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.
That is 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).
The damage runs real money. In the typical county, the median paid basement claim was $5,435, and county medians ranged as high as $34,100. That is what the NFIP paid the insured minority. Outside the floodplain, where almost nobody carries flood coverage, comparable damage is a bill the owner absorbs in full.
And that is a floor, not a rate. Read the next section before you quote it.
- 131,361basement flood claims analysed across 156 US counties (FEMA NFIP)
- 29%were filed outside the mapped 100-year floodplain, and that is a floor
- 33%of rainfall and snowmelt claims came from outside the zone
- $5,435median paid basement claim in the typical county; medians reached $34,100
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.
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.
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.
Use this data
Cite or embed this study
Alongside this, our water-table and freeze-thaw data show why basement water is a groundwater and frost problem, and permit fees for these cities are in our building permit fee report.