a roofing membrane which exhibits improved properties , particularly reduced propensity to blister , is produced by selecting a the group consisting of straight run asphalts with a rod & amp ; ball softening point in the range of from 80 to 130 degrees f . ; oxidized asphalts , solvent washed asphalts , road tars , refined tars , and blends thereof ; selecting a modifying polymer from a group consisting of : styrene - butadiene - styrene block co - polymer , atactic polypropylene , and a combination of sbs and app ; modifying the bitumen by adding the polymer while the bitumen is in a molten state in a sealed mixing vessel ; exposing the modified bitumen to a pressure less than ambient during or after modification while still in a molten state to allow release of entrained gases ; and forming the roofing membrane from the molten modified bitumen .