PURPOSE: To prevent rainwater from penetrating under a floor, by providing a fitting groove opened oppositely to the wall face of a building, at the side edge of the floor material and fitting an elastic material in the fitting groove to push out the wall face.

CONSTITUTION: A fitting groove 31 opened oppositely to the wall face 11 of a building is longitudinally formed at the opposite side edge of the floor material 3 against the wall face 11. An elastic material 4 made of synthetic rubber of which end face is positioned to contact the wall face 11 is fitted in the fitting groove 31. The elastic material 4 is provided with a protrusion 4a and a hollow part 4b to be easily compressed when it gets in contact with the wall face 11. Accordingly, the construction is easily and rapidly carried out and water dripping between the wall face 11 and the floor end is secured and hence, secure water tightness can be kept.