{{attachment:4645023}}

What is the cause of marble tile discoloration in a shower?

Is Carrara marble shower floor darkening normal or it indicates an underlying problem?

How to successfully install light-colored marble on shower floors with water-in, water-out method?

The topic of marble moisture discoloration on shower floors remains extremely "hot" for the tile & stone industry in the U.S. I often receive phone calls and emails from people who try to understand what happened with their recently installed marble shower floor and whether it is possible to remove moisture discoloration from it. The biggest problem, as I can see it, is that many even very skilled and experienced installers fall into that issue.

If you consider yourself to be a professional installer, please do not blame marble as stone not suitable for wet areas.

>> See MARBLE MOISTURE DISCOLORATION: DON'T BLAME THE STONE!

Some professionals install it with great quality and results. If you do not believe me, check the work of K Nardone Custom Tile Work LLC, for example (https://www.facebook.com/KNardonecustomtilework/), and there are many other tile & stone mechanics who also have a long record of successful marble installations in wet areas.

According to my reasonable research and testing, there is two methods that, when properly followed, will provide predictable great results with light-colored marble on shower floors (not to mention maintenance & care products and instructions customers have to be provided with by a professional contractor).

Method #1: traditional dry-packed mortar bed shower pan (also known as water-in, water-out method).

Method #2: bonded waterproof membrane shower pan with epoxy adhesive and grout.

Bellow you will find my recommendations on how to successfully install Carrara marble on a shower floor with Method #1.

1) The benefits of the "old school" shower pan method for light-colored marble.

A properly installed dry-packed mortar bed provides a bedding of the connected porosity when all pore space is available for flow of liquid allowing water to be “taken away” from the underside of marble mosaic and be evacuated via weep holes in the clamping collar.

2) The mortar bed

Stone has to be installed "in a dry-packed mortar bed consisting of one part Portland cement to four to five parts sand" (Dimension Stone Design Manual Version VIII, Chapter 16, Stone Tile Residential Showers, Steam Rooms, and Steam Showers, 2.2).

Pre-pitch and final slope must be done by industry standards with a minimum 2% (or 1/4" per foot) slope when measured from the farthest point to drain.

3) Mixing

Avoid over watering the sand while mixing. All you need is that the squeezed clump of mixed "mud" should hold its shape without sagging or falling apart. Use a water measuring cup and carefully follow mortar manufacturer's instructions for mixing and water ratio. Preferably, do not install floor tile the next day. Instead, give an extra 24 hours for drying of the final layer of mortar bed.

4) Waterproofing membrane

The waterproofing membrane must be smoothly continued all the way to the drain opening passed the weep holes so that there is no "dams" or elevations in water's way to evacuate.

Pay attention that there are NO wrinkles, bird baths/low spots in the PVC/CPE pan liner or lead/copper/hot-mop system because even a 1/16" elevation on the pre-slope can cause water to build up which can be enough for moisture to start going upwards and saturate the stone through capillary action.

The pre-slope must be finished either flush with the drain flange or even a little higher to avoid any water/moisture accumulation around drain. When a loosely laid pan liner is installed, make sure that the bead of sealant that is applied to provide a water-tight connection between the membrane and the drain flange is sufficient to seal the seam but doesn't cause any build up or clogs the weep holes.

For more information see TCNA Handbook 2020, Wet Areas Guidelines, pages 41-42.

5) Finished surface of the mortar bed

To provide better water absorption and evacuation, do not over compact the dry-packed mortar and do not finish it too smooth. "Polished, glossy, honed, or smooth backup surfaces shall be roughened by sanding or scarifying" (Dimension Stone Design Manual Version VIII, Chapter 13. Installation, General Information. 3.1.2.4. Suitable backings).

6) Rapid-set mortar screeds

Do not use rapid-set polymer modified mortar screeds because they have reduced porosity which can significantly slow down water absorbency, migration, and evacuation.

7) Weep holes protection

Weep holes must be protected from clogging and, in my opinion, tile spacers would be a better choice than pea gravel that can become calcified over time. Plastic positive weep protectors, produced by some manufacturers, can become laying flat being pressed by the dry-packed mortar thus creating an impervious layer around drain that can prevent water to freely evacuate through weep holes in the clamping ring. If possible, additional weep holes can be made/drilled in the clamping ring to maximize water evacuation.

