Repairing a roof vent stack involves replacing the rubber grommet that prevents water from seeping in around the base of the stack. The old rubber is cut away from the base of the vent, and then the grommet and flashing are pulled over the stack and secured.
Grommets are damaged by exposure to the sun. They crack, crumble and pull away from the metal roof stack, creating places for water to get in. Using a utility knife, cut the old grommet away from the flashing. The flashing is the metal square that holds the grommet in place, and the entire assembly is called a roof boot.
The old flashing can remain in place, but the shingles on the top end need to be loosened so that new flashing can be tucked under them. The top end, which holds the shorter end of the collar, is inserted under the shingles. The rubber grommet is pushed over the roof vent stack until the flashing is flush with the roof.
Some of the shingles around the top might have to be trimmed for a neater fit. The flashing is then nailed down on the two sides and along the bottom. The new rubber grommet fits snug against the stack, but use a caulking agent to strengthen the seal.