Bathroom renovation: the plumbing to plan before the work starts

A bathroom is planned from what you can see: the vanity, the shower, the tile. But it is the plumbing that decides what is possible and what it will cost, depending on whether the fixtures stay where they are or move.

CAA-Québec says it plainly in its renovation advice: among the costly projects worth reconsidering, it gives “moving the plumbing” as an example. The same guide lists “outdated plumbing or electrical problems” among a project’s common surprises, and suggests setting aside 10 to 15% of the budget for them.

A drain works by gravity. For a relocated fixture to empty properly, its pipe has to reach the existing system while keeping a continuous slope, which depends on the distance to cover, the thickness of the floor and the direction of the joists. In a basement, on a concrete slab, it is often the slab itself that has to be opened.

Venting is the other constraint, and the less familiar one. Chapter III, Plumbing, of Québec’s Construction Code requires that “every trap be protected by a vent pipe” (article 2.5.1.1, as the Régie du bâtiment explains it). The trap is the water seal under each fixture that keeps sewer odours from coming back up. Without venting it can empty, and the new shower smells of sewer. Moving a fixture therefore also means planning how its trap will be vented.

All of this is settled at the rough-in, with walls and floor open, before the waterproofing and the tile: it is the only time a drain moves easily.

On most projects, the general contractor or renovator handles demolition, walls, waterproofing and tile. Plumbing falls under a separate licence. The Régie du bâtiment du Québec (RBQ) is clear: “to carry out plumbing work in Québec, you must hold the appropriate licence”, namely subclass 15.5, Plumbing contractor. Those licences are issued by the Corporation des maîtres mécaniciens en tuyauterie du Québec (CMMTQ), which acts on the government’s behalf.

The RBQ also spells out the owner’s responsibility: plumbing work must be entrusted only to contractors holding that licence. Before signing, ask your general contractor who will do the plumbing, and check that person’s licence in the RBQ’s licence holder registry. According to the Régie, hiring a licensed contractor also gives you access to a financial guarantee in case of a dispute.

There is a nuance for owners who do the work themselves. The CMMTQ explains that an individual may carry out plumbing work in the single-family home they live in with their family, with no rental income. They must still meet the codes and declare their plumbing work to the RBQ, and as soon as someone else does the work, it must be “entrusted to a CMMTQ member contractor holding the appropriate licence”.

We hold RBQ licence 5825-6405-01 and are CMMTQ members.

It is the municipality, not the plumber or the contractor, that decides whether your work needs a permit. The RBQ and CAA-Québec give the same advice: check with your municipality before you start.

In Boucherville, the official permits and authorization certificates page expressly covers construction or renovation work, interior as well as exterior, and points out that “only obtaining the permit or authorization certificate allows you to begin your work”. Some applications are made online; for a residential project, the City answers questions at 450 449-8625.

If you live in another South Shore town, do not assume Boucherville’s rule is yours: call your municipality’s planning department. And, as the RBQ recommends, tell your insurer too before the work begins.

A toilet, shower or vanity installed below street level sits exactly where the sewer backs up when it is overloaded. Chapter III covers this at article 2.4.6.4: according to the RBQ and CMMTQ Bonnes pratiques sheet PL-12, sanitary fixtures located below the level of the street adjacent to the building must have a backwater valve on their drain pipe where that pipe connects to a main collector subject to backflow.

Boucherville goes further than the Code. Article 37 of its by-law 2015-232 applies “whatever the year of construction of the building” and requires a valve on every private horizontal connection at ground-floor level or below street level. The same article bans the insertion (squeeze-in) valve and any valve on the main collector connection, requires valves to stay accessible and be serviced at least once a year by the owner, and states that a plug on a floor drain does not exempt you from the obligation.

A renovation, with walls and floor open, is the best time to install that valve or check the one already there, and to plan its access hatch before the tile seals it in. If the valve is new, it may qualify for the Adaptation component of Rénoclimat, which grants up to $700 for installing a backwater valve after 1 October 2026 by a contractor holding the appropriate licences. Our article on backwater valve subsidies sets out the conditions and the order of the steps.

One last point from CAA-Québec, useful when choosing fixtures: “low-flow toilets are not suited to basement installations”. Discuss the model with your plumber before buying it.

A deep tub or a more generous shower changes the household’s hot water demand. According to the Government of Québec, a bath takes 75 to 150 litres of water, whereas a six-minute shower with a low-flow showerhead needs only 35.

Natural Resources Canada, in its Water Heater Guide, notes that “certain types of appliances, such as large deep tubs and whirlpool baths, require a large amount of hot water”. The guide recommends judging a water heater by its first-hour rating (FHR), the amount of hot water it can supply in one hour starting from a full tank, rather than by tank size alone. That rating should match or exceed the home’s expected peak consumption over one hour.

The guide also warns against the opposite excess: a tank oversized for occasional needs costs more to buy and loses more heat while idle.

Any plumbing work starts with shutting off the water. The RBQ and CMMTQ Bonnes pratiques sheet PL-11 sums up what Chapter III requires: an accessible shut-off valve on the main water service, as close as possible to where it enters the building; a shut-off valve at the supply point of every toilet; a shut-off valve near the water heater, on the pipe feeding it; and, in a building with more than one dwelling, a shut-off valve where the water enters each dwelling.

Access matters as much as the valve itself. The RBQ points out that plumbing equipment must be usable, cleanable and maintainable, and that a shut-off valve must be readily accessible: you must be able to shut off the water quickly, without a step ladder or heavy objects to move. An access panel in a wall or floor is accepted to conceal a valve, provided it allows quick access at all times.

Use the open walls to confirm the main valve still turns and to plan the access panels before the finishing. Our article on the main water shut-off valve explains how to test it.

The right order avoids most bad surprises: tile laid over a badly placed drain, a permit applied for too late, a valve forgotten behind a finished wall.