How to Replace a Cracked Cast Iron Drain Pipe with PVC

Before beginning any work, gather all necessary tools and materials. Put on your Personal Protective Equipment (PPE), including safety glasses, heavy-duty gloves, and a respirator. Lay down a bucket and drop cloths under the work area to catch any water or debris. To prevent unexpected water flow, advise household members not to use any plumbing fixtures that drain into the line you are working on.

Cast iron is extremely heavy and brittle. To prevent the pipe from sagging or crashing down when cut, you must support it securely on both sides of the intended repair area. Use pipe hangers, sturdy wood blocking, or metal pipe stands to brace the pipe against the floor joists or ground. Ensure the supports are snug and stable before proceeding to the next step.

Identify the cracked section of pipe. Using a tape measure and a permanent marker or soapstone, mark two cut lines. The lines should be on either side of the crack, extending at least 2 inches past the visible damage on each side to ensure you are removing the entire weak area. Make the marks go all the way around the pipe to serve as a clear guide for your cutting tool.

This is the most challenging step. Equip your reciprocating saw with a new blade specifically designed for cutting cast iron, such as a diamond grit or carbide-tipped blade. Carefully begin cutting along the first marked line. Let the weight of the saw do the work; do not force it. It will be a slow process that generates heat and sparks. Once the first cut is complete, repeat the process on the second mark. Be mindful of the saw binding or kicking back.

With both cuts complete, the damaged section is now free. It will still be very heavy. Carefully lower and remove the cut piece of cast iron from the line. Place it directly into a heavy-duty trash bag or onto your drop cloth. Be prepared for residual water and sludge to spill out as you move the pipe.

For the new couplings to seal properly, the ends of the remaining cast iron pipe must be clean and relatively smooth. Use a wire brush and rags to clean the outside surface of the pipe ends, removing any rust, scale, and grime. If there are any sharp burrs from the cut, use a metal file to smooth them down gently.

Measure the exact distance between the two ends of the cast iron pipe. Now, measure your two shielded transition couplings; specifically, measure the distance from the end of the coupling to the internal pipe stop. Subtract the combined space that the two couplings will occupy from your initial gap measurement. This final number is the exact length to cut your new PVC pipe. Use a PVC handsaw or tubing cutter to cut a piece of Schedule 40 PVC pipe to this length. Deburr the inside and outside of the cut ends with a deburring tool or sandpaper.

Take your two shielded transition couplings. Loosen the band clamps on each coupling, but do not remove them completely. Slide one coupling fully onto one end of the existing cast iron pipe. Then, slide the second coupling fully onto the other end of the cast iron pipe. The rubber sleeve should be in contact with the cleaned surface of the pipe.

Take your pre-cut piece of PVC pipe. Insert one end of the PVC pipe into one of the couplings already on the cast iron pipe, pushing it until it hits the internal pipe stop. Now, slide the second coupling from its position on the cast iron pipe over the other end of the PVC pipe until it is centered over the joint. Ensure both ends of the PVC pipe are fully seated inside their respective couplings.

Using a torque wrench with the correct size socket (usually 5/16" hex), tighten the band clamps on both couplings. Most shielded couplings require a specific torque, typically 60 inch-pounds, which is usually printed on the band itself. Tighten the bands evenly, alternating between them to ensure a uniform seal. Do not overtighten, as this can damage the coupling or the pipe.

The final step is to test your work. Go to an upstream fixture (like a sink, toilet, or tub) and run a significant amount of water through the drain line. While the water is running, carefully inspect the new couplings and connections for any drips or leaks. Place a dry rag or paper towel under each coupling to make it easier to spot even the smallest leak. If no leaks are present, the repair is complete. You can now remove your pipe supports and clean up the work area.