What a stack is, and why it fails from the inside out
Sewer stack pipe lining starts with what a stack is. A waste stack is the vertical spine of a building drainage system. Fixtures on each floor drain into horizontal branch arms, the branch arms tie into the stack, and the stack carries everything down to a base elbow where it turns and becomes the building drain. Above the top fixture the same pipe continues upward as a vent through the roof. In buildings put up before the mid-1970s that entire assembly is usually cast iron, joined with hub-and-spigot fittings packed with oakum and lead.
Vertical pipe does not fail the way buried horizontal pipe does. There is no soil load, no root intrusion and very little settlement. What a stack gets instead is decades of waste falling down one side of the bore. The flow clings to the wall rather than filling the pipe, so corrosion concentrates in a narrow vertical band, and the metal there thins while the rest of the circumference still looks sound. Add the sulfide attack that happens above the waterline in a warm building and the result is a pipe that is close to full thickness on three quarters of its circumference and nearly gone on the other quarter.
The symptoms that point at a stack rather than a drain
Blockages that move between floors, gurgling at a fixture when something two stories up is used, and slow drains that clear and return are stack symptoms rather than branch symptoms. The one that settles it is staining: a rust-colored weep on a ceiling or in a chase, appearing at a floor line and not at a fixture, means the stack is leaking at a joint or through a thinned wall rather than a trap or a supply line being at fault.
Why replacing a stack is so expensive
A vertical pipe is buried in the building rather than in the ground. Reaching it means opening chases, cutting into closet walls and bathroom tile on every floor it passes, and doing that in units somebody lives or works in. The pipe itself is a small part of that budget. Restoration, protection, dust control and the loss of the spaces involved are most of it, and the schedule is set by access to occupied floors rather than by plumbing. Lining exists to take almost all of that away, since CIPP lining for buildings in Denver works through openings that already exist.
A stack is lined from the top down, not the bottom up. Gravity is doing the work. The liner is introduced at a roof vent or an opened section near the top of the run and drawn or inverted downward through the stack, which means roof access and a clear path through the vent are prerequisites, not details to sort out on the day.
How the liner goes in
The sequence begins with cleaning, and on cast iron that means mechanical descaling rather than water alone. Chain flails or a rotating cutting head are run down the bore to take the tuberculation and hardened scale back to sound metal, and the debris falls to the base of the stack where it is collected. A camera then runs the full height to confirm the wall is continuous and to mark the footage of every branch arm. The liner is impregnated with resin, introduced at the top, pressed against the host wall by air or water pressure, and cured in place. Once cured, the old cast iron is a jacket around a new jointless pipe.
The two hard parts: branches and the base
- Branch arms. Every fixture connection is a hole the liner covers over. Each one has to be cut back open from inside with a robotic cutter, located against the footage marks from the pre-lining video, and each reinstatement has to be confirmed on camera afterward.
- The base elbow. The turn at the bottom takes the impact of everything that falls down the stack, so it is usually the most eroded fitting in the building and the hardest place to get a liner to seat cleanly.
- Offsets. Stacks in older buildings jog sideways to clear structure. A liner negotiates a gentle offset and does not negotiate a sharp one.
- Vent transitions. The pipe above the top branch carries air rather than waste and is often thinner, which changes where the lined section should sensibly start and stop.
What the building has to give up while it happens
Water does not have to be shut off, because the stack carries drainage rather than supply, but drainage does. Every fixture that ties into the stack being lined is out of service from the start of cleaning to the end of reinstatement, which for a single stack is generally a working day. In a building with several stacks the work is phased so that only one line of bathrooms is down at a time. Understanding how a cured-in-place liner is installed and cured in advance makes the notice to occupants much easier to write, and verified lining installers serving Denver will walk the risers with a building manager before quoting rather than after.
Closing the job out on paper
A stack liner should close out with a pre-cleaning video, a post-cleaning video showing bare sound metal, a post-lining video running the full height, and a separate confirmation of every branch reinstatement with its footage mark. That last item is the one most worth insisting on, because a branch that was only partly reopened will drain acceptably for a while and then stop, at which point the evidence needed to diagnose it no longer exists.
Common questions in Denver
Can a vertical sewer stack really be lined?
Yes. The liner is introduced at the top, usually through a roof vent or an opened section near the top of the run, and drawn downward through the stack before curing. Gravity assists the installation, and the cured liner becomes a jointless pipe inside the old cast iron.
Do the bathrooms have to be gutted?
No, which is the point of lining. The work is done through the existing pipe and through the roof vent, so chases and tiled walls stay closed. Access is needed at the top of the stack and at the base, not on every floor.
How long is a stack out of service?
For a single stack, generally one working day covering cleaning, lining, curing and reinstatement of the branch connections. Fixtures tied into that stack cannot be used during that window, so buildings with several stacks are phased one at a time.
What happens to the fixture connections the liner covers?
Each one is cut back open from inside by a robotic cutter, guided by footage marks taken during the pre-lining camera run. Every reinstatement should be confirmed on the post-lining video before the crew leaves the building.
Is the base of the stack lined too?
Usually it needs separate attention. The elbow at the bottom absorbs everything falling down the stack and is often the most eroded fitting in the system, so it may require a spot repair or replacement before the vertical run above it is lined.