A floating cover roof for a storage tank has a frame assembly and multiple floating units. The frame assembly has a holding frame and an outer rim. The holding frame has multiple longitudinal beams and transverse beams. Each longitudinal beam has a connecting seat, a first holding segment and a second holding segment. Two of the transverse beams are connected to two adjacent longitudinal beams to form a locating area between the beams. The floating units are mounted in the locating areas of the frame assembly, are connected to the beams of the holding frame and each has an outer casing and a body. The outer casing is mounted in one of the locating areas of the holding frame, is connected to the adjacent beams and has a casing body and a top board. The body is mounted in the outer casing between the casing body and the top board.