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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.

Once vapor enters stone, it can condense and become a liquid. Since impregnators form a protective barrier against water in its liquid phase, this condensation-turned-liquid becomes trapped within the stone's pores. It will not escape unless it returns to its vapor form and evaporates.

Here’s a link to the full article on Pavlo’s website.

All high quality sealers are breathable, meaning they allow vapor permeability. What can happen is that water vapor can go into the stone, vapor then condenses into liquid and liquid becomes trapped because the sealer isn’t permeable for liquid, only for vapor. It will take a long time for liquid to evaporate and this can cause the discoloration that may last a very long time.

Some key points about drains in shower systems: [1:05:44]

- It’s very important that the weep holes are unplugged. Key is that the water hits the pan and exits the weep holes. Weep hole is the sub drain system that allows water to slowly drain out.

- In newer drains, the weep system is bigger which is important for these types of technical installations. Don’t just grab whichever drain is on the shelf; do some research.

- Weep holes can get plugged up without some type of protection system. When installer is doing drain test, if it’s draining slow, the installer needs to deal with it immediately or the problem will arise again in the future.

- The cement industry has changed; there’s more free lime than ever before which causes more calcium carbonate which means your shower is basically a limestone factory.

- mixing mortar too rich is a problem (i.e. 3:1); better to do 4:1, 5:1, or even 6:1.

- Overpacking the shower – overpacking with a steel trowel brings cement to the surface which seals the mortar bed shut. You want to use a wood float which leaves the pores open.

- Clogged weep holes can lead to more/bigger problems than just discoloration. This also can lead to wicking water up the walls.

More information about drains and weep holes can be found through the TCNA. All tile setters should familiarize themselves with these different kinds of assemblies.

Method 2: Bonded membrane. Bonding flange drain. Epoxy adhesive. Epoxy grout. Penetrating sealer. (Bulletproof sealer – 2 coats, with proper cure time between). [1:20:51]

It’s very important to have proper slope and full mortar coverage. Any moisture that goes below the tile will just evaporate will full coverage. After burning mortar into substrate, comb all the lines in a single direction (directional troweling), then need to take flat side of trowel going perpendicular to mortar lines and collapse all mortar ridges (becoming a uniform surface), the ensure 100% coverage with no voids. Perimeter joint needs to be fully filled with backer rod and proper 100% silicone sealer before installing the wall tile. With this method, water enters the stone (in the form of vapor) but not the grout or the mortar and because water is only in the stone, it quickly evaporates. So tile may darken temporarily after getting wet but will lighten up within a couple hours.

This method is not designed to have internal water evacuation (like the traditional method). Using incorrect drain can result in water vaper going under the stone, converting into liquid, and then it will hit the thin-set mortar. Using this method you can use a resin backed stone.


1. What is the mesh / adhesive backing on Z Collection Mosaics? [1:42:28]

Water based latex. If submerged, it is water soluble but just running the mosaic through the saw won’t cause it to fall apart.

2. What is the generally preferred method between traditional cement mortar and the epoxy method? Or is it always case-by-case? [1:43:46]

Always case by case. i.e. curbless shower and you have to use one of these membranes rather than the traditional pan method. Curbless showers are growing in popularity.

3. For a marble shower where stains (hair dye, etc) are a concern, would you tend to recommend the epoxy method? [1:49:21]

Pavlo recommends epoxy method for any Carrara application. It’s necessary to provide customers with maintenance information - how to treat their products, for example with regular cleaning using PH neutral cleaner.

Chris recommends for showers that are sealed to reseal them annually. Also, homes with very hard water may not be good candidates for marble showers.

4. Are dark spots / moisture just an aesthetic problem or does this compromise the integrity of the tile installation? Does your answer vary depending on the type of stone? [1:54:47]

Chris: no, the shower could be functioning properly and it’s just aesthetic. It could mean there are other problems with the installation though so it may require further investigation.

Dirk: if darkening (water) is going up the wall, you have a bigger problem (likely the weep holes are clogged) and it needs to be resolved.

5. Any issues with unmodified mortar bonding to meshed tile (polyester or otherwise)? [1:58:31]

Chris: I would do it. I would use modified. (if white marble, then use white thinset)

Dirk: If a mosaic, I would use modified.

Jason: I would use modified. Check the data sheets and manufacturer recommendations.

6. Do any of these recommendations change if the shower is a steam shower? [2:00:37]

Jason: Everything changes when it’s a steam shower. They are extremely technical.

Chris: Schluter (Kerdi) is a fantastic product for steam showers.

Pavlo: Red Guard is approved for steam showers (residentially about 3 coats to meet the minimum mil thickness requirement)

7. If using epoxy method, when the sealer wears off, what’s the process to dry out the shower and reseal it? Would you advise a homeowner do this themselves or should it only be handled by a professional? [2:02:58]

Pavlo recommends professional services. A homeowner can do a simple sealer test: drop few drops of water, if beads up is ok, if it goes into the stone then it’s time to reseal.

8. If using the traditional mortar method with unsealed tile, how would you advise homeowners to maintain their tile / grout? Would you caution against white grout in this scenario? [2:07:06]

Jason: I would not recommend white or black grout

Pavlo: previously there was no white grout, only grey. Ok to use white, in fact it’s his preferred choice but it has to be maintained. He recommends an annual alkaline deep clean.

Pavlo summarizes the key point: [2:14:58]

Don’t blame the stone when it comes to Carrara marble on shower floors. It can be successfully installed with predictable great results if using the two methods discussed today.

Download the PDF here:

ZBIZ Recap Technical
Download PDF • 804KB

More resources:

Natural Stone Council

Tile Council or North America

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