(MONEY Magazine) – As this fall's Hurricane Floyd so graphically demonstrated, 90% of all natural disasters in the U.S. are flood-related. Yet the Federal Emergency Management Agency (FEMA) estimates that only 25% of high-risk homes have flood insurance. Misconceptions about what homeowners policies cover may be one reason. Here's what you need to know about the arcane insurance rules for floods.

A typical homeowners policy covers damage from a burst pipe or a broken washing machine hose. If a hurricane blows off your roof and lets in rain, that's covered by a homeowners policy. Yet sewer or drain backups are covered only if you have a special endorsement, which costs $30 a year on average. And if your home is damaged by a rise in the outside water level--including rain runoff, storm surges, mud slides or a river overflowing--only flood insurance will help. This government-backed policy, sold by insurers and FEMA, averages $340 a year.

If you're in a floodplain, flood insurance is a necessity. But keep in mind that 25% to 30% of claims come from outside of high-risk zones. To gauge your risk, check a flood-insurance rate map--available from insurers and lenders--to see how near you are to a floodplain.

One catch: To be eligible for flood insurance, your community must participate in the National Flood Insurance Program (NFIP). If it doesn't--a possibility if you live in a low-risk or sparsely populated area--you have little recourse. For more information, you can call NFIP at 800-427-4661.

--ADRIENNE CARTER