AO and AH are the shallow-flooding high-risk zones. Here is the difference. Paste an address below to see its FEMA flood zone, or a whole list to check them all at once.
Quick answer
Flood zones AO and AH are high-risk areas of shallow flooding, with a 1% or greater chance each year of flooding 1 to 3 feet deep. In zone AO the water flows as a sheet down sloping ground, and the map gives a flood depth instead of an elevation; in zone AH the water ponds, and the map gives a base flood elevation. Both are in the Special Flood Hazard Area, so flood insurance is required for mortgages from federally regulated or insured lenders.
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US addresses with a house number and a state or ZIP. Only the address is used for the check.
In zone AO the lowest floor must be raised above the highest ground next to the building by at least the depth on the map (2 feet where none is shown), and the site must drain water away from the building. In zone AH the lowest floor must be at or above the base flood elevation.
As in every zone starting with A, flood insurance is required with a mortgage from a federally regulated or insured lender. Under Risk Rating 2.0 the premium reflects the property's own risk.
A guide and a planning check, not an official flood zone determination: a lender's determination, or FEMA's own map, is the official answer for a single property.
What does flood zone AO mean?
High-risk shallow flooding, 1 to 3 feet deep, usually sheet flow on sloping ground. The map shows a flood depth rather than an elevation.
What does flood zone AH mean?
High-risk shallow flooding, 1 to 3 feet deep, usually ponding, with a base flood elevation shown.
Is flood insurance required in AO and AH zones?
Yes, for mortgages from federally regulated or insured lenders.