Every failed shower we've ever been called out to inherit had the same thing in common. The tile looked fine. The grout looked fine. Somewhere below the surface, the waterproofing had failed, and by the time the homeowner noticed the musty smell or the soft spot on the floor outside the shower, there was black mold behind the drywall and a joist under the tub that we could push a screwdriver through.
The waterproofing layer is the single most important part of a shower build, and it's the part nobody talks about until it fails. This post is the honest comparison of the three methods that are legal and defensible in Ontario in 2026. For the broader bathroom scope, see our GTA bathroom renovation cost breakdown.
One thing to get out of the way first. Tile is not waterproofing. Grout is not waterproofing. Neither is thinset. Water passes through all three. If water gets past your tile — and it will, because grout is porous and the silicone joint will eventually crack — it has to meet a true waterproof membrane behind the tile or it's going to find your joists. The membrane is what matters. The tile is a wear surface.
Schluter Kerdi — the sheet membrane we default to
Schluter Kerdi is a 4-mil polyethylene sheet membrane laminated with a fleece layer on both sides that bonds to thinset. It comes in rolls that we cut to size, install over cement board or drywall with unmodified thinset, overlap seams with Kerdi-Band, and terminate at inside and outside corners with pre-formed Kerdi-Kereck fittings. The drain is a Kerdi-specific unit (Kerdi-Drain or Kerdi-Line) that bonds directly into the membrane for a continuous waterproof layer from the wall to the drain. That's the whole system.
We use Kerdi on roughly 85% of the bathroom jobs we build in the GTA. The reason is that Kerdi is the closest thing to a foolproof waterproofing method available in the Ontario market. It doesn't depend on drying time, humidity, applicator skill, or thickness — if the sheets are overlapped correctly and the seams are taped with Kerdi-Band, the waterproofing is correct.
A first-year apprentice can lay Kerdi correctly on day one because the system has almost no margin for user error. That's the opposite of liquid membranes, which we'll get to next.
The downsides of Kerdi are cost and install time. A full Kerdi system on a standard mid-size shower (the walls, the tray, the drain, the Kerdi-Band, the corner fittings) runs $450 to $780 in material alone for a 5-foot by 3-foot walk-in shower. A bottle of RedGard for the same square footage is $80.
The labor on Kerdi is about 30% longer than a liquid membrane because you're dry-fitting the sheets, cutting them with a utility knife, and troweling thinset in controlled amounts. A three-man bathroom crew can finish a Kerdi install in a day on a standard walk-in. On a complex curved or stepped shower, it's two days. This is the same membrane we default to on the curbless conversions covered in our aging-in-place curbless shower post.
- Polyethylene sheet membrane with fleece on both sides, 4 mil thick
- Installed with unmodified thinset, seams overlapped with Kerdi-Band
- Kerdi-Drain or Kerdi-Line bonds directly into the membrane
- Near-zero margin for installer error if the seams are overlapped and taped correctly
- Cost: $450 to $780 in material alone for a standard 5x3 walk-in shower (2026 pricing)
- Warranty: Schluter offers a 10-year limited warranty on the system when installed per spec
RedGard — the liquid membrane that works when the applicator does
RedGard is Custom Building Products' trowel-on or roll-on liquid waterproofing and crack-isolation membrane. It's a pink latex liquid that goes on over cement board in two coats, dries to a flexible red rubbery membrane, and gets tile applied directly over the cured surface. Mapei Aquadefense and Laticrete Hydro Ban are similar products from competing brands that we treat as functionally equivalent. All three are legitimate waterproofing systems when applied at the correct thickness — which is the part most DIY and cut-rate contractor jobs get wrong.
The correct spec on RedGard is 40 mils wet thickness (two coats) across 100% of the substrate, with no thin spots, pinholes, or uncoated corners. The problem is that 40 mils is hard to hit consistently by eye. A roller takes off as much product as it puts on, and applicators tend to apply too thinly in the corners (where water collects) and too thickly in the field.
We have pulled RedGard-waterproofed showers apart and seen pinholes the size of a BB, which is plenty for a decade of slow water intrusion to rot a joist. If RedGard is going on, we specify the mil gauge, we apply two full coats, and we check with a wet-film thickness gauge after the first coat. Most contractors skip the gauge.
