PURPOSE: To obtain an appropriate roof in a large scale building with large openings for an entrance and exit, by contacting a number of balloons filled with a gas having a lighter specific gravity than air under a water proof sheet to support the water proof sheet and by laying a coarse-opening net under the balloon.

CONSTITUTION: The peripheral edge of a water proof sheet 1 composing the upper face of a roof is fixed along the upper edge of the wall 2 and a coarse- opening net 3 is laid in parallel with each other at an appropriate interval under the water proof sheet 1. And a number of balloons 7 having the diameter shorter than the distance between the water proof sheet 1 and the net 3 but larger than the opening of the net 3, are contained between the sheet 3 and the net 3. In this inside of the balloon 7, helium gas or the like is charged and sealed. The water proof sheet 1 and the net 3 are fixed at some points by fixing members 4 like rope crossing to the both faces to keep a certain interval all along the face.