main stack in basement - cast iron stack repair

How to Make a Cast Iron Stack Repair (in 4 Steps!)

Have you identified a plumbing problem that requires a cast iron stack repair?

If so, take heed. It’s one of the most important pipes in your home. And replacing or repairing a cast iron stack isn’t an easy or fast project. It takes effort and at least an intermediate level of plumbing know-how.

But if you’re up for the challenge, here’s how to do it. Let’s start with the basics.

What is a Plumbing Stack?

A main plumbing stack is the primary vertical drain that smaller drains flow into to remove waste from your home. The length of the stack is from the highest fixture (such as a faucet or shower) to the lowest fixture (such as a washing machine in the basement). Its width in residential use is typically 3 or 4 inches in diameter.

cast iron stack repair - removing cleanout plug
Example of a portion of a cast iron plumbing stack located in a basement. The stack in this case goes into the concrete flooring and connects to the main sewer line. It also goes to the top of the house and sometimes vents through the roof.

How Does a Plumbing Stack Work?

The stack collects wastewater from smaller, horizontal drains that connect to it.

Being a vertical pipe, it takes full advantage of gravity to deliver wastewater to your sewer line and away from your property. Because it’s a straight pipe that uses gravity, it rarely clogs, unlike the horizontal drains that connect to it.

diagram of plumbing system - how to make a cast iron stack repair
The plumbing stack (vertical pipe on left) has many names: main stack, drain stack, soil vent pipe, soil stack, drain-waste-vent, ventilated discharge pipe, and more. But whatever you call it, this large vertical pipe’s purpose is to collect wastewater coming from smaller drains and then deliver it to your main sewer line.

Cast Iron Stack Repair in 5 Steps

Because this isn’t an easy project, we encourage you to get assistance from a friend. If you have a plumber friend, even better. In any case, this is a solid DIY method for making a cast iron stack repair.

1. Support (Brace) the Stack

  • You are about to cut out a piece of a heavy, damaged pipe. Without any support of the existing stack, gravity will bring the rest of the stack down. Not a good idea.
  • Purchase nylon straps at your local hardware store or plumbing supply house.
  • Attach the straps to the vertical pipe at the nearest fitting or joint.
  • Secure the top end of the pipe to a joist (a supporting beam).

2. Remove the Broken Pipe

  • Do not run water in the house during this repair.
  • The easiest way to cut cast iron pipe is with a ratchet pipe cutter, available at an equipment rental center.
  • Before cutting vertical drainpipe, support it with riser clamps or 2x4s braced against a joint that is above the section to be removed.
  • With chalk, mark the area to be cut, wrap the chain around the pipe and hook it into the body of the tool.
  • Tighten the knob, turn the dial to “Cut,” and work the handle back and forth until the cutting discs bite through the pipe.
  • If badly corroded pipe crumbles under the pipe cutter (there’s a good chance it will), rent an electric saber saw and metal-cutting blade instead.

3. Cut the Replacement Pipe

  • Immediately after removing the damaged section, stuff newspapers or paper towels into the open ends of the pipes to block dangerous sewer gas.
  • Measure the gab in the pipe and transfer that measurement — less 1/4 inch — to a cast-iron or PVC replacement pipe.
  • Lay cast iron pipe across two level 2x4s, spaced to support the pipe ends, and cut it to size with the ratchet cutter or saber saw.

Pro Tip: Although this article is about a cast iron stack repair, we recommend replacing at least the damaged section with PVC pipe. It’s durable but lighter and easier to work with (cutting only requires a handsaw).

cast iron stack repair - plumbing stack before and after
The stack on the left is cast iron. On the right, you can see that the cast iron (the portion in the basement) has been replaced with PVC pipe. This is a lighter but durable plastic material that is perfect for use in a stack repair or replacement.

4. Fit the Replacement Pipe

  • The replacement section joins to the two standing pipes by means of hubless fittings. These are neoprene sleeves secured by stainless steel clamps.
  • Slide a clamp onto each standing pipe and tighten them temporarily in place.
  • Slip the neoprene sleeves onto each pipe until the ends bottom out inside the sleeves.
  • Fold the lip of each sleeve back over the pipe.
  • Work the replacement pipe into the gap between the sleeves until it is properly seated.

5. Complete the Repair

  • Pull the folded lips of the sleeves over the replacement pipe. Loosen and slide the clamps over the sleeves, center them over the joints, and tighten with a nut driver or socket wrench.
  • Run water through the stack to test the repair.
  • If a joint leaks, take apart and reassemble the fitting.

Pro Tip: The above is considered a major repair. But what if you simply want to patch a leak in a cast iron pipe fitting? First, turn off all the water. Now use a chisel or wire brush to remove any corrosion around the leak. Next, fill the area with cast-iron repair paste and let dry (per instructions on the label). Turn the water back on and test for leaks.

cast iron repair paste for leaky stack
Caution: cast-iron repair paste should be reserved for minor leaks around fittings. Even then, keep a close eye on it after you’re finished. If leaks continue, try a full-scale repair as instructed above or call a plumber.

Final Thoughts on Cast Iron Stack Repairs

If you’ve gotten this far, congratulations. You are a true DIYer who’s with no fear of a big bad pipe like a plumbing stack. 😉

Just remember, if you need help or run into unforeseen problems with your cast iron stack repair, reach out to a trusted plumber. They’ll be happy to help.

Call 1-Tom-Plumber

Don’t hesitate to contact us here or call us at 1-Tom-Plumber (1-866-758-6237) if you need help with a drain repair or replacement, including your stacks and vents.

1-Tom-Plumber’s certified team of plumbers and drain technicians respond immediately to any emergency plumbing, drain cleaning, or water damage problem. We also handle the excavation of underground water lines and sewer main lines. Our immediate-response team is available every day and night of the year, even on holidays.

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