In high-risk areas, there is at least a 1 in 4 chance of flooding during a 30-year mortgage. All home and business owners in these areas with mortgages from federally regulated or insured lenders are required to buy flood insurance. They are shown on the flood maps as zones beginning with the letters 'A' or 'V.'

In moderate- to low-risk areas, the risk of flooding is reduced but not completely removed. These areas submit more than 20 percent of National Flood Insurance Program claims and receive one-third of Federal disaster assistance for flooding. Flood insurance isn't federally required in moderate- to low-risk areas, but it is recommended for all property owners and renters. They are shown on flood maps as zones beginning with the letters 'B', 'C' or 'X' (or a shaded X).

Some flood maps also include areas where there are possible but undetermined flood hazards or unstudied areas. Flood insurance rates reflect the uncertainty of the flood risk. These areas are shown on flood maps as areas beginning with the letter 'D.'