Where RedGard is legitimately the right call: a tile tub surround where we're just waterproofing the tub-to-tile joint and the walls above a tub that already has a proper acrylic pan. The liquid membrane is fast, cheap, and sufficient for that scope. We still use it on 10 to 15% of our bathroom jobs, almost always on tub-shower combos where the tub is the floor pan and we only need to waterproof the walls above. We don't use it on full curbless showers because the risk of an applicator miss on the floor is too high.
- Latex liquid waterproofing, trowel or roll-on, cures to flexible red rubber
- Requires 40 mils wet thickness (two coats) — measured with a wet-film gauge
- Full cost on a 5x3 shower (two coats + tape + primer): $180 to $280 in material
- Best fit: tub-shower combos where the tub is the pan and you're waterproofing walls above
- Weakness: applicator skill variance, pinholes and thin spots in corners
- RedGard, Laticrete Hydro Ban, and Mapei Aquadefense are functionally equivalent
Hot mop — the traditional method we almost never use
Hot mop is the oldest legal shower pan waterproofing method in North America. The installer heats asphalt to roughly 200°C, mops it onto a felt-covered shower floor in alternating layers — usually three layers of asphalt with two layers of felt paper between them — and lets it cool to a flexible waterproof layer. It's labor-intensive, smells terrible during application, requires open windows and a 24-hour cure, and is legitimately effective when done by a specialist. The shower pan warranty from the Tile Council of North America (TCNA Handbook) still recognizes hot mop as a compliant method for showers where the pan is a concrete bed over the waterproofing.
We almost never use hot mop in 2026. The reasons are: the crew that specializes in hot-mop in the GTA is shrinking (it's a dying trade), the material is genuinely hazardous to handle safely without specialized equipment, and the cure time pushes a bathroom schedule back by a full day.
We still do it occasionally when a homeowner is rebuilding a 1950s bathroom and wants to match the original waterproofing method, or when we're working on a commercial steam room where hot mop is still the premium spec. Residential bathroom jobs — basically never, anymore.
The honest note on hot mop is that it's not worse than Kerdi or RedGard when it's done right. It's comparable, possibly better in certain commercial high-temperature applications. It's just much harder to find a crew that does it correctly in 2026 and much harder to defend the additional risk and scheduling complexity to a residential client when a Kerdi system is equally bulletproof and twice as fast to install.
- Traditional asphalt and felt layered waterproofing for shower pans only
- Legitimately effective but requires specialized crew and cure time
- Cost: $600 to $1,200 on a standard shower — comparable to Kerdi
- Use case in 2026: heritage restorations, commercial steam rooms, almost never residential
- Installer pool in the GTA has shrunk dramatically — hard to find qualified crews
What makes a shower waterproofing job actually fail
Failures are almost always at the transitions, not in the middle of a wall. The drain-to-membrane junction, the wall-to-floor inside corner, the pipe penetrations for the valve and showerhead, the niche shelves built into a wall — these are the spots where water finds a way past any waterproofing system if the installer was sloppy. The reason we're so obsessive about pre-formed Kerdi corner fittings and Kerdi pipe collars is that these are the details most installers skip or fake, and they're the exact failure points we find when we tear out a leaky shower.
The second most common failure is the slope. A shower floor has to slope at 1/4-inch per foot minimum to the drain, per OBC 9.33. If the slope is wrong, water pools, grout stays wet, and eventually a pinhole in the membrane becomes a leak. We check slope with a 4-foot level and a digital inclinometer at three points on every pre-tile inspection.
Most jobs we take over from other contractors have at least one area where the slope is closer to 1/8-inch per foot. That half-inch of sag per four feet doesn't sound like much, but it's the difference between a shower that drains in ten seconds and one that holds water for forty.
“You spent half a day taping seams I was never going to see. I almost asked you to skip it. I'm glad I didn't.”
The short version of this whole post: for any shower that isn't a tub-shower combo, use Kerdi or a comparable sheet membrane system. It's the fastest to install correctly, the hardest to install incorrectly, and the cheapest insurance against a ten-year disaster we've found. For tub-shower combos, a liquid membrane like RedGard is fine if the applicator knows what they're doing with a wet-film gauge. Hot mop is a specialist play for a specific job type.
If you're getting a shower built in the GTA in 2026 and the contractor can't tell you which of these three methods they're using and why, you have the wrong contractor. Our bathroom remodels page is where we walk homeowners through the membrane decision in person.
