PURPOSE: To cover a roof easily by forming rooftop hardening thermally jointable sheet material with nonhardened polymer composition consisting of semicrystalline polymer, filler, working material, and hardening package.

CONSTITUTION: The semi-crystalline polymer of a 100 pts. weight to be formed with polyolefin to be made from monomers showing the crystallization of about 2wt.% or more and having two carbon atoms is regulated. Besides, the filler of carbon black, crushed lime, low-temperature crushed rubber, and the like of about 20-300 pts. weight for the polymer 100 pts. weight is regulated. Besides, the working material of the processing oil, wax, and the like of about 20-150 pts. weight is regulated. Besides, the hardening package of about 1.5-10 pts. weight to be formed with sulfur and sulfur vulcanizing agent and for hardening composition at the temperature of about 50°C at least is regulated. Then, with nonhardened polymer composition consisting of the semi-crystalline polymer, the filler, the working material, and the hardening package, roofing rooftop hardening thermally jointable sheet material is formed.