To repair an existing roof or an existing roof floor by the mutual thermal welding of thermoplastic resins in a portion in which edges of a plurality of improvement bodies made of the thermoplastic resins including fibers overlap one another.

When heat is applied to the improvement body 1, thermoplastic resin yarn 3 is melted and put into fiber yarn (fiber body) 2. When the improvement body 1 is cooled, it becomes rigid. When a portion 4, wherein the edges of the plurality of improvement bodies 1 overlap one another, is heated, the improvement bodies 1 are connected and integrated together, so as to cover the existing roof or the existing roof floor with no space between. A hook bolt F of the existing roof or the existing roof floor is inserted through a hole 7 of the improvement body 1, covered with a cover 8, held by a cylindrical leaf spring 9, and covered with a cap 10; and the cap 10 is thermally welded to the improvement body 1, so that the connected and integrated improvement bodies 1 can be firmly fixed onto the existing roof or the existing roof floor.

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