Concrete slab or wood joist: two different realities
In a wood floor, the drain runs between two joists, inside a single bay. Moving it from one side of that bay to the other is usually simple; crossing several load-bearing joists is a different matter, one that can call for reinforcing the structure.
In a concrete slab — a recent tower in Ville-Marie, or an old industrial building converted in Le Sud-Ouest — the drain is cast into the concrete. Moving it means cutting the slab along the new path, resetting the drain, then patching the slab back. It's doable, but the price and the timeline change completely.
| Floor type | What's generally possible | What complicates a move |
|---|---|---|
| Wood joist | Moving within the same joist bay, often without touching the structure | Crossing several load-bearing joists |
| Concrete slab (tower, converted industrial building) | A move is possible by cutting the slab along the new path | Higher cost and timeline, the slab has to be made good again |
Slope and venting limit the distance
A drain needs to keep a constant slope toward the main stack; the farther it moves from that stack, the lower it has to drop, and the space under the floor isn't infinite. A sink, with its smaller drain, generally tolerates more distance than a toilet.
The vent needs to stay within a reasonable distance of the fixture so the drain flows properly instead of siphoning the water out of the trap. A plumber works that distance out before confirming a move, not after the floor has already been opened.
Condos add their own rules
When the drain crosses a structural slab shared with the unit below, or ties into a common stack, the condo syndicate often has a say: notice to neighbours, allowed work hours, the plumber's insurance. It isn't a structural engineering question but a building-bylaw one, and it gets checked before work starts, not during.