When a tiled shower fails, the visible symptom is usually grout cracking, tiles coming loose, or water appearing in the room below. The cause is almost always behind the tile, not in it. Waterproofing is the layer between the tile and the substrate that prevents water from reaching the framing. When it is done correctly, it is invisible and permanent. When it is skipped or done poorly, the tile installation above it is on borrowed time.
What waterproofing actually involves ¶
In a properly built shower, the waterproofing layer is applied to the substrate before any tile adhesive or tile is installed. The most common system we use is Schluter Kerdi, a polyethylene membrane that is bonded to the substrate with unmodified thinset. The membrane covers the floor, all walls, and critically, the corners and transitions where water is most likely to find a path through. The drain is integrated into the waterproofing system so there is no gap between the membrane and the drain body.
What happens when waterproofing is skipped ¶
Standard drywall, even moisture-resistant drywall, is not a waterproof substrate. Water vapour penetrates grout joints and tile adhesive over time. Without a waterproofing membrane, that moisture reaches the drywall or cement board behind the tile, promotes mould growth, and eventually causes the substrate to deteriorate. The tile begins to flex, grout cracks, and tiles come loose. By the time the failure is visible, the damage behind the tile is usually significant.
Cement board is not waterproofing ¶
This is a common misconception. Cement board, products like Durock or HardieBacker, is a stable, non-organic substrate that does not deteriorate when it gets wet. But it is not waterproof. Water passes through it. It needs to be used in combination with a waterproofing membrane, not instead of one. We see a lot of shower failures in renovations where cement board was installed without a membrane.
Linear drains and waterproofing ¶
Linear drains, which are increasingly popular in walk-in shower conversions, require careful waterproofing at the drain-to-floor transition. The floor is sloped in a single plane toward the linear drain rather than in four planes toward a centre drain, which changes the geometry of the waterproofing installation. We use Schluter Kerdi-Line drains, which are designed to integrate directly with the Kerdi membrane system and eliminate the transition gap.
How to verify your contractor is waterproofing correctly ¶
Ask your contractor what waterproofing system they use and ask to see it before tile is installed. A reputable contractor will have no objection to showing you the membrane before it is covered. If the answer is that cement board alone is sufficient, or that the grout will keep the water out, those are answers worth questioning.
Waterproofing is the most important step in a bathroom tile installation and the one that is easiest to skip because it is never visible in the finished product. We apply it on every wet area, without exception.