Only residential structures are permitted to be wet flood proofed, meaning water is permitted to flow into the lower grade level. However, commercial structures are not permitted to be wet flood proofed, but they are permitted to be dry flood proofed, which means keeping the water out of the structure. Therefore, a dry flood proofed mixed-use building is permitted to have the lowest finished floor within the floodplain at grade level.
However, dry flood proofed structures must be designed and built to withstand the hydrostatic pressure of flood waters up to the level of the Design Flood Elevation. There are significant and complex regulations that must be followed for structures of this nature, and they require substantial planning and specialized construction techniques. Dry flood proofing, although available as an option, is not always recommended in a floodplain.