QUESTION

We installed a limestone over hardy backer board with crackbuster pro crack isolation membrane & hybrid grout.
I figure the limestone cracked because it is such a soft tile.

The owner is now wanting us to install porcelain. What do you suggest the assembly under the porcelain tile in order to reduce risk of tile cracking.

ANSWER

ANSWER -

Limestone isn’t so soft that it will crack easily.   The ASTM C568 standard for limestone breaks it down into low-density limestone, medium-density limestone and high-density limestone.  So limestone can be very durable.

I point that out because you want to be sure you understand what caused the cracking, so that the porcelain tile doesn’t end up cracking too.

Porcelain tile being a man-made ceramic tile is much more consistent and durable over a limestone, so it will be more forgiving, but it compensate for defective substrate or installation.

How to install it depends on the application and the substrate conditions.

Since it is in an elevator.  Depending on what the substrate is, I would recommend using an epoxy adhesive to bond the Hardibacker board to metal substrate or an ANSI A118.11 thin-set to bond it to a wood substrate.

I would use a liquid applied crack isolation membrane over the Hardibacker board and not the peel and stick, which can be more problematic.

I would use the ANSI A118.15 thin-set or an epoxy adhesive to bond the tile to the Hardibacker board.  Make sure you get 95% thin-set contact with no voids larger than 2 square inches (size of a golf ball), and make sure the corners and edges of the tile are fully supported.

I would use an epoxy grout, and along the perimeter of the elevator leave a ¼” gap and fill with backer rod and the ASTM C920 silicone sealant.

Make sure to use the same manufacturer for all of the setting materials to get the extended warranty that will provide you with at least a 10 year labor and material warranty.