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  • Lisa Rattan

ZBIZ Review - Technical Recap

This year we hosted a panel of expert certified tile installers to discuss marble moisture discoloration in shower floors. This problem can appear as a darkening in all or part of the shower floor. It’s not a stain, but an indication of moisture that is trapped under the marble. Often, given enough time, the moisture will evaporate but it can take up to six months to do so. Most commonly the problem presents itself around the drain, around and up walls, or sometimes around the perimeter.


The following is a summary of many of the points in our discussion. You can watch the full length of the discussion on our Youtube channel. Alternately if there are particular segments from this document that interest you, you will find time stamps in hours, minutes, and seconds shown bracketed in green like this: [0:00].

The conversation of marble discoloration centered around the research done over the past two years by installer Pavlo Starykov of Star Tile & Stone. He showed us six modules of shower floors, each utilizing different installation methods and materials, and walked us through the results. Adding to the conversation, we heard great insight from Dirk Sullivan of Hawthorne Tile, Chris Campbell of Campbell’s Tile Concepts, and Jason McDaniel of StoneMan Construction. We are so grateful to these gentlemen for taking the time to share their expertise with us.


Marble Facts [0:33:54]

1. Key Carrara marble properties: density, compressive strength, and hardness

2. Architectural definition: carbonate rock that can be finely carved and polished, so this also includes dense limestones that do not comply with the exact geological definition (i.e. Crema Marfil).

Is Marble Suitable for Wet Areas? [0:35:41]

Answer from Dimension Stone Manual from NSC:

“The action of water in areas such as fountains and showers is a factor to be reckoned with. Stones must be able to withstand frequent or continuous water projections, and in the case of showers, the presence of hot steam. Again, the best results are obtained with a dense, resistant stone, such as a granite, or a compact stone with a low absorption coefficient.

Carrara is translucent so darkening will be more obvious. However the stone is not porous. [0:37:24]

Absorption: ASTM C97 absorption coefficient by weight for most white marbles ranges from 0.9% to 0.11% which is equal to many granites.

Density: ASTM C97 test results often range from 2.716 to 2.722 kg/cu.m which is even higher than many granites.

In our strong opinion, most of the problems with marble shower applications arise due to inappropriate installation methods/techniques.

Related: [0:40:12]

TCNA Handbook for Ceramic, Glass, and Stone 2019 – Translucent Glass Mosaic in Showers

Bonding translucent glass tiles directly to membranes or other impervious surfaces is not recommended because any moisture trapped between the tile and membrane would be visible. Membranes should be placed behind or below the tile setting substrate where translucent glass tile will be installed. Select only opaque glass tile if the tile will be bonded to membrane.

Water Transfer in Brick, Stone, and Concrete - Porosity of Building Materials [0:41:47]

“In a porous medium such a sand pack, formed from solid particles, it is obvious that all the porosity is connected. A nanoscopic ant could wander throughout the void space and eventually visit all points within it. This means that all pore space is available for flow of gas or liquid and is in communication with the environment in which the material finds itself. However, as processes such as cementation and sintering act to reduce the porosity, it is possible for part for the pore space to become disconnected. Disconnections can be of several kinds, but most important for our purposes here is to recognize that some of the pore space may be entirely visited by and ant. Such closed porosity does not contribute to the transport of gases and liquids.”

There are two different methods to creating a shower pan that can both lead to successful installations. The key is that each method must follow a very specific corresponding setting technique and material specification.

(1) bonded waterproofing membrane that is right under the marble

(2) dry pack shower pan with a mud set bed; the water goes through the mortar and drains through the weep holes.

Pavlo demonstrates how water behaves when introduced to each method. [0:44:09]

(1) 3” traditional dry pack mortar – water immediately goes through dry pack (w/in 2-3 seconds). Water is pushed by gravity and exits underneath. It will take about 10 minutes for 80% of the water to pass through the dry pack mortar. This is called connected porosity.

(2) latex-modified mortar complying with ANSI 118.11 – It will take a few hours for water to pass through the mortar.

There are benefits and drawbacks of each but there are two methods that provide predictable and successful results with Carrara marble.

Method 1: Traditional dry pack mortar bed. Basic unmodified thinset. Unmodified grout. No sealer.

The principal behind this method is to provide water with a substrate that water can freely pass through; moisture is not trapped. Water seeps through stone and continues to pass through the dry pack mortar; it will hit the waterproofing membrane, and then will go through the weep hole channels in the drain. Any moisture left in the dry pack will evaporate.

Cons to this approach: nowadays many marble mosaics have a resin backing which has been added to reinforce the stone during handling, transport, and installation. The problem is that traditional mortars are not suitable for resin backed stone because there is not a good bond between tile and mortar. Tile can delaminate and moisture can collect under the marble because it can’t evaporate through the resin layer.

Some highly breathable sealers can be used with the traditional setting technique but the success of this method relies on the breathability of the installation.

Subject of Sealers – they are not a cure for all maladies. Sealer doesn’t prevent moisture discoloration. [0:53:38]

An informative article by Dr. Fredrick Hueston on the topic of sealers [0:59:50]

About Penetrating Sealers

Penetrating sealers, or impregnators are designed to penetrate below the surface of the stone and deposit solid particles into the pores of the stone and coat the individual minerals below the surface. With the pores filled, water, oil, and dirt are restricted from entering the stone. Impregnators can be solvent or water based, and most impregnators are vapor permeable or breathable, which means water vapor is able to pass through the sealed stone.

Why not seal stone in wet environments?

Although most impregnators on the market today are breathable, this does not mean they are suitable for wet environments. Stone sealed with a penetrating sealer is protected from water entering the pores of the stone in liquid form, but these sealers will still allow water vapor in the form of humidity, steam, and other forms to pass into the stone. Moisture absorption inevitably occurs when stone is exposed to unregulated humidity, temperature fluctuations, and the like.