Disclosed herein are various waterproof membranes that may be applied to a sill surface of an opening in a building structure to waterproof that surface as well as direct water that penetrates through the opening out of the structure. Generally, one or more flexible waterproof membranes are adhesively applied to the sill surface for purposes of waterproofing that surface. Such membranes may further incorporate a back dam structure that is disposed proximate to the rearward edge of a sill plate (i.e., disposed toward the interior of the structure) to direct water towards the front edge of the sill plate (i.e., toward the outside of the structure). Further, such membranes may include wicking material on one or more surfaces to allow for directing water out of a building structure. In various arrangements, one or more flexible membranes may be utilized to create an open fronted sill pan.