Base flood elevation (BFE), explained

What the base flood elevation is

The base flood elevation (BFE) is the height floodwaters are expected to reach during the one percent annual chance flood.

The so called 100-year flood is not a flood that comes once every 100 years. It is a probability, so it could happen more than once in a single year.

The BFE printed on a flood map is rounded to the nearest whole foot. The profiles and tables in the Flood Insurance Study give more exact elevations.

BFEs on a community's flood map are measured against a stated vertical datum, such as the National Geodetic Vertical Datum of 1929 or another datum shown on the map.

Sources: The base flood is the 1 percent annual chance flood; What a base flood elevation is; BFEs on the map are rounded; study profiles are more exact

Special flood hazard areas

A special flood hazard area (SFHA) is land in the floodplain with a 1 percent or greater chance of flooding in any given year.

Along rivers and streams, special flood hazard areas appear on flood maps as Zones A, AE, AH, AO, AR and A99. Older maps may use Zones A1 to A30.

In special flood hazard areas, federal floodplain management rules apply, and so do the mandatory flood insurance purchase requirements.

Sources: What a special flood hazard area is; Which zones are riverine SFHAs; Federal rules and mandatory purchase apply in SFHAs

Building at or above the BFE

New and substantially improved homes in Zones A1 to A30, AE and AH must have the lowest floor, including any basement, elevated to or above the base flood level.

A nonresidential building may either be elevated or be made watertight below the base flood level, with walls substantially impermeable to water.

Where BFE data are used in Zone A, the community must obtain the lowest floor elevation of new and substantially improved buildings and keep a record of it.

Under the minimum criteria, enclosed areas below the lowest floor need at least two flood openings totaling one square inch for every square foot of enclosed area.

A substantial improvement is any rebuild, rehab, addition or other improvement costing 50 percent or more of the building's market value before the work starts.

A building is substantially damaged when restoring it to its condition before the damage would cost 50 percent or more of its market value.

Local officials, usually the floodplain administrator, decide whether a building has been substantially improved or substantially damaged.

The NFIP rules are minimum standards. State or local floodplain rules that are more restrictive are encouraged and take precedence.

Sources: A zone homes: lowest floor at or above BFE; The 50 percent rule for improvements; Local officials make the call; Stricter state and local rules take precedence

How the elevation is documented

FEMA describes elevation certificates as an optional NFIP tool for checking a building's First Floor Height and the elevation around it.

A community may require an elevation certificate both before and after construction.

A request to change a flood map finding, such as the flood zone, through a Letter of Map Change must include an elevation certificate.

Before paying for a new one, ask around: former owners may have a copy, and the developer or builder may have been required to get one.

In Florida, the elevation certificate must be completed and sealed by a Florida licensed surveyor, though community officials may complete it for approximate Zone A and Zone AO sites.

In Maricopa County, Arizona, ECs are usually completed by a registered land surveyor or engineer, but a homeowner may complete one in Zones AO and A without a BFE.

Florida permit holders must submit an EC once the lowest floor is placed, before further vertical construction.

Since January 1, 2023, Florida law requires surveyors and mappers to submit a digital copy of every EC they complete to the state Division of Emergency Management.

Sources: Elevation certificates are optional rating tools; Florida: completed and sealed by a Florida licensed surveyor; Homeowners may complete it in AO and A without BFE; Florida surveyors file every EC with the state