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What industry document supports installing stone over tile in a wet area?

QUESTION

I was curious if you could direct me to an industry document supporting the proper technique for installing Jerusalem limestone in a shower wall (stone over tile). The stone is 3/4” thick, and approximately 4’ x 5’. I don’t know the weight per foot. Can this be direct bonded or would it need mechanical fasteners? I’m looking for a document that would support one or the other.

ANSWER 1

When it comes to a large stone slab like this I need to direct you to the Natural Stone Institute (NSI) (naturalstoneinstitute.org). When 3/4” stone exceeds 24” on a side, it becomes outside the realm of typical tile installation and the NSI are the persons that best know how to handle this material. Their Dimension Stone Design Manual may address installing a slab over existing tile in a wet area, but I am not certain.

Adding mechanical fasteners may likely penetrate the waterproofing membrane, which can/will create problems with the system’s water management integrity. If the stone is not direct bonded with 95% coverage over the existing tile layer or membrane, any voids behind the stone may trap moisture, bleed through, and stain the stone (possibly permanently). NSI can speak to you about the suitability of limestone in a wet area. I am guessing this is a project you are involved with as the setting material manufacturer. You might want to advise the client that limestone can be a tricky stone for a wet area.

There are gauged porcelain tile panels (GPTP) that can be applied in a tile-over-tile, direct-bond situation without additional fasteners. You may want to suggest your client considers GPTP as an alternative. I can put you in contact with GPTP manufacturers/distributors if needed.

In the meantime, I am sure you know these things – but here are a few reminders for any tile-over-tile installations in a shower situation:

  • Ensure the existing tile installation/substrate is suitably well bonded, plumb, flat, etc., to support the new installation
  • Ensure the waterproofing membrane is intact or a new membrane is applied and connected to the pan
  • Take a look at TCNA Handbook methods TR418 and TR 420 for Shower Receptor Renovation details
  • Follow EJ-171 Movement Joint Guidelines

– Mark Heinlein, NTCA Technical Director

ANSWER 2

Here are a few more items for your consideration.

That slab of Jerusalem limestone would weigh approximately 205 lbs. dry and would increase when wet.

There are three classifications for limestone under ASTM C568 as follows:

  • Class I (low density) 7.5% to 12%
  • Class II (medium density) 3% to 7.5%
  • Class III (high density) up to 3%

The finish of the stone (normally honed) and the density of the stone will determine its porosity.

When it is installed in a wet environment (follow Mark’s advice on the installation), it should (must) be sealed using a breathable penetrating sealer per the manufacturer’s recommendations on the number of coats and dry time. Jerusalem limestone will require ongoing maintenance to keep it sealed and more easily cleaned. 

– Scott Carothers, Director of Certification and Training, CTEF

Mark Heinlein

Mark Heinlein is Training Director for the National Tile Contractors Association. He is Certified Tile Installer #1112 and currently a Ceramic Tile Education Foundation evaluator for the Certified Tile Installer program. Heinlein was the owner of Mark Heinlein Surfaces of Negaunee, Michigan.