PURPOSE: To improve waterproof performance by integrally forming a paint film waterproof layer of a fiber-reinforced plastic on the bed face of a roof or a balcony floor.

CONSTITUTION: Floor joists 8 are provided along the girder direction of a balcony, and a cushion sheet 9 is stretched on the floor joists 8. A lower bed plate 10 is mounted on the sheet 9, and an upper bed plate 11 is installed on the lower bed plate 10. A slender wood 12 is fitted to a corner portion between the upper bed plate 11 and a siding member 6. Fiber sheets made of glass fibers are installed at the rise section of the siding member 6, the inside face and bottom face of a trough gutter 16, the inside of a spandrel wall 15, and the rise section of a siding member 14. A polyester resin is sprayed on the fiber sheets and is infiltrated and hardened, then a heat insulating material 25 is provided on the back face of the lower bed plate 10 to form a balcony floor 7.