What is a 100-year floodplain?
A 100-year floodplain is land with a 1% chance of flooding in any given year. It does not mean the land floods once every hundred years.
In more detail
FEMA calls this the 1% annual-chance flood or the base flood, and maps it as the Special Flood Hazard Area: zones A, AE, AH, AO and the coastal V zones.
Because the chance resets every year, two 100-year floods can happen in back-to-back years. Over a 30-year mortgage, the chance of at least one is about 26%.
Maps also age. Many were drawn from older rainfall records and land that has since been paved, so the real floodplain can be wider than the map.
What it means when you buy
Buildings in the 100-year floodplain need flood insurance for most loans, and local rules limit how you can build or expand. Check which buildings are inside it, not just the parcel.
How the free flood check shows it
The free flood check shows the 100-year and 500-year floodplains over the parcel and lists which buildings fall in each.
Check the flood zone for an address, free
Enter any US address to see its FEMA flood zone and a map of the flood areas around it. Add your email to see which buildings sit in the flood zone and the risk numbers.