To provide a method of forming a waterproofing membrane on a roof garden, according to which a urethane waterproofing membrane having water vapor permeability is formed over the entire waterproofing objective surface having a large area, on which the roof garden is to be formed, while adjusting the film so as to have a positively uniform thickness of 10 mm or more, preferably 20 mm or more.

According to the method, a film thickness setting member 52 having a predetermined thickness or a predetermined height is placed on the waterproofing objective surface 23 on which the roof garden is to be formed, of a building, like a roof floor of a concrete building, and a liquid mixture 23 containing a fast curable urethane base compound 3 and a hardener 4 thereof which are mixed beforehand is sprayed to the waterproofing objective surface 23 along a current of compressed air. The spraying process of the liquid mixture 22 is continued until the accumulated height of the liquid mixture 22 becomes almost the same thickness or height of the film thickness setting member 52, and by hardening the liquid mixture 22 accumulated until the film thickness setting member 52 is embedded therein, the urethane waterproofing membrane 57 is formed.