To improve workability by supporting a floor material laid on a drainage ditch by arranging a support member in the drainage ditch composed of a drainage slope inclined face of a steep slope arranged on a backing layer.

A flat roof F of a concrete structure is constituted by laying a walking floor material 5 on the upper surface of a waterproof backing layer 1 formed on a building frame C so that a drainage ditch 7 is arranged on the backing layer 1 in the vicinity of a parapet P. In this case, a steep slope-shaped drainage slope inclined face of a downward slope is formed toward the parapet P on a heat insulating material 2a of the bottom part of the drainage ditch 7. Next, a panel-like walking floor material 5a is horizontally supported in a flush shape with an adjacent walking floor material 5b on the upper surface by arranging a cross-sectional L-shaped floor support member 8 in the drainage ditch 7. Water flowing along the reverse of the floor material 5 is flowed down in the drainage ditch 7 by arranging plural water passing holes in a support member 8 when necessary. Therefore, a cost can be reduced by efficiently/easily performing execution work without requiring a complicated-shaped gutter member and complicated connecting work as usual.