A waterproof liquid membrane is a high quality sealant, a thin layer of water-tight material that is laid over a surface. This layer is designed to fill, seal and level grout lines, voids, seams and surface damage on construction. Simply put, it does not allow water to pass through it. For example, on a flat terrace, a waterproofing membrane could be laid above the structural slab and then layed over with the finished tiles.