A well-renovated bathroom is an investment that lasts 20 years or more. A badly renovated bathroom is a set of problems that start within 6 months and often cost more to repair than they would have cost to do properly the first time.
The difference comes down to details most homeowners never see — and that a number of contractors underestimate. Not because they're dishonest, but because they aren't plumbers. And plumbing is precisely what determines whether your new bathroom ages well or badly.
In this article I'll walk you through the pitfalls I see in the field, when to bring a plumber into your project, and why hiring a CMMTQ member isn't only a matter of quality — it's also a legal matter.
First things first: the fundamentals you can never skip
Before we even talk plumbing, we need to talk preparation. This is where a lot of renovations start off on the wrong foot.
Walls must be straight and square. If you install a shower or a tub against walls that aren't perfectly aligned, you'll have sealing problems at the joints. The caulking will crack, water will get behind the tile, and three years later you'll have mould inside the wall.
The floor must be level. This is especially critical in older houses or older apartments, where floors have often developed a slope over the years. You can correct it two ways:
- With a self-levelling compound: it's fast, gives an excellent result, and leaves you a perfectly flat surface for laying tile and setting your tub or shower.
- By building the floor up: more laborious, but sometimes necessary when the slope is too pronounced or you want to adjust the overall height.
Why does this matter so much? Because your fixtures (tub, shower, toilet) are designed to be installed level. Set them on a sloped floor and you create mechanical stresses that eventually turn into leaks, cracks and failed joints.
Once your walls are straight and your floor is level, the lifespan of every fixture multiplies. That's the foundation. Without it, everything else is compromised from the start.
Do you need a permit?
A lot of people wonder whether a bathroom renovation requires a municipal permit. The honest answer: it depends on the scope.
A simple cosmetic refresh (paint, changing a faucet, replacing a toilet) generally doesn't require a permit. But as soon as you touch the wall structure, move supply lines or drains, or redo the whole bathroom, a permit becomes likely, if not mandatory.
Rules vary between South Shore municipalities. Before you start, check with your city — Saint-Bruno, Longueuil, Brossard and Boucherville each have their own requirements. A permit that should have been pulled and wasn't can complicate the sale of your property, and even cause problems with your insurance in the event of a claim.
The legal argument everyone forgets: the CMMTQ plumber
Here's a point most homeowners aren't aware of: in Quebec, plumbing work is a regulated activity. Whether the renovation is done by you, a general contractor, a handy brother-in-law or a subcontractor, the plumbing itself must be performed by a plumber who is a member of the CMMTQ (Corporation des maîtres mécaniciens en tuyauterie du Québec).
General contractors carry exactly the same responsibility you do on this: if they touch the plumbing without being qualified, they're in violation. Some try to do the connections themselves anyway to save money. That's exactly where the problems start.
Why does this regulation exist? Because bad plumbing work can cause:
- Hidden leaks that destroy the structure over years
- Sewer backups that contaminate your home
- Pressure problems that damage your appliances
- Code violations that block you at resale
- Denied insurance coverage in the event of a claim
If you hire a general contractor for your renovation, ask who does their plumbing. A serious contractor works with a CMMTQ plumber they know and have an established relationship with. A contractor who answers vaguely is a red flag.
The 5 most common bathroom plumbing pitfalls
These are the mistakes I see most often when I inspect renovations. None is catastrophic on its own — but each one can save you an expensive repair later.
1. Bad copper-to-PEX connections
In a renovation, you often need to join existing copper piping to new PEX (flexible plastic) lines. That connection has to be made with the right fittings and the right technique, or it leaks or loosens over time.
I've seen under-tightened fittings, transitions without support sleeves, and sharp angles that create stress. At first it seems watertight. Six months later, moisture starts appearing behind the wall — and you won't see it until the damage is substantial.
2. A missing or badly placed shut-off valve
Every fixture (toilet, sink, bathtub, shower) should have an accessible local shut-off valve. Why? Because the day you have a leak or a repair to do, you want to be able to cut the water to that specific fixture without shutting down the whole house.
Far too often, I see:
- Installations with no shut-off valve at all
- Valves placed behind non-removable panels
- Valves forgotten behind the tile
It's a $50 saving that will cost you hundreds of dollars the first time you need to work on it.
3. A badly seated wax ring under the toilet
The wax ring under the toilet is what seals the fixture to the drain. If it's badly seated — off-centre, poorly compressed, set on a flange that's too low or too high — it leaks progressively.
The worst part? The leak is invisible. Water seeps under the toilet, into the subfloor, and you don't find out until the floor starts to buckle or the ceiling below stains.
It's one of the most common renovation mistakes. Plenty of people set their own toilet thinking it's simple. Technically, it is. Except that the small details (alignment, flange type, wax type, compression) make all the difference over 10 years.
4. Drain lines without proper strapping
When you install new bathroom drainage (drains, venting), the pipes must be solidly fastened to the joists and walls with pipe straps. That's a plumbing code requirement.
Why? Because an unsecured drain can shift, fall out of alignment, eventually lose its correct slope, and create backups or leaks at the joints. Over the years, vibration and temperature changes do the rest.
I've seen installations where the drain line simply rested on a wood shim. It holds for a while. But it isn't compliant, and it isn't durable.
5. Code non-compliance
This is the catch-all category, but it counts. The Quebec Construction Code defines precisely how every plumbing element must be installed: minimum diameters, drain slopes, required venting, distances between elements, supply line heights.
A renovation done by a non-plumber almost never meets 100% of the code. Not out of bad faith — simply from not knowing the rules. And the day you sell your house, or your insurer investigates after a claim, those violations can cost you dearly.
A CMMTQ plumber knows the code by heart. It's the job. That's why the law requires their involvement.
When to bring the plumber into your project
A lot of people think you only call the plumber at the end, to "hook things up." That's exactly backwards. Here are the moments a plumber needs to be there:
Before demolition — for planning
This is the most important moment, and often the one people skip. Before you even demolish the old bathroom, a plumber can help you:
- Verify the feasibility of your changes (moving a shower may require modifying the main drains — a major job)
- Anticipate surprises (old corroded piping, badly sloped drains)
- Establish a real budget for the plumbing, not a guess
That 30 to 60 minute consultation can save you thousands of dollars in surprises during the build.
Before the walls close — for the rough-in
Once demolition is done and the walls are open, that's the moment to install the new plumbing. All the supply lines, drains and vents have to be in place before the walls are closed back up. Otherwise you end up having to reopen everything later.
It's also the right moment to run pressure tests on the new connections, before everything is concealed.
Before tiling — for accessible valves
Before tiling starts, the plumber verifies the location of the access valves and shut-offs. If a valve is badly placed and ends up behind the tile, you'll be forced to break your beautiful new finish the first time you have a problem.
This step is too often overlooked, and it's exactly the kind of detail that gets expensive a few years down the road.
My final advice
A bathroom renovation is a project that requires several trades: demolition, carpentry, plumbing, electrical, tiling, finishing. Each has to do its work at the right moment, in the right order.
If you're working with a general contractor, confirm they work with a real CMMTQ plumber, rather than "managing" the plumbing themselves. If you're running the project yourself, bring the plumber in at the planning stage — not at the end, when it's too late to correct mistakes.
A good renovation is one that gives you 20 years of peace. A bad one brings problems back in the first year. The difference comes down to details that look insignificant — but change everything.
Planning a bathroom renovation? I can advise you from the start of the project, do the code-compliant installations, and coordinate with your contractor. Call me at 263-382-8612 or request a free quote.