On your roof the vent pipes for your plumbing system extend through the roofing material. When the rubber collar goes bad a leak will occur. To replace the flashing and rubber collar follow these steps.
First you'll need to remove the shingles that cover your vent flashing's flange. To remove shingles, use a shingle ripper or hacksaw to cut any nails holding them in place and remove them. If any are damaged, it is a good time to replace them with new ones. Then cut the nails anchoring the vent flange and lift it off.
Place the new flashing and flange over the vent pipe and secure with galvanized screws Place them to the outside so that they will be covered when you replace the shingles.
Replace the shingles, making sure to cover any nail and/or screw heads with roofing cement. Place a generous bead of cement around the top of the flashing where it meets the vent pipe.