Method of preparing moisture barrier sheet material

A method of preparing a urethane-modified bitumen sheet material adapted for use as a moisture barrier, which method comprises:

• a) preparing a hot, melted bitumen composition which comprises bitumen and a minor modifying amount of a urethane polymer, which urethane polymer is prepared by the reaction of a polyisocyanate with a long-chain polydiene hydrocarbon having terminal groups which react with the polyisocyanate to form the urethane polymer to provide stable nonflow properties and surface tackiness properties to the bitumen layer over the range of about 10.degree. F. to 125.degree. F.;
• b) applying a layer of the melted bitumen composition onto a releasable film, which film may be easily removed from the cooled surface of the bitumen layer prior to use;
• c) applying an outer moisture barrier polymeric film to an exposed surface of the bitumen layer to bond the outer polymeric film to the bitumen layer;
• d) cooling the bitumen layer;
• and
• e) recovering the cooled sheet material for use as a moisture barrier sheet material.