Waterproofing is the most consequential decision in any bathroom renovation — and the one most homeowners never ask about. A beautiful tile shower installed without proper waterproofing will begin failing within two to three years. Water infiltrates through grout and tile joints, reaches the wall assembly behind, and creates mold and rot that’s invisible until the tile is pulling away from the wall or the floor underneath is soft and spongy. By then, a repair requires complete demolition of everything installed.

The tile conversation most homeowners have is about color, size, and pattern. The conversation that actually protects them is about what goes behind the tile.

“In fifteen years of bathroom renovation, the most common failure I’m called in to remediate is a shower that wasn’t properly waterproofed. The tile looks fine. The grout looks fine. The water is getting behind it and the wall is rotting. When I ask the homeowner about the waterproofing system, they look at me blankly — they had no idea that conversation needed to happen. It needs to happen before any tile is ordered.”

— Marcus Chu, renovation cost consultant and project manager with 14 years of experience reviewing and managing residential kitchen and bathroom renovation budgets in the local area

Waterproofing Questions

Modern bathroom with white marble and bathtub.

What waterproofing system will be used in the shower?

This is the most important question in any bathroom renovation. Ask specifically — not “will you waterproof it?” But “what waterproofing system will you use and how will it be installed?”

The main waterproofing systems used in Canadian residential bathrooms:

Schluter KERDI system:
A sheet membrane bonded directly to the substrate (drywall or cement board) with thinset mortar. Very reliable when installed correctly. KERDI requires compatible substrates and specific installation sequence. Schluter also offers complete shower system solutions (KERDI-BOARD substrate, KERDI-DRAIN, KERDI-BAND for seams) that, when used as a complete system, provide excellent performance.

Liquid-applied membranes (RedGard, Laticrete, Mapei Mapelastic):
Applied by roller or trowel in one or two coats directly to the substrate. Effective when applied at the correct thickness — most manufacturers specify two full coats to achieve the required mil thickness. Ask specifically how many coats will be applied and how thickness will be verified.

Traditional mortar bed (mud bed) with sheet membrane:
The oldest method — a sloped mortar bed forming the shower floor, with a PVC or CPE sheet membrane embedded within it. Still used by experienced tile setters. Extremely durable when done correctly. Requires a skilled installer.

Foam shower system (Wedi, Schluter KERDI-BOARD):
Extruded polystyrene foam panels that are inherently waterproof, used as both the substrate and waterproofing layer. Pre-sloped shower floors available. Faster installation than traditional methods.

Ask specifically which system is being used, whether the installer has training or certification for that system, and how the seams and corners will be detailed.

What a good answer looks like: A specific product named, an installation method described, and confirmation that seams, corners, and penetrations will be properly detailed.

Will the waterproofing be inspected or tested before tile is installed?

Responsible bathroom contractors test waterproofing before tile installation — either by visual inspection of all seams and corners, or in some cases by a flood test (filling the shower pan with water and confirming no leakage over 24 hours). Ask whether any testing or inspection will be done before tile installation begins.

What a good answer looks like: A described verification process before tile is installed.

Is the shower floor properly sloped to the drain?

A shower floor must slope toward the drain at approximately 1/4″ per foot — enough to drain completely without leaving standing water. Ask specifically how the floor slope is achieved — a mortar bed, a pre-sloped foam panel, or a pre-sloped shower tray — and how it will be verified.

What a good answer looks like: A specific slope method described, with a confirmation that the slope will be checked before tile is installed.

What substrate is being used on shower walls?

Standard drywall is not appropriate behind tile in a shower. Ask specifically what substrate will be used on shower walls:

  • Cement board (HardieBacker, Durock): Concrete-based panel. Doesn’t degrade with water, but not inherently waterproof — still requires a waterproofing membrane over top.
  • Glass mat gypsum (DensArmor, DensShield): Moisture and mold resistant, more workable than cement board. Requires waterproofing membrane over top.
  • Foam board (Wedi, Schluter KERDI-BOARD): Inherently waterproof. No additional membrane required if properly seamed and detailed.

Standard drywall — even moisture-resistant greenboard — is not appropriate as a substrate behind shower tile.

Tile Questions

What tile are you recommending for the shower floor, and why?

Shower floor tile has specific requirements beyond aesthetics:

Slip resistance: Shower floors must have adequate coefficient of friction (COF) when wet. Smaller tiles (mosaic, 2″x2″, 4″x4″) with more grout lines provide more grip. Large-format tiles (12″x24″ or larger) require verified slip-resistant texture ratings (DCOF ≥ 0.42 for wet areas per ANSI A137.1). Ask what the slip resistance rating is for the proposed floor tile.

Grout line width: More grout lines mean more slip resistance but also more grout to maintain. Smaller tiles naturally produce more grout lines.

Ask the contractor to explain why the proposed floor tile is appropriate for a wet, slip-resistant application.

What grout is being used?

Grout type significantly affects maintenance and longevity:

Sanded grout: Used for joints wider than 1/8″. Contains sand aggregate. More porous than unsanded grout — benefits from sealing.

Unsanded grout: Used for narrow joints (1/16″–1/8″). Smoother texture. Used for polished or delicate tiles where sanded grout would scratch.

Epoxy grout: A two-part epoxy system. Extremely stain-resistant and waterproof. More expensive and more difficult to install. The best-performing grout for shower applications.

Modified polyblend grout: Many quality grouts now include polymer additives that improve stain and water resistance — a middle ground between traditional and epoxy grout.

Ask what grout type is being used in the shower and whether it needs sealing.

Will grout be sealed after installation?

For non-epoxy grout in wet areas, sealing is strongly recommended — it significantly improves stain resistance and water penetration resistance. Ask whether sealing is included and what product is being used.

What tile adhesive (thinset mortar) is being used?

In wet areas, polymer-modified thinset mortar is required — standard unmodified thinset can fail in the presence of prolonged moisture. For glass tile and some large-format tiles, specific thinset formulations are required. Ask whether the thinset being used is appropriate for the tile type and the wet application.

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