In a house built before plastic, the main stack is a vertical run of cast iron from the roof to the basement, with every upstairs fixture branching into it through a hub and a lead joint at each floor. It carries waste down and air up, it is buried inside a wall for its whole height, and when it fails the result is a leak inside the wall or a gurgling, smelly house. It is also the one large pipe in the house that a camera can inspect from top to bottom without entering any fixture, because the vent terminal on the roof is the top of the stack, and a camera dropped through it sees everything down to the base.
Quick answer: a cast iron stack is inspected by feeding a camera from the roof vent down to the point where it turns horizontal. The findings are scale, which is normal; corroded or loosened lead joints at each floor, which leak and admit gas; holes and channeling at the base, where water hits the invert hardest; a cracked or broken hub at a branch; and a collapsed or missing section. A stack with sound joints and a sound base has years left; one with a corroded base or several failing joints is a replacement, usually in plastic, floor by floor.
Why the stack is inspected from the top
The roof vent terminal is the open top of the stack. A camera fed through it drops with gravity, passes each branch hub, and reaches the base where the stack becomes the building drain. No fixture needs pulling and no wall needs opening. A 23 mm head on a 7 mm rod drops well; a 17 mm head on a thin rod also works and is less likely to catch at a hub. The rod is fed, not pushed, and withdrawn slowly so the hubs are seen on the way up as well. Entering from a roof vent covers the roof work and its safety.
What a sound stack looks like
A dark, scaled wall with a hub at each floor where a branch enters, the lead joint visible as a grey ring in the hub, the branch opening square and dry. Light scale and staining below each branch where water has run. A base that turns smoothly into the building drain with the invert intact. The lead joint guide shows what sound and failed joints look like.
What fails
Joints. The oakum dries and the lead loosens, and the joint leaks outward into the wall and lets gas inward. On camera: loose fibre, a gap at the lead ring, staining below the hub, sometimes an offset where the branch has moved. The base. Where the vertical stack turns horizontal, falling water hits the invert constantly, and this is where cast iron corrodes fastest. On camera: channeling, holes, soil, and often the worst condition on the whole line. Branch hubs. A hub cracked by a renovation, a heavy fixture or a plumber’s hammer shows as a broken opening. The wall itself. Rarely, a vertical crack or a hole in the stack’s own wall, usually from external corrosion where the wall was damp. The terminal. Nests, ice and debris, covered in the vent stack guide.
Judging what is left
A stack with light scale, sound joints and a sound base is not a repair item, and the report should say so. One or two failing joints at upper floors, with the base sound, is a localised repair: open the wall at that floor and replace the joint. A corroded base is a replacement of the base section, which is often done from the basement without touching the upper floors. Several failing joints and a bad base is a full stack replacement, which is a major job in an occupied house and is usually done floor by floor in plastic. Repairing cast iron drains covers the options, including lining a vertical stack, which some contractors do and some do not.
The branches off the stack
Each hub is a branch to a bathroom, a kitchen or a laundry, and the camera from the stack can look into the first few feet of each. Galvanized branches show their rust lip at the hub; plastic replacements show a rubber coupling. A branch that enters at a bad angle or protrudes into the stack catches debris. The stack inspection should note the condition of each hub and what enters it. The galvanized guide covers the old branches.
What to record
The depth from the terminal of each hub and its condition. The condition of the base. Any crack, hole or collapse with its depth. A still at each joint that is failing. The depth readings are what a plumber uses to decide which wall to open, and a stack report without them sends the plumber in blind.
Common questions
How long does a cast iron stack last?
Often a century, because it is not wet all the time. The joints and the base go first; the camera shows how far along they are.
Can a stack be inspected without going on the roof?
From a test tee at the base, the camera can look up a short way. The full inspection is from the terminal.
Can a cast iron stack be lined?
Some contractors line vertical stacks; branches must be reinstated afterward. Replacement in plastic is more common.
Related guides
See sewer line problems in houses built before 1950, the best sewer cameras for cast iron pipes, and cleanout vs roof vent vs toilet entry.