rated zone beginning A or V, the Special Flood Hazard Area. Everything else counts as outside. 4,932 claims carry no rated zone and are excluded from the percentage rather than assigned to either side.
Geography is county, not city. FEMA redacts the city field, so county is the finest honest level available. The county containing a city is not the city: it includes suburbs and unincorporated land, and cities sharing a county share a row here. We have not pretended otherwise anywhere on this page.
Percentages are only ranked for counties with at least 100 claims. Below that a share is noise. Claim counts span 1978 to 2026.
Free to cite or reproduce with a link to this page.
Where basement flooding least respects the map
Counties with at least 100 basement claims, ranked by the share filed outside the mapped floodplain. The spread is the story: river-flooding counties sit at the bottom, and suburban counties with no river worth the name sit at the top.
| County | Cities covered | Basement claims | Outside floodplain |
|---|---|---|---|
| Mahoning County, OH | Youngstown | 156 | 92% |
| Montgomery County, MD | Gaithersburg, Rockville | 442 | 92% |
| Arapahoe County, CO | Aurora | 131 | 91% |
| Denver County, CO | Denver | 153 | 82% |
| Riley County, KS | Manhattan | 205 | 70% |
| Alexandria city, VA | Alexandria | 240 | 68% |
| El Paso County, CO | Colorado Springs | 547 | 67% |
| Oakland County, MI | Farmington Hills, Rochester Hills, Southfield, Troy | 305 | 66% |
| Oklahoma County, OK | Edmond, Oklahoma City | 111 | 64% |
| Wyandotte County, KS | Kansas City | 113 | 63% |
| Boulder County, CO | Boulder | 916 | 63% |
| Hennepin County, MN | Bloomington, Maple Grove, Minneapolis, Plymouth | 421 | 61% |
| Bexar County, TX | San Antonio | 100 | 61% |
| St. Joseph County, IN | South Bend | 107 | 59% |
| Broome County, NY | Binghamton | 2,037 | 57% |
| Erie County, PA | Erie | 144 | 56% |
| Dane County, WI | Madison | 203 | 55% |
| Winnebago County, WI | Oshkosh | 103 | 55% |
| Larimer County, CO | Fort Collins | 175 | 55% |
| Jefferson County, CO | Arvada, Lakewood | 249 | 53% |
| Baltimore city, MD | Baltimore | 498 | 52% |
| Frederick County, MD | Frederick | 145 | 52% |
| Waukesha County, WI | Waukesha | 268 | 50% |
| Jackson County, MO | Blue Springs, Independence, Kansas City, Lee's Summit | 857 | 50% |
| Philadelphia County, PA | Philadelphia | 768 | 50% |
| Albany County, NY | Albany | 211 | 49% |
| Tulsa County, OK | Broken Arrow, Tulsa | 130 | 49% |
| Cuyahoga County, OH | Cleveland, Euclid, Lakewood, Parma | 777 | 48% |
| Luzerne County, PA | Wilkes-Barre | 3,066 | 48% |
| Dallas County, TX | Dallas, Garland, Irving, Mesquite | 144 | 46% |
| Fayette County, KY | Lexington | 205 | 46% |
| Knox County, TN | Knoxville | 179 | 46% |
| Johnson County, KS | Olathe, Overland Park, Shawnee | 695 | 43% |
| Genesee County, MI | Flint | 307 | 42% |
| Kalamazoo County, MI | Kalamazoo | 160 | 41% |
| Johnson County, IA | Iowa City | 194 | 41% |
| Westchester County, NY | New Rochelle, White Plains, Yonkers | 7,233 | 41% |
| Rutherford County, TN | Murfreesboro | 117 | 41% |
| York County, PA | York | 902 | 40% |
| Franklin County, OH | Columbus | 671 | 39% |
| Butler County, OH | Hamilton | 219 | 39% |
| Macomb County, MI | Sterling Heights, Warren | 623 | 39% |
| Lake County, IL | Waukegan | 1,101 | 39% |
| Hamilton County, IN | Carmel, Fishers | 176 | 39% |
| St. Louis city, MO | Saint Louis | 375 | 39% |
| Middlesex County, MA | Cambridge, Lowell | 2,459 | 39% |
| Summit County, OH | Akron, Cuyahoga Falls | 496 | 38% |
| Berks County, PA | Reading | 746 | 38% |
| Lucas County, OH | Toledo | 565 | 37% |
| Milwaukee County, WI | Milwaukee | 1,995 | 37% |
| Washington County, MD | Hagerstown | 138 | 37% |
| Montgomery County, OH | Dayton, Kettering | 166 | 36% |
