Older home plumbing ranges in material make-up from copper and lead to cast iron. These pipes are being replaced in new construction with PVC pipes, which last a long time with very few complications. If you must replace an old section of cast-iron pipe, PVC is easily installed and connected to the iron with a coupling fitting and PVC glue.
Turn off the water to the house. Turn the valve clockwise to shut it off.
Cut out the damaged piece of cast-iron pipe with a diamond-tip blade and remove it.
Place the cut pipe next to PVC pipe of the same diameter. Mark the length of the iron pipe onto the PVC pipe. Cut the PVC pipe with a fine-tooth saw, so the segment's length matches that of the damage iron piece.
Clean the ends of the PVC pipe with PVC cleaner. Rub it on and let it dry.
Insert the couplings onto the cast-iron pipe openings.
Brush PVC adhesive onto the ends of the PVC pipe and slide the pipe onto the couplings to close the gap in the plumbing.
Tighten down the bands of the couplings to seal them to the PVC and the cast-iron pipe ends. Turn the water back on and test the pipe for leaks. If you see a drip, tighten the couplings down a little more.
Wear safety glasses when cutting the pipes to avoid getting shards in your eyes.