Replacing a cast iron stack means removing the old pipe; lining it keeps the old pipe as the form for a new one to cure inside. Both end with a stack that carries waste the way it should, but they ask completely different things of the iron already behind your wall or under your floor. Replacement doesn't care what shape that iron is in, because none of it stays. Lining depends on it: on the wall holding its thickness, the joints holding their position, and nothing downstream left out of the plan.
The Pipe Wall Sets What Either Path Can Do
Replacement sidesteps the wall-condition question entirely. Cut the old cast iron out and the new PVC or ABS starts at full strength, with nothing inherited from what came before. Two different methods get sold as lining, and they ask different things of that same wall. Cured-in-place pipe pulls or inverts a resin-saturated tube through the cleaned host and cures it into a new, close-fit pipe that can be designed to stand on its own once it hardens; it needs a continuous, round path to cure against, not a surface to stick to. A brush-applied epoxy coating is different: it's built up in layers directly on the cleaned iron and depends on bonding to that wall, which makes it the method where wall condition matters most.
For either method, that distinction matters most where the wall has actually thinned rather than just scaled. A liner bridges a soft spot or even a small hole, but it doesn't put iron back where corrosion already ate through the pipe wall. If the host pipe has lost enough of its original thickness along the bottom, where waste sits longest, a brush coat has nothing sound to bond to, and a cured liner is left carrying loads the design may not have been sized for. Where the wall reads solid on camera, and the loss is mostly interior scale, lining has real material left to work with, and the line still has to be cleaned to bare, sound iron before either method goes in. That prep step is also where a marginal wall gets exposed: a section that looked intact under years of buildup sometimes turns out thinner than expected once it's cleaned down, and at that point the plan shifts from lining to replacing that section outright.
Shape matters as much as thickness. A liner has to take the form of whatever it's pulled through, so a section that has sagged into a belly deep enough to hold water, or a section that has partly caved in, doesn't provide a consistent host for it to conform to. That kind of deformed run is a replacement candidate regardless of how the rest of the stack scores, since a liner can only reproduce the shape of the pipe it's installed in, not correct it.
The Joints and Branch Tie-Ins Decide What a Liner Can Reach
An original cast iron stack is built from hub-and-spigot sections, each joint historically packed with lead and oakum, with branch fittings tying in the wye and closet-bend connections from every floor along the way. A liner is pulled or inverted through a continuous run between two access points, so it passes over every branch opening along that run. Getting each one working again is a separate step once the liner has cured. A joint that has separated or shifted out of line is the real problem for a liner, not one that has simply corroded in place, because the liner reproduces whatever step or gap is already there instead of bridging it invisibly.
That cuts both ways. A liner run through clean, aligned joints leaves each joint sealed beneath a continuous new surface, closing off the gas and seepage paths that a lead-and-oakum joint can open over time. Replacement remakes every joint and every branch connection with new fittings as part of the job, so there's no question of whether the new material can reach a tie-in; it's built to reach all of them by definition. A stack with clean, tight joints and only a few branch connections is a much easier lining candidate than one with several tie-ins stacked close together or a joint that has already shifted out of line.
Downstream of the Stack: The Building Drain and Its Trap
Where the stack lands at the bottom matters as much as the stack itself. A stack in a house built before about 1960 is usually cast iron, and the building drain past its base often still runs through a U-shaped house trap set in the same years, so a plan that covers only the stack is really covering half the job: the trap has carried the same waste for the same decades and does not improve on its own. A house built after about 1970 usually has no trap past the stack, so there the stack is the whole decision.
That coupling belongs in the plan from the start, because a liner run through the stack stops where the stack ends.
Lining Opens Less Wall Until the Branches Cluster
Replacement generally opens the wall bay or floor cavity along the stack's full run because the old pipe has to be removed in sections before the new pipe goes in. Lining works through existing cleanouts and access points wherever the run allows it, which is why it is often pitched as the less invasive option: fewer new openings, less patching afterward, less disruption to finished space around the stack.
That gap narrows quickly on a run with several branch tie-ins close together, since each one still needs to be exposed and reconnected once the liner is in place.
Don't cut or knock loose a section of cast iron stack yourself. Everything above the cut can drop once it loses support, and old lead joints shed lead when disturbed. A plumber supports the stack above before any cut.
A Lined Stack Hides Its Failure Longer Than a New One
A new replacement stack fails, if it fails at all, the way any new installation does: a workmanship issue at a joint, usually within the first stretch of use, and usually visible fast because it is fresh work someone is watching. A lined stack fails differently. The failure point tends to be a branch reconnection or a spot where the liner didn't cure cleanly against the host wall, and because the liner presents a smooth interior right up until that point, water can be working its way between the liner and the old pipe for a while before anything reaches the surface as a backup or a stain.
That gap widens further in a house that is only used in stretches, with weeks or months passing between visits. A branch reconnection that starts weeping behind the liner during one stay leaves the whole gap soaking the framing until the next visit, so what greets the next arrival is already a stain, not a sound anyone was there to catch. A house lived in every day works the other way: a gurgle or a new damp patch gets flagged within days, because somebody is standing at the fixture when it happens.
| Decision Dimension | Replacement | Lining |
|---|---|---|
| Pipe wall condition | Not a factor; old material is removed | Needs a continuous, round host to cure against |
| Joints and branch tie-ins | Rebuilt new; every connection remade | Liner spans and seals them; each branch reopened afterward |
| What's downstream in the building drain | Addressed only if scoped separately | Not touched at all; stops where the stack ends |
| Access required | Wall or floor opened along the full run | Mostly through existing access points |
| How a failure shows later | Usually early, at a fresh joint | Can stay hidden behind a smooth liner for a while |
Frequently Asked Questions
No, not in practice. A house trap is a tight U-shaped turn, and a liner needs a run it can be pulled or inverted through smoothly. The sharp reversal in a trap does not give the resin tube a path to follow the way a straighter run does, so trap work almost always means physically cutting it out and re-piping that section, even on a job where the stack above it gets lined.
Yes. The liner's wall sits a little inside the old iron, so each branch opening is cut back open from the inside with a cutting tool, leaving a small lip at the junction. A good crew trims that lip smooth and seals the junction, because an unsealed edge is where water gets behind a liner.
Yes, more than most people expect. Cast iron's mass dampens the sound of water moving through a drain stack, and a liner cured inside that same iron keeps that mass in the wall. A full PVC or ABS replacement carries the same flow noticeably louder unless the new pipe is wrapped or insulated as part of the job, something a stack replacement doesn't automatically include.
Yes, and it's common in practice. One clean, straight section is lined, while a section with a bad joint or a branch cluster elsewhere on the same run is cut out and replaced in the same job.
At a tight bend near a tie-in, a liner is often stopped short of that point rather than pushed through it, and the fitting is cut out and replaced with a new one, joined to the end of the liner with a mechanical coupling. Forcing a liner through a sharp turn next to a branch risks folding or thinning the resin unevenly right where it needs to be strongest.
If the house has one, it's a vent pipe tied into the building drain at or just before the trap, running up to a small capped opening outside near the front foundation. It only does its job when paired with the trap that holds water on one end. Pull the trap and leave that inlet open, and there's nothing left between it and the sewer, so it becomes an open vent for sewer gas right at the front wall. It gets capped or removed in the same job the trap is removed from.
Settle stack versus lining before a bid gets written — an assessment of wall condition, joints, and the trap downstream, done in one visit. AT Plumbing Services serves Scarborough, South Portland, Portland, and surrounding areas. Call (207) 707-3170.