What is the function of a crack isolation membrane?

• A. Isolate tile from substrate cracks and prevent crack transfer to tile surface.
• B. Provide waterproofing beneath tiles.
• C. Improve color uniformity of grout.
• D. Help tiles cure faster.

Answer: (A)

Crack isolation membranes act as a flexible barrier between the substrate and the tile to stop movement in the substrate from showing up as cracks in the tile surface. They’re applied over the substrate, then the tile is set over the membrane with thinset. The membrane stretches and moves with the substrate, absorbing and transferring movement within its own layer rather than letting a crack propagate into the tile and grout. This keeps tiles and grout from cracking when the substrate cracks or shifts.

Waterproofing is a separate requirement, and while some products may offer both features, the primary purpose here is decoupling movement to prevent crack transfer. The membrane doesn’t control grout color or speed up curing of the tile set.