Standard thinset mortar is water-resistant, meaning it will not dissolve when exposed to moisture, but it is not inherently waterproof on its own and requires a dedicated waterproofing membrane beneath or over it in a shower. Water can slowly seep through porous tile grout lines and standard mortar beds, potentially reaching vulnerable wood framing or drywall if a proper waterproof barrier is absent. To build a truly waterproof shower enclosure, professionals install liquid waterproofing membranes or sheet waterproofing systems over cement backer boards before setting the tiles with a high-performance polymer-modified thinset. This multi-layered approach ensures that any moisture penetrating the tile assembly is safely contained and directed toward the shower drain without causing structural damage.
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