Standard homeowner policies exclude flood, and the exclusion is not an oversight. Flood breaks the statistical assumption that private insurance pricing depends on.
Pooling needs losses that are not correlated
Insurance works because the insured events strike a small, scattered fraction of policyholders in any year, funded by premiums collected from everyone else.
Flood does the opposite. A single river or coastal event damages every exposed property in an area within hours of itself.
Losses that arrive together defeat the pooling arithmetic, because there is no unaffected majority left to fund the payouts.
The same difficulty applies to earthquake and to war, which is why those perils are excluded from standard cover or handled separately as well.
Adverse selection concentrates the buyers
Flood risk is unusually visible to the owner. People in low-lying areas know what they face, and those on higher ground know they are largely safe.
If cover is optional, mainly the exposed buy it, so the pool ends up consisting largely of properties that expect to flood.
Pricing that pool accurately produces premiums close to the expected loss itself, which very few buyers regard as worth paying.
An insurer that priced below that level would attract still more of the exposed properties, which makes the underlying problem worse rather than better.
Public programmes filled the resulting gap
Government-backed flood programmes were established because private markets withdrew, offering cover in participating communities that adopt floodplain management rules.
Coverage limits, waiting periods and the definition of flood come from programme rules rather than from a standard homeowner contract.
Those rules differ by jurisdiction and change over time, so the terms in force at the date of purchase are the ones that govern a claim.
Flood is narrower than water damage
A burst pipe is not a flood. Surface water rising and entering a property generally is, and that distinction decides which policy responds to the loss.
Sewer and drain backup usually sits in a third category requiring its own endorsement on the homeowner policy.
Claims are frequently disputed over which category applies rather than over whether damage occurred at all.
Where water arrives from more than one source during a single event, the allocation between policies becomes a question of engineering evidence.
Private capacity has returned selectively
Improved mapping and modelling have allowed private insurers to write flood cover on properties they can price with reasonable confidence.
That capacity concentrates on the better risks, which leaves the public programme holding a growing share of the most exposed properties.
Mortgage requirements keep purchase rates high inside mapped high-risk zones and low everywhere else, even though a substantial share of flood claims arise outside those zones.