Most homeowners evaluating a bathroom remodel spend their time choosing tile and vanity styles. The tile is what you see. The waterproofing membrane behind the tile is what determines whether you’re doing this again in seven years because of a mold problem that started invisibly in year three. This guide covers the things your contractor should be telling you — and that many don’t.
Why bathroom waterproofing matters more than the tile
The tile you see in a shower is not a waterproofing barrier. Ceramic and porcelain tile are not waterproof — they absorb some moisture through the grout joints and at the grout-tile interface, especially with sanded grout in wet applications. The actual waterproofing happens behind the tile: at the substrate and the membrane layer.
When a shower fails — when water infiltrates the wall, soaks the framing, and promotes mold growth — it’s almost never because the tile cracked. It’s because the waterproofing layer was inadequate, improperly applied, or simply wasn’t there. In older KC homes where the original shower was a “mud-set” construction (a layer of mortar bed, usually done in the 1950s–1970s), you may have a system that’s been working for 60 years or one that’s been leaking for 20. Demo reveals the difference, and there’s no way to know without taking it apart.
The three waterproofing systems we use
Schluter KERDI system. A polyethylene foam sheet membrane applied over cement board or directly over wood framing in specific applications. Bonded with unmodified thin-set, seamed at corners and seams with KERDI-BAND. This is our default system for most shower builds in KC. The foam core provides some insulation, the polyethylene is a true vapor barrier, and the system is well-documented and well-inspected. An average shower installation with KERDI runs $400–$700 in materials beyond standard tile installation costs. The labor is a separate skill set — a tile setter who doesn’t know the KERDI installation sequence will create problems. Ask specifically.
Wedi board system. An isocyanurate foam core bonded between two glass-fiber-reinforced mortar faces, with a factory-applied surface treatment that’s inherently water-resistant. The board itself is waterproof; you still need to tape and seal joints and penetrations. More expensive in materials than cement board + KERDI, but faster to install for experienced crews. Common in high-end bathroom builds where the full system cost is less visible relative to the tile and fixture budget. Materials cost: roughly $5–$8 per square foot for Wedi board vs. $2–$4 for cement board.
Sheet-applied membranes (Redgard, Mapelastic). Brush- or roller-applied liquid membranes that cure to a flexible sheet. Cost-effective and widely available. The limitation: proper application requires consistent thickness and complete coverage at corners and penetrations — areas that get inconsistent application when speed is prioritized over quality. When done right, sheet-applied membranes work. When done by a crew that’s moving fast on a low-bid job, they fail at exactly the spots moisture concentrates. We use sheet-applied as a secondary treatment (at shower pans, around fixtures) rather than a primary system for full wet-area enclosures.
What we never use: Backer board alone. There is no scenario where cement board without a membrane constitutes adequate waterproofing for a shower application. If a contractor’s proposal doesn’t specify a membrane system, ask why. The answer will tell you a lot.
Plumbing relocation: when it’s worth it and when it isn’t
The honest baseline: every plumbing relocation costs money. Moving a shower valve from one wall to another costs $800–$2,500 depending on access and pipe routing. Moving a toilet flange costs $1,500–$3,500 depending on whether it’s a wood-framed floor or a slab. Moving a drain from one location to another — especially a shower drain — can run $2,000–$6,000+ in slab situations.
The question is whether the functional improvement justifies the cost.
Move plumbing when:
You’re converting a tub-shower combo to a standalone shower. The original tub/shower combo drain is usually at one end of a 60-inch tub alcove. A freestanding shower with a zero-threshold entry typically needs the drain positioned differently for proper slope. This relocation is almost always worth doing as part of a full gut bathroom — it’s a $1,500–$2,500 add when the floor is already open.
You’re adding a second sink. A bathroom that’s going from one sink to a double-vanity needs a second drain stub and a second supply rough-in. If the floor is open and the walls are stripped for a full remodel, this is $800–$1,500 in additional plumbing. It’s substantially more expensive if you’re trying to add a second sink without opening the floor.
The existing valve is genuinely failing. Older compression valves and single-handle cartridge valves from the 1980s–1990s that are weeping or impossible to get parts for are worth replacing during a full gut. It’s not just convenience — a failing shower valve behind a tile wall is a future leak.
You’re expanding the footprint into adjacent space. If the bathroom is growing — taking square footage from a closet or adjacent bedroom — new plumbing locations for the shower or toilet often come with the territory.
Don’t move plumbing when:
The existing position is functionally fine. Moving a toilet 14 inches to the left to visually center it in a wider bathroom might cost $2,500 and produce no meaningful functional improvement. We’ve had homeowners make this decision and immediately realize the visual improvement was worth it — and others who, looking at a finished bathroom, couldn’t see the difference. Price it out, then decide with the number in front of you.
You have slab plumbing and a limited budget. Slab core drilling and patching in a 1960s KC ranch home adds cost and timeline to any plumbing relocation. If the budget is constrained, the right move is usually to work with the existing drain location and adjust the shower pan geometry accordingly.
The slab situation in KC
A significant portion of Kansas City’s ranch-era housing stock — built roughly 1955–1975 — has slab-on-grade foundations with plumbing embedded in or below the slab. This is particularly common in the flat-lot subdivisions of Independence, Blue Springs, and parts of south KC and Lee’s Summit.
Before you finalize a bathroom scope in a home you’re not certain about, it’s worth understanding whether your bathroom plumbing is slab-embedded. Signs include: the bathroom is on the ground floor of a slab-foundation home, there’s no basement, and the drains run horizontally rather than dropping through a crawl space. A plumber can usually assess this with a camera inspection of the drain line.
Slab plumbing isn’t a problem in itself — it just means that any drain relocation requires concrete cutting, which adds cost, timeline, and concrete dust to the project. Budget for it rather than discovering it mid-demo.
Aging-in-place features: the cheapest time to add them is now
This is the section that contractors often skip because it’s not exciting to talk about. But if you’re over 45 and you’re gutting a bathroom, the marginal cost to add certain features during construction is genuinely small compared to adding them later.
Blocking for grab bars. A 2x10 flat-blocked behind tile is $200–$400 during construction. A grab bar added to a finished tile shower without backing is an anchoring nightmare that often results in failed fasteners in the grout joint. Do it now.
Zero-threshold shower entry. A barrier-free shower entry (no curb) costs roughly the same as a traditional curbed entry in a full gut remodel — the slope-to-drain geometry is adjusted during the pan phase. The threshold is not a meaningful design element; it’s only a construction convenience. Zero-threshold is universally considered better design practice and requires no accommodation for future mobility changes.
Wider doorways. Taking a 28” doorway to 36” during a full gut — when the framing is already open — is a half-day framing task and $400–$800. The same change in a finished bathroom with intact framing and drywall is a $2,500–$4,000 structural + finish job.
These aren’t concessions to aging. They’re design-forward features that happen to also prepare the bathroom for use by any member of the household at any age. Build them in once.