How to Replace a Main Waste-Vent Stack - Part 1 of 2
A new main waste-vent stack is best installed near the location of the old stack. In this way, the new stack can be connected to the basement floor cleanout fitting used by the old cast-iron stack.
Although a main waste-vent stack rarely rusts through entirely, it can be nearly impossible to join new branch drains and vents to an old cast-iron stack. For this reason, plumbing contractors sometimes recommend replacing the iron stack with plastic pipe during a plumbing renovation project.
Be aware that replacing a main waste-vent stack is not an easy job. You will be cutting away heavy sections of cast-iron, so working with a helper is essential. Before beginning work, make sure you have a complete plan for your plumbing system and have designed a stack that includes all the fittings you will need to connect branch drains and vent pipes. While work is in progress, none of your plumbing fixtures will be usable. To speed up the project and minimize inconvenience, do as much of the demolition and preliminary construction work as you can before starting work on the stack.
Because main waste-vent stacks may be as large as 4" in diameter, running a new stack through existing walls can be troublesome. To solve this problem, our project employs a common solution: framing a chase in the corner of a room to provide the necessary space for running the new stack from the basement to the attic. When the installation is completed, the chase will be finished with wallboard to match the room.
From: The Complete Guide to Plumbing, 978-1-58923-378-2
How to Replace a Main Waste-Vent Stack - Step 1
Secure the cast-iron waste‑vent stack near the ceiling of your basement, using a riser clamp installed between the floor joists. Use wood blocks attached to the joists with 3" wallboard screws to support the clamp. Also clamp the stack in the attic, at the point where the stack passes down into the wall cavity. Warning: A cast-iron stack running from basement to attic can weigh several hundred pounds. Never cut into a cast-iron stack before securing it with riser clamps above the cut.
How to Replace a Main Waste-Vent Stack - Step 2
Use a cast-iron snap cutter or a reciprocating saw to sever the stack near the floor of the basement, about 8" above the cleanout, and near the ceiling, flush with the bottom of the joists. Have a helper hold the stack while you are cutting out the section. Note: After cutting into the main stack, plug the open end of the pipe with a cloth to prevent sewer gases from rising into your home.
How to Replace a Main Waste-Vent Stack - Step 3
Nail blocking against the bottom of the joists across the severed stack. Then, cut a 6"-diameter hole in the basement ceiling where the new waste-vent stack will run, using a reciprocating saw. Suspend a plumb bob at the centerpoint of the opening as a guide for aligning the new stack.
How to Replace a Main Waste-Vent Stack - Step 4
Attach a 5-ft. segment of PVC plastic pipe the same diameter as the old waste‑vent stack to the exposed end of the cast-iron cleanout fitting, using a banded coupling with neoprene sleeve.
How to Replace a Main Waste-Vent Stack - Step 5
Dry-fit 45° elbows and straight lengths of plastic pipe to offset the new stack, lining it up with the plumb bob centered on the ceiling opening.
How to Replace a Main Waste-Vent Stack - Step 6
Dry-fit a waste -‑fitting on the stack, with the inlets necessary for any branch drains that will be connected in the basement. Make sure the fitting is positioned at a height that will allow the branch drains to have the correct 1⁄4" per ft. downward slope toward the stack.
How to Replace a Main Waste-Vent Stack - Step 7
Determine the length for the next piece of waste-vent pipe by measuring from the basement T-fitting to the next planned fitting in the vertical run. In our project, we installed a T-fitting between floor joists, where the toilet drain was connected.
How to Replace a Main Waste-Vent Stack - Step 8
Cut a PVC plastic pipe to length, raise it into the opening, and dry-fit it to the fitting. Note: For very long pipe runs, you may need to construct this vertical run by solvent-gluing two or more segments of pipe together with couplings.
How to Replace a Main Waste-Vent Stack - Step 9
Check the length of the stack, then solvent-glue all fittings together. Support the new stack with a riser clamp resting on blocks attached between basement ceiling joists.
How to Replace a Main Waste-Vent Stack - Step 10
Attach the next waste T‑fitting to the stack. In our demonstration project, the waste-T lay between floor joists and was used to connect the toilet drain. Make sure the waste-T is positioned at a height that will allow for the correct 1⁄4" per ft. downward slope for the toilet drain.