looking inside old cast iron pipe

Kendall "aka Renos"

Founder, Lead Instructor of Renos 4 Pros and Joes and Bathroom Update Guide

When repairing a cast iron drain pipe, we have to locate and cut out the damaged cast iron pipe and replace it with new drain pipe. I’m standing here in the unfinished basement of a 65-year-old ranch style home, and I’m about to repair the cast iron waste line by replacing several sections of rotten cast iron with PVC pipe. (As an aside, if you are located in the Atlanta Metro area and need cast iron pipe repair, you can contact me here.) As I am making my material list, the homeowner asks me some additional questions about the cast iron repair process and one of his first questions was, “How to cut cast iron pipe?”

There are 3 types of tools that work well for cutting cast iron pipes: Soil Pipe Cutter, Angle Grinder, and Reciprocating saw. A soil pipe cutter is a heavy-duty tool that works with a ratcheting mechanism. It can make an unlimited number of cuts, but it is heavy (20 lbs or more) and expensive (over $1000), but it may be a good option if you can rent one. The next option is an angle grinder with a cutting wheel. This is a much more economical option, it’s also compact, and can efficiently cut cast iron pipe. The downside is that because it’s driven by a spinning wheel, it creates a lot of dust. The third option is a reciprocating saw with a diamond grit blade which is designed to cut cast iron. On average a reciprocating saw costs a bit more than an angle grinder. The reciprocating saw makes far less dust when it cuts. The reciprocating will more easily cut cast iron pipes that are in the ground either in a yard or under a concrete slab.  

Best Way to Cut Cast Iron Pipe

So now that I’ve told you the best tool to use to cut the cast iron, there is one more question that we need to answer: What type of saw blade?” I’m so glad that you asked. I end up doing these types of cast iron repair and replacement jobs off and on on a pretty consistent basis. Up until doing this job, I always reach for a diamond grit blade. But on this job I discovered a blade that cuts much faster and lasts a lot longer. The best blade for cutting cast iron is a Diablo Carbide Tipped Reciprocating Saw Blade (click here to see it on Amazon).

What is Cast Iron Soil Pipe?

In older homes, in the US generally built before the ’80s, it’s common to find big black heavy metal looking pipes below your floors and in your walls and sticking out from the top of your roof. This is cast iron pipe and it was used to create the plumbing drain systems in properties for many years. Over time cast iron can corrode and rot. Sections of the pipe that are horizontal to the floor are typically more susceptible to rot than vertical sections. But vertical sections fail also, particularly those that are under kitchen sinks, where many homeowners pour out all types of chemicals.

Cast iron was not just used above ground but also in yards and underneath concrete floors in homes and buildings. This pipe can fail over time as well. Repairing or replacing cast iron in the ground or under concrete has an added complexity of having to uncover the cast iron to repair or replace it, as well as putting back the soil and or concrete once the work is complete.

Before we begin cutting out any cast iron pipe we need to first determine whether not we will be replacing all of the cast iron pipe or only portions of it. There are several factors that should be considered when making this decision. The first factor to consider is the overall condition of the cast iron pipes. If most of the pipe is in good condition and there’re only a few spots that are visibly rotten you may be able to get by with replacing only a few sections of the cast-iron waistline. However, in other situations, you may be able to visibly determine that the majority of the plumbing waistline has failed and/or is rotten.

Determine the Location of the Leak

You want to make sure you know where the water is coming from before you jump in to repair mode. You don’t want to take on a larger project than you have to and then figure out later that you misdiagnosed the problem and have to do even more work.

If you happen to notice a leak under a bathroom you aren’t sure if the pipes are leaking or if the tub is leaking, be sure sure to check out my post about the different places a bathtub can leak.

The next factor to consider is the budget for the job. An ideal world we would always go in and replace all of the cast iron pipes with new materials. But in reality, the budget and inconvenience may not make a complete replacement a feasible option for every property owner.


Should I Repair or Replace Cast Iron Pipes?

In many cases, I repair actually does involve replacing some sections of the damaged pipe. The bigger question is what to do about the pipes in the floors and the walls? If you are doing a full scale remodel and the walls are already open, then replacing the vertical cast iron stacks makes sense. But if you aren’t then just do the repair. Replace the bad sections and get back to your life, but keep it on your radar.



How to Replace Sewer Pipe in the Basement?

The old tried and true method is to cut the concrete floor and remove and replace the pipe. This can be expensive and is pretty involved and can get messy. This is why most property owners never want to replace the sewer pipe that’s under concrete. Not even when they are doing a whole house remodel, or even when replacing all the rest of the drain pipes in the rest of the house. If you chose to wait for the drain pipe in the slab to fail before replacing it, you need to be aware that it is a possibility and that there is no telling when it could happen. Additionally, if and when it does happen, you will likely have to address the problem very quickly to keep from ruining your house, so at a minimum, you need to have an action plan in place.


