A pontoon is a flat-bottome boat or the floats used to support a structure on water. It may be simply constructed from closed cylinders such as pipes or barrels or fabricated as boxes from metal or concrete. These may be used to support a simple platform, creating a raft. A raft supporting a house-like structure is one form of houseboat. The basic design is usually implemented as a simple catamaran or, with three rows of floaes, a trimaran.