What is the primary purpose of an uncoupling membrane in floor tile installation?

• A. It decouples tile from substrate movement to prevent cracking; commonly used with plywood or gypsum substrates; example Schluter-DITRA.
• B. It provides a waterproof seal for concrete slabs.
• C. It is a decorative underlayment that adds texture under the tile.
• D. It bonds the tile directly to the substrate without adhesive.

Answer: (B)

Uncoupling membranes are designed to absorb and separate movement between the tile layer and the substrate, so the tile can move with the substrate without transferring stress that causes cracking. This flexible layer, used under tiles on substrates like plywood or gypsum, lets differential movement occur without breaking the tile or grout. While some membranes offer additional moisture management, their primary purpose is decoupling, not waterproofing. They’re not decorative underlayment, and they don’t bond tile directly to the substrate without adhesive.