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Why Your Shower Is Rotting From the Inside Out (And What We Use Instead)

What we found under a shower that looked perfectly normal on the surface — black rot caused by years of hidden moisture.

If your bathroom was built or remodeled more than 10 to 15 years ago, there's a good chance your shower is slowly failing — and you'd never know it just by looking at the tile.

We just kicked off a master bathroom remodel and what we found during demo is a perfect example. Beneath a shower that looked perfectly normal on the surface, the subfloor was covered in black rot. Moisture had been silently doing damage for years.

This isn't unusual. In fact, we see it all the time. And the culprit is almost always the same thing: the traditional vinyl pan liner and wet bed method.

How the Old Method Works (And Why It Fails)

For decades, the standard way to build a shower was the wet bed and pan liner method. Here's how it goes:

A layer of mortar — called the pre-slope — is floated onto the subfloor to pitch water toward the drain. A flexible vinyl or rubber membrane (the pan liner) is then laid over that pre-slope and folded up the walls several inches. The liner is the actual waterproofing layer. Another layer of mortar — the top mud bed — goes over the liner to create a flat surface, and then tile is set on top.

Two examples of the old wet bed vinyl pan liner system — the method we tore out of this bathroom. This is what's hiding under many showers built 10 to 20 years ago.

In theory, any water that makes it through the grout and tile hits the liner underneath and flows down to weep holes in the drain. In practice, this system has some serious weaknesses:

• The liner can be punctured during installation by a misplaced screw, a dropped tool, or even a sharp piece of gravel in the mortar.
• The seams where the liner meets the drain or folds at corners are vulnerable failure points.
• Because the liner is buried under inches of mortar, you'll never know it's leaking until the damage is already severe.
• Water gets trapped between the mortar layers, creating the perfect environment for mold and rot.

That's exactly what you see in these photos from our recent project:

The subfloor and framing around this shower had been absorbing moisture for years — all hidden behind tile that looked fine.

The moisture wicked into the subfloor and framing around this shower for what was likely years. By the time you'd notice a soft spot in the floor or a musty smell, the structural damage is already done.

The full scope of the master bathroom demolition — years of hidden moisture damage.

This method isn't wrong because contractors were being careless. It was the accepted standard for a long time. But building science has evolved, and we now have dramatically better options.

The Schluter Approach: Waterproofing That Actually Works

We made the switch to Schluter Systems for all of our tile and shower work, and we haven't looked back. Here's why — and what the key products are.

The Schluter KERDI system — a complete, bonded waterproofing assembly where every component works together.

Schluter Waterproofing Membrane (KERDI)

The Schluter KERDI membrane is a thin, flexible, sheet-applied waterproofing membrane made of pliable polyethylene with fleece webbing on both sides. You apply it directly over cement board or other approved substrates using thin-set mortar, and then tile directly over it. The waterproofing layer is immediately behind the tile — not buried under inches of mud where you can't see or access it. There's no cavity for water to pool and nowhere for moisture to hide and cause rot.

Schluter Waterproof Building Panels (KERDI-BOARD)

Schluter KERDI-BOARD is a rigid waterproof building panel that replaces cement board entirely. It's lightweight, easy to cut, and is waterproof right out of the box. You can use it to build shower walls, niches, curbs, and benches — all without needing a separate waterproofing step. It simplifies the whole build and eliminates a layer where something could go wrong.

Schluter Waterproofing Strip (KERDI-BAND)

Schluter KERDI-BAND is a waterproofing strip used to seal joints, corners, and transitions — anywhere two surfaces meet. Think of it as the detail work that makes the whole system watertight. Corners and seams are exactly where the old liner method was most vulnerable, and Schluter's KERDI-BAND addresses that directly.

Schluter Shower Drain (KERDI-DRAIN)

The drain is one of the most critical failure points in any shower. The Schluter KERDI-DRAIN integrates directly with the KERDI membrane through an integrated bonding flange, creating a fully bonded, watertight connection at the drain point. No relying on a clamping ring pressed against a vinyl liner and hoping the seal holds for 20 years.

Schluter Prefabricated Shower Trays (KERDI-SHOWER-T)

Schluter's prefabricated shower trays eliminate the need for a hand-floated mortar pre-slope altogether. They're made of expanded polystyrene with the slope already built in and KERDI waterproofing laminated right to the surface. They integrate directly with the Schluter drain system. This removes one of the most skill-dependent and error-prone steps in the traditional method.

A complete Schluter KERDI shower kit — everything needed for a fully waterproof shower assembly.

Why This Matters For Homeowners

The difference between these two approaches comes down to one thing: where the waterproofing lives.

In the old method, the waterproofing is hidden deep in the assembly and relies on gravity and weep holes to manage water that's already behind the tile. In the Schluter system, the waterproofing is bonded directly to the surface — right behind the tile — and water never reaches the framing or subfloor in the first place.

We've opened up too many showers that looked fine on the surface but were rotting underneath. That's why we're fully committed to Schluter's approach on every tile project we do.

It costs a bit more in materials, but it's faster to install, far more reliable, and it protects your home the way waterproofing should.

If your bathroom is due for a remodel — or if you're noticing soft spots, musty smells, or cracked grout in your shower — it might be worth finding out what's happening behind those walls before it gets worse.