A.Sean Gerolimatos, technical services manager for Schluter Systems, responds: To answer your question, the technical services department at Schluter recently did some simple testing. First, we installed porcelain tiles on a loose sheet of Kerdi with unmodified thinset mortar and filled the joints with two different epoxy grouts. Once the mortar and grout had cured, we placed the assembly on a sheet of cement backerboard. Then we applied heat per the grout manufacturer’s recommendations and removed the grout with various tools, including a grout remover, a utility knife, and a screwdriver. Since the Kerdi wasn’t bonded, we could inspect the back of the membrane for evidence of damage.

We found that gradual heat application softened the grout enough for removal without damage to the Kerdi. However, if we applied concentrated heat by holding the gun steady instead of waving it back and forth, the membrane “bubbled” off the back of the assembly after about 90 seconds of exposure. While this effect would likely vary with different heat guns, settings, and proximity to the tile surface, it’s possible that in an actual installation concentrated heat application for extended periods could damage the membrane.

So while I think that it’s okay to use a heat gun to repair epoxy grout applied over a Kerdi membrane, this method should be undertaken carefully. It’s not just the heat you need to worry about — the sharp grout-removal tool poses a risk too. You might get away with a small puncture on a vertical surface that sheds water quickly, but on a horizontal surface like a shower base or bench, any kind of damage to the membrane could cause big problems later on.

Don’t forget: You won’t be able to inspect the membrane after the repair, the way we did in our testing.