How to Replace a Whole-House Plumbing Trap

In this video, This Old House plumbing and heating expert Richard Trethewey shows tools and techniques for fixing corroded cast-iron pipe.

Steps:

1. Wrap the chain of the soil-pipe cutter around the cast-iron water main, positioning it close to the plumbing trap.

2. Tighten the chain, then ratchet the handle back and forth to cut through the pipe.

3. Break loose the opposite end of the plumbing trap by striking it with a 3-pound sledgehammer. 4. Remove and discard the old trap.

5. Use a ball-peen hammer and cold chisel to chip off the old lead and oakum from the end of the water-main pipe.

6. Use a no-hub coupling to join together an angled cast-iron fitting and a clean-out fitting. Tighten the stainless steel clamps with a torque wrench.

7. Use the soil-pipe cutter to cut a length of cast-iron pipe to fit between the existing water-main pipe and the new clean-out fitting.

8. Join the new pipe section to the existing water main with a no-hub coupling.

9. Set the clean-out fitting into place against the new pipe section.

10. Measure for a new length of pipe to span from the other end of the existing water main to the angled fitting attached to the clean-out fitting. Cut a length of cast-iron pipe to fit.

11. Join the cast-iron pieces together with no-hub couplings.

Expert Insight: Richard Trethewey, This Old House plumbing and heating expert, explains the old sealing method you’ll encounter when chipping off the joint: “Oakum is a sealing method used for the old lead connections. It’s actually oiled hemp.” Carefully chisel away both the lead and oakum to prepare a clean surface for the new no-hub coupling connection.

Why Replace with a P Trap: Richard Trethewey explains that the modern P trap — named because it resembles the letter P — is self-scouring: as water leaves, it cleans out the trap so no sediment stays there. It still maintains the critical water seal that keeps sewer gas from coming up inside the building, and it’s very easy to clear with a wire if a blockage does occur.

Pro Tip: Richard Trethewey notes that when using the soil-pipe cutter on old cast iron, there’s a risk the worn pipe may break away unevenly: “What I’m worried about is that this pipe may be worn and the bottom may break away and not break clean.” Take care to position the chain cutter precisely and tighten it evenly to get the cleanest cut possible.