To extend or repair cast-iron drainpipe, make the transition to PVC drainpipe. Several types of no-hub fittings are available to join iron and plastic; check local codes to see which are accepted. A no-hub fitting has a neoprene sleeve and clamps that are tightened around the cast-iron pipe and the plastic piece to hold them in place and make a watertight joint. Such fittings are considered a permanent joint.
Cast-iron pipe is heavy, so work carefully and use a helper. Support pipe with clamps and framing before cutting it. Add clamps above and below the new joint or leave the frame permanently in place.
A full day to support, cut, and install a replacement piece with two banded couplings; an hour or two to install a saddle-tee fitting
Carpentry tools, cast-iron snap cutter or circular saw with metal-cutting blade, felt-tip marker, PVC saw or backsaw and miter box or power miter saw, deburring tool, hex screwdriver or torque wrench
Good carpentry skills, measuring, attaching with bolts
Examine how the existing pipe is clamped; determine how best to hold it firmly as you work
Riser clamps, plastic pipe or fitting matching cast-iron pipe, banded couplings
If it's approved by your local inspector, a saddle fitting is easier and quicker to install. Cut a roughly circular hole in the cast-iron pipe using a grinder equipped with a metal-cutting blade. The hole should be slightly larger than the opening in the fitting.
Position the sheet over the hole and press the plastic fitting over it. Slip a U-clamp around the back of the pipe and slide its threaded ends through the plastic fitting. Screw on the two nuts finger-tight. Attach the other clamp the same way, then tighten all four nuts.