Tile isn't waterproof. Neither is grout. The waterproofing in a shower or tub surround is behind the tile, not the tile itself. This misunderstanding is the source of the most common and most expensive bathroom callbacks in residential construction.
Why Tile Fails
Grout is porous. Water penetrates it, especially at corners and transitions. Caulk shrinks and cracks over time. The tile itself may be impermeable, but the assembly isn't. Water that gets behind tile needs to drain out, hit a waterproof substrate, or both. When it does neither, it saturates framing, grows mold, and eventually destroys the wall assembly from inside.
The Traditional Method: Mortar Bed
A properly built mud-set shower with a pre-slope, liner membrane, and mortar bed is still the gold standard for longevity. The liner (PVC, CPE, or lead) is the waterproofing. Tile is installed over the mortar bed. Water that penetrates tile drains through the mortar bed to the liner and out the weep holes in the drain. This system, properly installed, lasts 40+ years.
Modern Waterproofing Methods
Tile backer with topical membrane: Cement board or foam backer (Kerdi board, Wedi, etc.) with a topical waterproofing membrane (sheet or liquid-applied) before tile. Systems like Schluter Kerdi or RedGard are widely used and code-compliant. The membrane is the waterproofing, not the backer. Foam backer systems: Some foam products (Wedi, Laticrete Hydro Ban Board) are inherently water-resistant — the board is the waterproofing. Seams and corners must still be taped and sealed.
What Doesn't Work
Regular drywall — even moisture-resistant "green board" — is not a suitable shower substrate. Neither is OSB. Fiber cement board (Hardibacker) is water-resistant but not waterproof — it needs a topical membrane for shower applications. Cement board without a topical membrane is not waterproof — it resists water but absorbs it. Use cement board with a membrane.
Critical Details
Corners are the most vulnerable points. Sheet membranes need fabric reinforcement at inside and outside corners. Liquid membranes need corner fabric embedded while wet. The transition from the shower pan to the wall system is the most common failure point — the pan liner must bond to or lap behind the wall waterproofing. Penetrations (shower niche, soap dishes, shower valve bodies) must be treated as special conditions, not afterthoughts.
The Bottom Line
Choose a waterproofing system and follow the manufacturer's installation instructions precisely. Don't mix systems from different manufacturers. The 20-minute you save on installation isn't worth the callback 3 years later when the homeowner finds mold behind their tile.