A roof flashing for forming a weather-proof seal about pipes of different diameters extending through openings in building structures, the flashing has a collar having a conical shape with a bottom edge and an apex end for passage of the pipe, the collar comprising a wall of elastomeric material wherein the wall has annular grooves forming tear lines for removing a portion of the collar allowing the collar to sealingly engage different diameter pipes. In addition, the roof flashing has a base having a tapered cylindrical shape formed by a wall of elastomeric material, the base having an opening formed by a top edge wherein the top edge is coupled to the bottom edge of the collar, the base further having a bottom edge. Furthermore, the roof flashing has a foot that has an annular opening wherein the annular opening is coupled to the bottom edge of the base, the foot having a substantially rectangular shape such a front portion of the foot is installed atop one or more shingles of a shingle roof and a back portion of the foot is installed beneath one or more shingles of the shingle roof, wherein the roof flashing has a longitudinal opening defined by opening members that extend from the apex of the collar to an edge of the foot thereby allowing the flashing to be spread apart and placed about a pipe, whereupon opening members are pressed together and secured by coupling members that seal the longitudinal opening.