Q: Is it okay to use Schluter's Kerdi material as a waterproofing membrane beneath a stone countertop? I plan to use it to cover the plywood substrate on top of the cabinets, and then turn it up the wall a few inches so that I can waterproof the area behind the planned tile backsplash at the same time.
A: Sean Gerolimatos, technical services manager for Schluter Systems, responds: Yes, the Kerdi waterproofing membrane (schluter.com) would be suitable for this application.
However, if you were planning to install tile on the countertop instead of the stone slabs, I would recommend using Schluter's Ditra uncoupling membrane instead. This is because stone slabs are basically self-supporting and are not as sensitive as tile to the inherent dimensional instability of the plywood deck. Ditra would provide better lateral flexibility than the Kerdi membrane, protecting tile and grout from movement in the plywood substrate caused by changes in moisture content over time.
You'll need to use a modified thin-set mortar to bond the Kerdi to the plywood substrate. If there are any seams, be sure to butt rather than overlap them, and seal them with Schluter's Kerdi-Band using unmodified thin-set mortar.
