The most expensive call I get is from a homeowner whose two-year-old bathroom leaked behind the tile and ate the floor joists.
It is always the same story. Cheap contractor, no membrane, "we just used green board." Two years later they smell something. The bathroom looks fine on the surface. Behind the tile, the wall cavity is wet, the studs are spongy, the subfloor below the shower has rotted, and the joists under the bathroom are starting to fail.
The bill to make it right dwarfs what it would have cost to do it correctly the first time. Proper waterproofing is a small line in the original scope; the remediation is a major project on its own.
This is the math nobody walks you through at the showroom. Let me do it now.
Why "just green board" fails
Green board is moisture-resistant drywall. The paper face is treated with wax, the gypsum core has additives. It is rated for "areas of high humidity," meaning above sinks, in laundry rooms, around toilets. The Ontario Building Code does not allow it as a tile backer in wet areas (showers, tub surrounds).
People use it anyway because it is cheap and goes up fast. Tile gets stuck on with thinset. Grout goes in the joints. Looks fine. Looks fine for a year. Looks fine until the grout cracks somewhere, water gets behind, and the green board starts to soften.
Tile is not a waterproof system. Grout is not waterproof. Thinset is not waterproof. The waterproof layer has to be behind the tile.
If your contractor is putting tile directly onto green board in a shower, the bathroom has a 2 to 5 year shelf life. After that you are running a slow leak you cannot see.
What proper waterproofing looks like
There are two systems we use on every bathroom renovation. Both are accepted by manufacturers and code, both work, and both add maybe 2 days and a modest materials-and-labour line to a typical bathroom.
Schluter-Kerdi (sheet membrane)
Schluter is a fabric-and-polyethylene membrane that gets bonded to the substrate with thinset. The shower wall goes up as 1/2-inch cement board (DensShield or Hardibacker), then the entire wet area gets covered in Kerdi sheets, with seams overlapped 2 inches and bonded with thinset. Penetrations (shower valve, drain) get manufacturer-supplied sleeves and corner pieces.
Done right, the system holds water. We have flood-tested Schluter showers and watched them sit full of water overnight without a drop coming through. That is the standard.
RedGard (liquid-applied membrane)
RedGard is a paint-on elastomeric waterproofing. Same goal, different application. Substrate goes up (cement board), then RedGard gets rolled or troweled on in two coats, with reinforcing fabric at all corners and seams. Dries red, then darker red. The colour change tells you the cure is complete.
RedGard is great for irregular shapes, curbless showers, and benches where Kerdi corners get tricky. We use both depending on the geometry.
The cost difference, in proportion
A typical Ottawa bathroom reno: tub surround plus a separate shower, 60-square-foot wet area total. A Schluter system on top of cement board adds only a small fraction to the project over the cheap green-board approach, and RedGard is similar. Across a full bathroom build, proper waterproofing lands at roughly 4-7% of the project cost. It is one of the smallest line items on the proposal and the one that protects everything else.
The bathroom in Nepean we ripped out
Couple in Nepean called us in early 2024. They had renovated their main bathroom in 2021 with a contractor they found through a friend. They chose the cheapest bid on the table. Cheap.
By 2023 they noticed two things. The grout in the shower had cracked along three lines. And there was a dark spot on the ceiling of the dining room directly below the bathroom.
They had a plumber check, no leak from the supply lines. The dark spot grew. They called us.
We opened the dining-room ceiling. The joists above were saturated, two were starting to rot at the bearing point. The subfloor under the shower drain was so soft you could push a screwdriver through it. The wall cavity behind the shower was wet from floor to ceiling. Mould had grown across the back of the green board for the entire shower wall.
What did the original contractor do? Tile directly onto green board. No membrane. Caulked the corners with silicone (which lasts 18 months in a wet area). When the silicone failed, water ran behind the tile, soaked the green board, ran down the back of the wall, and found the joist cavity.
The remediation scope, line by line:
- Full demolition of the bathroom
- Mould remediation by a certified company (3 days, sealed enclosure, HEPA filtration)
- Replace 4 floor joists, sister 2 more, replace the subfloor section
- Repair the dining-room ceiling, repaint
- Rebuild the bathroom (this time with Schluter)
Their original "savings" on the waterproofing line cost them a full structural remediation and rebuild, plus the dignity of explaining to their kids why they could not use the bathroom for 6 weeks. The remediation bill ran many times what proper waterproofing would have added at the start.
The signs your bathroom is leaking right now
If your bathroom is more than 5 years old and the contractor was on the cheap end, run through these checks tonight.
- Look at the grout. Any cracks, especially at corners and changes of plane? Water is getting in.
- Push on the tile. If any tile flexes or sounds hollow, the substrate behind has failed.
- Check the ceiling below. Stains, soft spots, peeling paint directly under the bathroom indicate active leaking.
- Smell the bathroom in the morning. A musty smell after 8 hours closed up means moisture in the wall cavity.
- Look at the silicone caulk. Black spots or peeling means the seal is gone.
If you check more than two of those, you have a problem. The earlier you act, the cheaper it is.
What to ask before signing a bathroom quote
Three questions stop bad bathrooms before they start.
- "What waterproofing system are you using behind the tile?" The answer should name a product (Schluter-Kerdi, RedGard, Wedi, Aquadefense). If the answer is "we use green board" or "tile and grout is waterproof," walk away.
- "Will the system be flood-tested before tile goes on?" A Schluter shower can be plugged at the drain and filled with 1 inch of water. If it holds for 24 hours, the membrane is good. Real contractors do this. Bad ones skip it.
- "Who supplies the warranty if the membrane fails?" Schluter has a 25-year written warranty when installed by a Schluter-trained installer. RedGard has a similar program. The contractor should be able to name the program.
The cheapest bathroom you ever build is the one you waterproof properly. Every other version is a deferred bill.
Pay for the membrane, not the regret
If a contractor is bidding noticeably less than the next guy on a bathroom, look at the substrate and waterproofing line items. Nine times out of ten, that is where the gap is. The cheap bid skipped the membrane.
You are not buying a discount on the bathroom. You are buying a delayed bill on the joists below it.
If you want to know what your existing bathroom's wet area looks like, or you are about to start a reno and want to make sure the spec is right, call us. Free assessment, no pressure. Better to know now than to find out from a brown spot on the ceiling below.