Drain relocation and venting

In a wood-joist floor, a drain can often move within the same joist bay without touching the structure. In a concrete slab — like the recent towers in Ville-Marie or the old industrial buildings in Le Sud-Ouest — the drain is cast into the concrete: moving it means cutting the slab, which changes the price and the timeline completely.

The slope needed to reach the waste stack and the distance to a vent limit how far anything can move, whatever the floor type. A sink can usually travel farther than a toilet before it loses its slope or its vent.

We assess the floor, the structure and where the existing stack sits before confirming a move is possible, and we say so plainly when it isn't.

The decision happens in stages, not at a glance. We open the floor to see which way the joists run relative to the move you want — a drain that has to cross several joists at a right angle costs more and sometimes means notching each joist within what's allowed, while a move that follows the joist direction usually stays inside a single bay. With roof trusses or I-joists, the answer can be no right away: those aren't solid lumber, and cutting them without an engineer's sign-off can weaken the structure. That's the kind of constraint you only see once the floor is open, never on a drawing.

The move has to be confirmed before the framer closes the new partition walls, and it can only be confirmed once demolition is done and the floor is exposed. A design locked in before the floor is opened — based only on the old layout — usually needs adjusting on site. Once the drain is moved and tested, the framer can close the walls around it, and the tiler and cabinetmaker follow the confirmed plan instead of the original sketch.

In a condo, moving a drain in a slab usually touches a shared stack serving several units, and cutting the slab can affect the ceiling of the unit below. The syndicate generally needs notice before the opening, water has to be shut off for the affected units at an agreed time, and noisy work — cutting especially — is often restricted to set hours under the building's rules. Building that coordination into the schedule keeps the job from sitting idle waiting on approval that could have been requested earlier.

Price mostly tracks floor type — a joist bay costs noticeably less to open than a slab does to cut — and the distance the drain has to travel while keeping its slope. When natural slope isn't enough over that distance, a lift pump (ejector) sometimes becomes the only option, which adds equipment to the project. Final fixture positions need to be confirmed before the floor is opened, because moving a drain a second time after the slab or floor is closed means redoing work that's already been paid for.

A site visit is the only reliable way to confirm what's possible. The plumber needs access to the floor already opened up — or, failing that, whatever joist or slab plan exists — to measure the real distance to the stack and the slope available under the current flooring. Coming to that visit with the new locations already marked on the floor, in chalk or tape, speeds up the answer, because confirmation or refusal can happen on the spot instead of after a second visit once demolition is further along. When the answer is no for the spot you wanted, there's almost always a nearby position that does respect the available slope and venting — it just needs to be checked before the rest of the trades move ahead.