Base flood elevation

Base flood elevation, the BFE, is the elevation floodwater is expected to reach during the base flood: the flood with a one percent chance of occurring in any given year. It is expressed as a height above a vertical datum, not as a depth of water.

The "100-year flood" nickname is misleading and worth discarding. It describes an annual probability, not a schedule. A one percent annual chance is roughly a 26 percent chance over a 30 year mortgage.

How it is used

In a mapped special flood hazard area, the lowest floor of a structure must be at or above the BFE. Many jurisdictions add freeboard, an extra margin of one to three feet, above the federal minimum. The local requirement is what governs, and it is frequently higher than FEMA's.

Freeboard is worth more than it costs

Elevating above the minimum reduces flood insurance premiums substantially, and the premium difference typically dwarfs the construction cost of the extra height. Where the choice exists, building higher than required is usually the better economic decision.

Establishing it on your site

The BFE comes from the Flood Insurance Rate Map, and comparing it to your structure requires a surveyor's elevation certificate, which reports the building's elevations on the same datum. Datums matter here: older certificates may reference NGVD 29 while current maps use NAVD 88, and the difference between them is large enough to matter. Conversions are routine for a surveyor and are not something to estimate.

In unnumbered A zones, no BFE is published, and the jurisdiction will specify how one is to be determined for your site.