What to Do About Cast Iron Drain Lines
How It Fails
Cast iron does not fail all at once. It corrodes from the inside, thinning along the bottom of horizontal runs where waste sits, until the base of the pipe is a shell. A vertical stack often has decades left while the horizontal branch beneath a bathroom floor is close to gone, and the two are frequently assumed to be in the same condition.
Finding Out Rather Than Guessing
A camera inspection is inexpensive relative to the decision it informs, and it is the only way to see the invert of a buried line. Tapping a pipe and hearing something solid proves nothing about the underside. This is the classic case where measuring beats assuming, because the wrong guess is discovered after the floor is closed.
Replace the Section, Not Always the System
When a bathroom floor is already open, replacing the branch under it is comparatively cheap and it removes the item most likely to fail next. Replacing an entire stack through occupied floors is a different order of disruption and is only justified when the inspection shows it is warranted.
Joining Old to New
Modern plastic waste joins to cast iron with shielded no hub couplings, which are reliable when the cut is clean and square and when the coupling is properly torqued. Both pipes need support close to the joint, since a heavy iron section hanging on a flexible connection is a leak with a timer on it.
The Other Thing to Check
While the floor is open, look at the vent. Bathrooms of this age were often altered by earlier owners in ways that left a fixture wet vented through something it should not be. Correcting it costs very little at that moment and is unreachable afterwards.