How to Replace Stacks and Roof Flashings

Removing the Old Stack

• 1

  Remove the metal flashing -- which helps keep moisture from getting into the house -- around the portion of the vent stack pipe that extends through the roof. Use a flat pry bar to lift up the shingles and remove any nails.

• 2

  Cut the old section of the vent stack off with a reciprocating saw, if the stack is PVC pipe. Use a snap cutter, a large tool that looks like a bolt cutter, if the stack pipe is cast iron.

• 3

  Place riser clamps, which will hold the cast-iron pipe in place after it's cut, between the joists. Have a helper assist you in removing the stack from the opening in the roof.

Installing New Pipe

• 4

  Place the new PVC stack pipe on top of the old pipe. Have a helper hold it in place. Go on the roof and have the helper push it to you through the opening. Ensure that the pipe is between 1 to 2 feet above the roof. Cut to length if necessary.

• 5

  Apply roofing cement to the underside of the new flashing. It should set in about 10 to 20 minutes. Place the flashing over the pipe. Lift up the shingles at the top of the flashing, and slide it under them.

• 6

  Go back to the attic and permanently connect the new pipe to the vent stack. Connect two PVC pipes with an appropriate size fitting. Apply primer and solvent to the pipe end and stick the fitting on the fixed section of pipe. Place the new section of pipe in the other end of the fitting. Use a special fitting called a "banded coupling" to connect old cast-iron stack pipe to PVC pipe.

• 7

  Return to the roof and press the flashing so it fits tightly against the roof shingles. Fasten the flashing to the roof with flashing nails.