Cast-iron drain stack repair and replacement

The cast-iron drain stack carries waste from every floor's toilets, sinks and showers down to the sewer. Installed from the 1920s to the 1950s, it lasts a long time, but electrolytic corrosion and the acidity of wastewater eventually thin it from the inside, especially in the bottom 60 cm near grade, where foundation moisture adds to the problem.

A crack at the base often shows up as a persistent smell rather than a visible leak, because the water seeps into the foundation rather than pooling on the floor. We confirm with a camera inspection before proposing either a full replacement or a partial replacement of the affected section, tied into modern PVC.

On a three-storey plex, replacing the full stack means cutting off plumbing access to every unit during the work — we plan the sequence to limit downtime and give each tenant notice before the date.

We confirm the material before proposing anything. Cast iron sounds dull and heavy under a wrench tap, it's painted or simply blackened with age, and a magnet grips it firmly — unlike PVC, which is light, white or beige, and sounds hollow. A section already swapped to PVC inside an otherwise cast-iron stack points to an earlier partial repair; we need to know where it ties in before planning the next step, because two poorly sealed joints on the same stack almost always act up again.

The call gets made with a camera, not a guess. One thinned section near the base, with the rest of the stack still sound, gets fixed with a partial replacement tied into PVC. Corrosion spread over several metres, or a stack that's already had more than one spot repair over the years, calls for a full replacement — patching a stack that's near the end of its life keeps costing more than just redoing it once.

Price mostly tracks the number of floors served and the length of stack being redone: full replacement runs roughly $1,800 to $3,500 per floor. A finished basement adds time because a wall or a dropped ceiling has to be opened and closed back up, where an unfinished one gives direct access. A plex where the stack serves three separate units also takes more coordination than the same job in a single-family house, mainly to limit how many days each unit goes without running water.

On streets close to the Lachine Canal, part of the ground was filled in during last century's canal works, and that fill keeps settling slowly under the foundations. That differential settling puts extra lateral stress on the stack's joints near grade, which is part of why base cracks turn up a bit more often there than in an area with more stable soil — one more thing a camera inspection can confirm before choosing between a partial repair and a full replacement.

One detail few homeowners anticipate: PVC carries the sound of falling water much more than cast iron, which is heavier and denser. On a stack that runs behind a bedroom wall, we often wrap the new section in an acoustic sleeve so that every flush upstairs doesn't come through clearly below — a small addition to the job that heads off a complaint after the fact.

More insurers now ask for the age and material of the drain stack at residential policy renewal, especially on properties that have never been renovated. A camera inspection report, even without immediate work, serves as a useful reference to answer that question without guessing.