Tile guy built a mud-set shower: laid the PVC liner flat on the plywood, packed the mud bed on top, screwed the liner to the curb 1/2 inch above the curb top, and slathered thinset around the drain base to 'seal it.' It flood tested fine.

This shower will fail because the waterproofing membrane is installed below the setting bed, violating the fundamental principle of a "water-in, water-out" system. Per IPC 417.5.2 / UPC 411.2 and TCNA Handbook Method B415, the liner must be sloped to the drain (¼" per ft) before the mortar bed is placed.

Specific failure modes:

1. No Sub-Drainage (The Killer): The liner is flat on plywood. Weep holes in the drain clamping ring are buried under 1.5"+ of mud. Water penetrating the tile/grout saturates the mud bed and cannot reach the drain. The mud stays permanently wet → rot, mold, efflorescence, thinset bond failure.
2. Curb Penetrations: Screwing the liner to the curb face ½" above the top creates holes below the flood level. Water wicks through screw shanks into the curb framing. Code requires the liner to fold over the curb top and fasten outside (exterior face) only.
3. Drain Seal: "Slathering thinset" on the drain base is not a code-approved seal. You need a clamping ring drain (mechanical compression) or a bonded flange drain (specific adhesive per manufacturer). Thinset cracks, shrinks, and leaks.
4. Plywood Substrate: IPC 417.5.1 requires the subfloor under the liner to be sloped to the drain. Flat plywood + flat liner = ponding.

The flood test passed because water hasn't saturated the mud bed yet. Failure manifests in 6–24 months.

Fix: Demo. Install sloped pre-slope (mud or foam) → Liner (sloped, folded over curb, no holes below top) → Clamping ring drain → Top mud bed → Tile. Or switch to a surface-applied membrane system (TCNA B422) on a sloped substrate.

This shower will fail due to critical violations of waterproofing principles, despite passing a flood test. Here’s why:

1. Liner Not Sloped to Drain: The PVC liner must be installed with a minimum 1/4" per foot slope toward the drain (TCNA Handbook, Section B422; IRC R708.2). Laying it flat on plywood creates a zero-slope plane. Water penetrating the mud bed (inevitable over time) will pool on the liner instead of draining, leading to saturation, mold, and substrate rot.

1. Incorrect Liner Attachment at Curb: Screwing the liner to the curb 1/2" above the curb top creates a dam. Water that reaches the liner (via cracks or grout) will be trapped above the drain level, unable to flow downward. The liner must be *folded down and secured to the outside face of the curb, with the top edge sealed above the finished curb height (typically 2" minimum) to allow drainage into* the shower pan (TCNA B422.2.1).

1. Thinset Is Not a Waterproof Seal: Slathering thinset around the drain base does not seal the drain flange. The PVC liner must be clamped to the drain body using the drain’s clamping ring (per manufacturer specs and IRC R708.2.1). Thinset cracks, shrinks, and offers zero long-term waterproofing—it’s a bond coat, not a membrane.

1. Flood Test Misleading: A flood test only checks for gross leaks immediately after installation. It doesn’t reveal design flaws that cause slow, hidden accumulation (e.g., flat liner, no slope). Failure may take months or years—by then, damage is extensive.

Correct Method:

• Slope the subfloor 1/4"/ft to drain.
• Lay liner with same slope, folded into drain and clamped.
• Extend liner up walls to at least 2" above shower head, sealed at top.
• Slope mud bed 1/4"/ft over the liner.
• Use proper drain clamping ring—never rely on thinset.

This build traps water where it can’t escape. It’s not a matter of if it fails, but when. A licensed tile setter or waterproofing specialist must rebuild the pan correctly. (Word count: 248)