IMPORTANT: If the section that you are replacing is vertical or in any way attached to a vertical cast iron stack, then extreme precaution must be taken to stabilize the vertical sections prior to any pipe removal. Not properly supporting the vertical sections could result in serious injury if the pipes slip or move.


For this repair, I replaced a section of cast iron pipe as opposed to the entire plumbing drains system. I identified the areas where the pipe was failing. I supported all the sections of pipe that I was cutting. I didn’t want the sections of pipe to break off and fall after I made my initial cut, which I knew was a possibility because the cast iron pipe was brittle and weak in areas where I was cutting. I marked my locations and then used a reciprocating saw to cut through the cast iron pipe.

I chose to use the reciprocating saw for several reasons. First, I already had one, so there was no additional cost to me outside of buying a pack of diamond grit reciprocating saw blades. I should also state that I also have access to an angle grinder, but I just didn’t want to create a bunch of dust, particularly since I was working inside of a house.


Additionally, I prefer to use a reciprocating saw to cut PVC pipe, so I was planning to use the saw twice instead of potentially bringing more than one tool to cut the different pipe materials. A reciprocating saw is a very versatile tool.  Be sure to check out my post where I share  14 different uses for a reciprocating saw.


After removing the large damaged sections of cast iron pipe, I had to come back in and do my precision work with a hammer and chisel and reciprocating saw. One section of cast iron pipe I replaced was in a joint. And it just so happened that one of the more severely damaged sections was close to a joint. With that said, the pipe ended up breaking off and leaving a jagged section and the joint hub.


How to Separate Cast Iron Pipe Joints?

Removing the cast iron at the joint was the most difficult part of this particular job. Sections of the cast iron pipe are assembled using lead and oakum. This is assembly is done with the high heat from a torch and creates a very very strong bond. The difficulty arises when you are trying to separate two sections of pipe without damaging the pipe you are saving.

This cast iron pipe hub removal had another layer of complexity to it because the hub was only about 5-6 inches below the floor joist. This made it impossible for me to access the top of the joint so that I could use my chisel to separate the two pieces. Instead, I had to cut and chisel and hammer the hub until I could get it to break away.

Using a hammer and chisel can be nerve-racking because you do not want to break or crack the cast iron that you are planning to save. But on the other hand, the pipes are very well assembled and are not easy to take apart, and therefore require some force behind your hammer strikes. There is definitely an art to it. I used hard taps. You also have to keep in mind that because this portion of the work was close to the ceiling I had to work with my hands and arms above my head. This can be really taxing on your shoulders.

Once I removed the hub, I cleaned up the undamaged sections of pipe to get them ready for the repair. Part of the section that I replaced was in a 90 degree turn off of a vertical section of pipe. I used a 90-degree elbow with a long sweep to ensure the best water flow through the direction change. This also helps prevent build up in the pipes.

I also installed a new cleanout, so that in the future if there was ever a blockage in that section of the pipe, the cap could be removed and the pipe could be inspected and drain cleaning tools can be inserted into the plumbing drain system from that point which should make it easier to clear a drain blockage.


Joining PVC Pipes and Fittings

I used a rubber coupling to join the remaining cast iron to the section of new PVC. I also used purple pipe cleaner to prepare the PVC joints that I glued together. I then used the red PVC multi-purpose glue to assemble the PVC. Once the glue goes on you have to work fast to get the pieces firmly together. The PVC pieces fit together snugly, and there are stops inside the elbows and couplings that only allow the pipe to go inside a certain distance. You have to make sure that the pipe is pressed all the way in to ensure that the pipes are properly seated. Next, I allow the assembled sections of PVC pipe to sit and cure.

Because a good fit between the pipes and fittings is essential, its also important that you cut your sections of pipe clean and straight. You want the sections of pipe that you cut to look just as smooth as the uncut ends as possible. Remove any burs and debris before applying pipe cleaner. Also, beware the purple pipe cleaner is really messy stuff. It drips easily and stains almost anything that you get it on.

Next, I dry fitted all of my assembled plumbing drains to ensure that everything fit together properly. I then used my level to check all the pipes to ensure that my horizontal sections have adequate slope. Its all about water flow. We want the pipes to have enough slope or fall on them to allow the water to drain out with gravity and not allow water to stand in the pipes.

Once I have everything the way I want it, I tighten everything down and then test for leaks. Once my leak test is complete and the repaired plumbing drains pass with flying colors the job is complete.