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In this video, This Old House plumbing and heating expert Richard Trethewey shows how to replace a badly corroded cast-iron flange.


1. Close the shut-off valve behind the toilet to stop the flow of water.

2. Remove the lid from the toilet tank. Flush the toilet and hold down the flush lever to drain as much water from the tank as possible.

3. Reach inside the tank and unscrew the ball float.

4. Use a large sponge to sop up the remaining water from inside the tank and from the bottom of the toilet bowl.

5. Remove the nuts from the closet bolts on each side of the toilet base. If the nuts are rusted in place, cut through the bolts with a close-quarter hacksaw.

6. Unscrew the water-supply line from the underside of the toilet tank.

7. Rock the toilet back and forth to break its wax seal with the drainpipe. Lift the toilet and carry it out of the room.

8. Use a stiff-blade putty knife to scrape away the wax gasket from the closet flange.

9. Unscrew and remove any extension rings to expose the cast-iron closet flange.

10. If the flange is badly corroded, use a hammer and cold chisel to chop out the flange.

11. Pull the loosened flange out of the cast-iron drainpipe with a pair of pliers.

12. Vacuum up all dust and debris from around the drainpipe and bathroom floor.

13. Set four wood blocks around the drainpipe to hold the new closet flange at the correct height.

14. Install a plastic expansion closet flange into the cast-iron drainpipe.

15. Use a ratcheting hex-key wrench to tighten the four screws on the inside of the flange until the flange is tightly wedged into the drainpipe.

16. Slide two new closet bolts into the slotted keyways in the flange.

17. Place a rubber toilet gasket over the bolts and onto the closet flange.

18. Set the toilet back into place on top of the gasket.

19. Onto each closet bolt put a plastic washer, stainless-steel washer, and hex nut. Alternately tighten each nut with a wrench, being careful not to crack the toilet base.

20. Trim off the excess bolt with the hacksaw.

21. Snap on the plastic caps to conceal the closet bolts.

22. Reattach the water-supply line to the underside of the toilet tank.

23. Screw the ball float back onto the flush valve.

24. Open the shut-off valve and flush the toilet to ensure it's working properly.