PURPOSE: To obtain an excellent heat-insulating waterproof effect, by superposing joints made of thin films of foamed styrene stuck to each other and sticking them on a roof of the uppermost floor and further, laying a waterproof sheet thereon.

CONSTITUTION: A bout 1mm thick foamed styrene 1, 2, 3, 4 on the market, on which thin films are stuck to each other, are laid on the roof of an uppermost floor with an overlap distance of about 10cm of the joint. Waterproof sheets 5, 6, 7 are stuck on the foamed styrene 1, 2, 3, 4 with thin films stuck to each other for a double-waterproof protection. Accordingly, the sheet- waterproofing safety is made double and hence, it blocks high humidity and low temperature outside air to improve the living environment.