PURPOSE: To rationalize execution of work in site and provide fireproofness to roof decking by a method wherein processes for building a roof deck in which roof decking materials including sheathing roof boards, rafters, purlines and others are normally assembled one after another are mechanized and the roof decking materials are made up in a panel structure formed of frames.

CONSTITUTION: A unitized panel 10 is constructed of a quadrilateral frame 30 serving as a structural body, a face panel 40 serving as a sheathing roof board and fixed to the surface on one side of the frame and a waterproof sheet 50 affixed to the surface of the face panel. The frame 30 is formed of vertical frames 31 and upper and lower frames 32 in a rectangular shape, and the face panel 40 is fixed so that edges 41 thereof can protrude outward beryond the periphery of the frame 30. A joiner 43 in an upside-down T shape is inserted from the underside into a connecting part in which the adjoining face panels 40 are butting each other at their ends, and the adjoining waterproof sheets 50 are overlapped (511 at their ends and joined together over the connecting part. One of four corners of the panel 10 is supported with a fixing plate 22 that is provided over the upper end of a column 20.