Overview
Here is the thing most homeowners never get told: tile and grout are not waterproof. Water passes right through cement grout and down the tile joints, so a tiled shower needs a real waterproofing system behind and under the tile. Skip it and the wall and pan rot, and mold grows inside the assembly where nobody sees it until the smell or the soft spot shows up. The cardinal rule on every shower I build is simple. The waterproofing is the system, the tile is just the finish. There are three modern approaches that work. The first is a membrane over a backer or foam board, the Schluter-Kerdi style sheet or a paint-on liquid like RedGard. The second is a foam board system where the board itself is the waterproofing. The third is the traditional mortar-bed shower with a pan liner and a sloped mud bed. All three keep water out if the seams and pan are detailed right. The number one failure I see is the worst one: no waterproofing at all, or plain greenboard hiding behind the tile, doing nothing. That shower was rotting the day it was finished.
Why Tile and Grout Are Not Waterproof
This is the misconception that kills showers. People look at a tiled wall and assume it sheds water like a countertop. It does not. Cement grout is porous, and water wicks straight through it. Even where the grout holds, water runs down the tile joints and reaches the substrate behind. Sealing the grout slows that down, it does not stop it, and the sealer wears off. So the real question is never whether water gets behind the tile. It does, every shower, every day. The question is what is back there to handle it. If the answer is drywall or greenboard, you have a problem. Greenboard is moisture-resistant drywall, which means the paper is treated to resist humidity in a bathroom. It is not waterproof and it does not belong behind shower tile. Soak it daily and the gypsum core turns to mush, the paper feeds mold, and the framing behind it stays wet and rots. Unsealed cement backer board is better as a substrate, but bare backer is not waterproof either. Treat the tile and grout as a wear surface. The waterproofing behind them is the part that actually keeps water out of your wall, and that is the part people leave out. See our ceramic-vs-porcelain-tile guide for the finish side of this.
The Waterproofing Systems
Three approaches do the job, and I have built all of them. First, membrane systems. You hang a substrate, cement backer board or a foam board, then waterproof it with either a sheet membrane like Schluter Kerdi or a liquid, paint-on membrane like RedGard or Mapei AquaDefense. The sheet bonds to the wall with thinset, the liquid rolls on like thick paint over two coats. Either way you waterproof the seams, the corners, and every penetration, and you tile directly to the membrane. This is the current pro standard because it is fast, reliable, and the waterproofing sits right behind the tile where the water is. Second, foam board systems like Wedi and Kerdi-Board. The board itself is waterproof, so you screw it to the studs, seal the joints and fasteners with the maker's sealant, and tile. Lighter, faster, no separate membrane coat. Third, the traditional mortar-bed shower: a sloped mud bed over a PVC or CPE pan liner, with backer on the walls. Durable and proven, but it takes real skill and labor. Whichever you pick, the pan must slope to the drain, the drain has to match the system, and the corners, seams, and penetrations have to be waterproofed. The seams and the pan are where every system leaks.
The Pan, Slopes, and the Details That Leak
The shower pan is the highest-risk zone in the whole assembly, because gravity sends every drop of water there. It has to slope to the drain, a quarter inch per foot, so water actually moves to the drain instead of pooling. In a traditional build that means a pre-slope under the liner, so any water that reaches the liner also runs to the weep holes instead of sitting in a puddle for years. The drain has to match the system: a clamping drain that grips the liner in a mortar-bed build, a bonding-flange drain that the sheet membrane laps onto in a Kerdi-style build. Then come the details that actually leak. The curb, the inside and outside corners, the niche you cut into the wall for shampoo, and the valve and pipe penetrations all have to be waterproofed with preformed corners, banding, or extra membrane. That is where leaks start, not in the middle of a wall. Before any tile goes on, pros run a flood test: plug the drain, fill the pan with a couple inches of water, mark the level, and leave it overnight. If the level holds, the pan is tight. The details, not the open field, are where showers fail.
FAQ
Can I just seal the grout instead of waterproofing behind the tile? No. Grout sealer slows water absorption on the surface, it does not stop water from passing through grout and down the joints, and it wears off. Sealer is maintenance for the finish, not a substitute for a waterproofing system behind the tile. See our grout-types-and-sealing guide for what sealing actually does.
Is cement backer board waterproof on its own? No. Cement backer board is a stable, rot-resistant substrate, but bare backer lets water pass through to whatever is behind it. You still need a membrane over it, sheet or liquid, or a vapor barrier behind it in older mortar-bed assemblies. The board is the base for the waterproofing, not the waterproofing.
How do I know if my existing shower has waterproofing? You usually cannot tell from the finished side. Warning signs are loose or hollow-sounding tile, persistent musty smell, grout that stays dark, or soft spots in the wall or floor. If a wall opens up during a remodel and you find greenboard or bare drywall behind the tile, there is no waterproofing and the shower needs to be rebuilt.
Sheet membrane or liquid membrane, which is better? Both work when installed right. Sheet membranes like Kerdi give a consistent, known thickness and are forgiving over backer board. Liquid membranes like RedGard are faster on complex shapes and tight spots but depend on getting full, even coverage across two coats with no thin spots. I use sheet on big flat walls and liquid where the geometry gets fiddly.