| Shawnee County, KS | Topeka | 217 | 36% |
| DuPage County, IL | Naperville | 2,250 | 35% |
| Camden County, NJ | Camden | 839 | 35% |
| Harris County, TX | Houston | 2,631 | 35% |
| Lorain County, OH | Elyria, Lorain | 273 | 34% |
| Kenosha County, WI | Kenosha | 131 | 34% |
| Linn County, IA | Cedar Rapids | 979 | 34% |
| Monroe County, NY | Rochester | 437 | 34% |
| Onondaga County, NY | Syracuse | 358 | 34% |
| Worcester County, MA | Worcester | 483 | 34% |
| Hampden County, MA | Springfield | 160 | 34% |
| Kane County, IL | Aurora, Elgin | 560 | 33% |
| Richmond city, VA | Richmond | 170 | 33% |
| Wayne County, MI | Dearborn, Detroit, Livonia | 1,795 | 32% |
| Cook County, IL | Arlington Heights, Chicago, Cicero, Evanston, Schaumburg | 10,618 | 32% |
| Sarpy County, NE | Bellevue | 183 | 32% |
| Allegheny County, PA | Bethel Park, Pittsburgh | 3,273 | 32% |
| Lehigh County, PA | Allentown | 553 | 32% |
| Vanderburgh County, IN | Evansville | 281 | 31% |
| Monongalia County, WV | Morgantown | 126 | 31% |
| Travis County, TX | Austin | 156 | 31% |
| Davidson County, TN | Nashville | 1,372 | 31% |
| Tippecanoe County, IN | Lafayette | 101 | 30% |
| Erie County, NY | Buffalo | 1,079 | 30% |
| Sedgwick County, KS | Wichita | 620 | 29% |
| Schenectady County, NY | Schenectady | 247 | 29% |
| Hamilton County, OH | Cincinnati | 807 | 27% |
| Stark County, OH | Canton | 260 | 27% |
| Ingham County, MI | Lansing | 251 | 27% |
| Northampton County, PA | Bethlehem | 998 | 27% |
| Lancaster County, PA | Lancaster | 1,079 | 27% |
| Middlesex County, NJ | Edison | 3,525 | 27% |
| Mercer County, NJ | Trenton | 1,643 | 27% |
| Will County, IL | Joliet | 1,016 | 26% |
| Buchanan County, MO | Saint Joseph | 189 | 26% |
| Blair County, PA | Altoona | 717 | 26% |
| Niagara County, NY | Niagara Falls | 109 | 26% |
| Essex County, NJ | Newark | 3,746 | 26% |
| Kenton County, KY | Covington | 179 | 26% |
| Hamilton County, TN | Chattanooga | 310 | 26% |
| Brown County, WI | Green Bay | 232 | 25% |
| Centre County, PA | State College | 174 | 25% |
| Union County, NJ | Elizabeth | 5,901 | 24% |
| Cabell County, WV | Huntington | 245 | 24% |
| Polk County, IA | Des Moines | 1,028 | 23% |
| Lackawanna County, PA | Scranton | 1,161 | 23% |
| Jefferson County, KY | Louisville | 1,867 | 23% |
| Kent County, MI | Grand Rapids | 363 | 22% |
| Allen County, IN | Fort Wayne | 705 | 22% |
| Douglas County, NE | Omaha | 354 | 22% |
| Lancaster County, NE | Lincoln | 222 | 22% |
| Dauphin County, PA | Harrisburg | 3,883 | 22% |
| Richland County, OH | Mansfield | 211 | 21% |
| Racine County, WI | Racine | 126 | 21% |
| Olmsted County, MN | Rochester | 390 | 21% |
| Oneida County, NY | Utica | 698 | 21% |
| Kanawha County, WV | Charleston | 1,106 | 21% |
| Plymouth County, MA | Brockton | 5,620 | 20% |
| Marion County, IN | Indianapolis | 1,039 | 19% |
| Suffolk County, MA | Boston | 2,996 | 17% |
| Rensselaer County, NY | Troy | 330 | 16% |
| Roanoke city, VA | Roanoke | 521 | 16% |
| Winnebago County, IL | Rockford | 923 | 15% |
| Lake County, IN | Gary, Hammond | 1,931 | 13% |
| Woodbury County, IA | Sioux City | 214 | 13% |
| Peoria County, IL | Peoria | 1,165 | 12% |
| Scott County, IA | Davenport | 1,311 | 12% |
| Hudson County, NJ | Jersey City | 3,259 | 12% |
| Black Hawk County, IA | Waterloo | 1,096 | 11% |
| Passaic County, NJ | Clifton, Paterson | 8,769 | 8% |
| St. Charles County, MO | O'Fallon, Saint Charles | 3,627 | 7% |
Using this data
The raw data is public, FEMA's FimaNfipClaims endpoint, and our harvester is in the repository, so the analysis is reproducible. Cite this page for the county breakdown. Per-market cuts and the underlying claim records are available on request.
Permit fees for these cities are in our building permit fee report.