Bitumen-based pressure sensitive adhesive compositions are provided which comprise bitumen, oil, and an ionomeric elastomer. The adhesive compositions are useful as self adherent waterproofing layers, particularly in roofing applications. The use of the ionomeric elastomer permits formulation of the adhesive to have improved tack at the lower temperatures encountered in such applications while maintaining desired flexibility, shear strength, and flow resistance. Also provided are waterproofing membranes comprising the inventive adhesive compositions.