Q&A / 

Replace Cast Iron With PVC

Replace Cast Iron with PVC TIPS

Tim the Master Plumber

Before we go any further, you need to know I'm a master plumber as well as a builder.

I got my master's certification back when I was about 29 years old. I've always loved to install plumbing as well as threaded black iron pipe. I cut my teeth doing many a remodeling job where I had to crack into a cast-iron drain stack and plumb in a new branch line for a bathroom or kitchen remodel job.

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I discovered in no time that cast iron drain piping was durable, it was soundproof and it lasted for decades.

What's more, modern cast iron pipe is made differently than the old cast pipe. A hundred years ago they actually had molds they poured the molten iron into to make the pipe. These molds could have places where the side walls of the pipe were thinner than other spots. These thin sections could rust away.

Modern cast-iron pipe is made by spinning the metal and the pipe wall thickness is uniform.

Cast Iron vs PVC

If I were building a new home today, all of my drain lines would be modern cast iron. All my vent lines would be PVC. Watch my plumbing vent line video below to see this complex system of pipes that exists in the walls and ceilings of your home!

Plumbing Vent Video

PVC pipe is noisy. If a person flushes a toilet and the drain stack is not wrapped with sound insulation, you'll hear the water crashing against the sides of the pipe as it goes down the wall.

You'd think there's a waterfall inside your home it's so loud. You never hear this if you use cast iron.

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Lanny's Question

Lanny, who lives in Campbellsville, KY, asked me a great question about old cast iron drain pipe.

"I'm going to renovate my bathroom and my main drain line for the water and sewer is the old black cast iron pipe. Is it against the code to keep the cast iron pipe or do I have to replace it with the PVC pipe?"

My Answer - You Already Know What I'll Say

Well Lanny, since it's impossible for me to memorize all the different plumbing codes across the USA, I can't tell you what the inspectors in your city would require. While there's a model plumbing code baked into the building code, cities and towns can and do modify it to their own local practices.

But here's the bottom line. Cast iron is a wonderful material. If I was building a new home today, and I plan on doing one in the next few years, it will have cast iron drain lines.

Cast iron is quiet. The issue with PVC is that when water cascades down a vertical stack it sloshes around and it's very noisy. Some who hear it think there's an actual leak inside a wall it's so loud.

It can last for hundreds of years. It's the most durable interior drain pipe you can install.

Newer cast iron pipe is made so the wall thickness is uniform. Old true cast iron pipe could suffer from thin-wall syndrome, but that's no longer an issue.

It's also easy to work with if you're creating a new branch line. You can use stainless-steel no-hub clamps to make leak-proof connections in case you want to run PVC into the cast iron.

I vote that you keep the cast iron and add new branches as needed. You can add new cast iron to old as well as blend PVC into the system if you want.

I want to add that my vent system in my new house will be PVC. But any drain line that handles water and waste flow will be cast iron.

How to Glue PVC Pipe Video

PVC Fittings Video

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7 Responses to Replace Cast Iron With PVC

  1. What about the iron drain pipes in the wall to drain sinks and tubs in old buildings. They have 90 degree junctions and very if not impossible to snake. They accumulate layers of rust and crud. Can they be cleaned ? I had one rusted through at the threaded joint and the whole pipe fell off. I presume you were just talking about the large 3 to 4 black iron pipe that was put together with lead and oakum.

    • I've worked on a number of homes that have been rehabbed. Typically the cast iron/galvanized is replaced with PVC except for the portion under the concrete floor which should be cleaned out at the same time the new is added to the system.

      Sometimes when an older home was converted from septic to (city) sewer the older cast would remain in place and the slope would be changed alluding to a situation Tim describes below.
      That is an accident (stoppage) waiting to happen.

      Running a second floor PVC toilet drain horizontally in the ceiling above the living room is a recipe for jokes at best (about the noise) and the potentially embarrassing sounds at worst.
      Won't happen with cast iron.

  2. I had a cast iron drain pipe from my septic tank to the drainage fields. At the time I bought the house it was 13 years old. At the time of closure all the drains in the house were backup. After investigation it was found that the cast iron pipe in the septic system had rusted on the inside and had block the water flow. Rotor router reuttered it out and it was fine. Ever several years I would have the same problem and till finally I had cast iron pipe replaced with PVC and the problem went away. So cast iron maybe good inside but not for outside.

    • John,

      I can tell you for a fact the cast iron pipe was not clogging due to rust. Those clogs were probably due to a flat piece of pipe or one where the slope was not enough to get the waste to the tank. Cast iron has a proven track record with respect to being a pipe that does *not* create blockages. If you want a pipe that does do that, then use galvanized iron pipe! It's the worst pipe to use for drains because it does corrode and block itself off.

  3. Please comment on the use of new cast Iron connected to a 3"
    copper drain line. The 50 year old copper drain line leaked(corrosion or
    electrolysis ?) about 10 years after bath remodel using cast iron ahead
    of the original copper drain line. Should remaining copper drain line be
    replaced? Thanks, Gordon

  4. It's funny how people in the US worry about loud PVC pipes but totally noisy and loud A/C or heating is absolutely ok.

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