A roof flashing having a base and a flexible collar operatively connected thereto is configured to prevent water infiltration around an exhaust pipe extending through a roof. The base includes a flat plate having a connecting mechanism extending upwardly from the plate. The collar includes either a fixed-diameter opening or a multi-diameter opening thereto, wherein the opening is configured to receive and seal against a pipe extending from the roof to which the roof flashing is attached. The collar is fixedly attached to the connecting mechanism of the base.