8) Setting material

Marble has to be installed with full white mortar coverage by collapsing the ridges before stone tiles are placed. "100% mortar coverage between floor and sub-slab is recommended" (Dimension Stone Design Manual Version VIII, Chapter 16, Stone Tile Residential Showers, Steam Rooms, and Steam Showers, 2.2). In my opinion, unmodified (ANSI 118.1) thin-set is the best choice to install marble over a dry-packed shower floor due to its higher porosity and drainage capability than of modified mortars that are more water-repellent and could cause moisture entrapment within the marble shower floor.

9) Perimeter soft joint

If shower floor is tiled before the walls, the perimeter joint has to be FULLY filled with an appropriate 100% silicone sealant complying with ASTM C920 and installed per manufacturer's directions and industry standards for expansion/soft joints (see ANSI A108.01, Section 3.7.4.1 and EJ171 Movement Joint Guidelines for Ceramic, Glass, and Stone in the TCNA Handbook 2020, pages 430-433). Such precaution will not allow moisture to accumulate in the perimeter of the shower floor causing discoloration along the walls and curb. Always read and follow sealant manufacturer's instructions recommendations before applying silicone sealants. Some products can stain natural stone and require the use of a special primer (see TCNA Handbook 2020, Natural Stone Tile Selection and Installation Guide, Considerations When Selecting Installation Materials, pages 12-13).

{{attachment:4645040}}

Will 100% silicone sealant in the perimeter joint create a water build up by preventing it from going into the mortar bed from behind the wall tiles?

It is important to understand that very little amount of water will go behind the wall tiles that are properly installed with sufficient mortar coverage (in dry and wet areas, the minimum coverage is 95% with no voids exceeding 2 square inches and no voids within 2" of tile corners, see TCNA Handbook 2020, Mortar Application and Coverage, Mortar Coverage for Natural Stone Tile, page 35).

10) What about natural stone on walls?

In my opinion, while simple setting materials with higher porosity are the best choice to install marble over a water-in, water-out shower pan, a premium water-repellent (hydrophobic) modified thin-set or medium bed mortar can be used to install wall stone tiles. Such mortars will provide a longer open time, better coverage for large format stone tiles.

It is important to apply such setting material with an appropriate notch trowel over a waterproof backer/foam board using directional troweling method that allows to achieve almost full mortar contact with substrate and eliminate large voids (see TCNA Handbook 2020, Mortar Application and Coverage, Directional Troweling, page 35). Applying a water-repellent adhesive over a non-waterproof masonry wall or cement backer board with only a vapor-retarding membrane behind it can eventually lead to moisture entrapment in the wall assembly.

It is important to apply such setting material with an appropriate notch trowel over a waterproof backer/foam board using directional troweling method that allows to achieve almost full mortar contact with substrate and eliminate large voids. Applying a water-repellent adhesive over a non-waterproof masonry wall or cement backer board with only a vapor-retarding membrane behind it can eventually lead to moisture entrapment in the wall assembly.

{{attachment:4645041}}

More information about the availability of hydrophobic mortars can be obtained from local mortar manufacturer's representatives. Some synthetic additives that can be used with premium modified mortars are also available on the market.

Since the water absorption of a premium modified mortar, even reinforced with a synthetic additive, will still most likely be higher than of epoxy adhesive, we would recommend to limit the use of it only on walls and ceilings and to not use it as a floor thin-set mortar for Method #2 mentioned earlier.

11) Resin-backed stone

A VERY IMPORTANT LIMITATION: "Stone tile with adhered fiberglass mesh reinforcement on their back surfaces are not to be used for shower applications" (Dimension Stone Design Manual Version VIII, Chapter 16, Wet Areas, Stone Tile Residential Showers, Steam Rooms, and Steam Showers, 1.3).

{{attachment:4645042}}

Such "adhered fiberglass mesh reinforcement", also known as "resin backing", is used to reinforce stone tile and to reduce the likelihood of fracture. The adhesive used to attach the mesh is not standardized, but is commonly an epoxy or polyester-based adhesive that truly seals the back of the stone, eliminating vapor transfer and significantly reducing the ability of Portland cement-based thin-set mortars to adhere to it (see TCNA Handbook 2020, Natural Stone Tile Selection and Installation Guide, Fiberglass Mesh Reinforced Stone, page 10, and Dimension Stone Design Manual Version VIII, Chapter 21, Restoration and Maintenance, 4.3.3).

Again, such backing adds an impervious/water repellent layer to the back of stone reducing its "breathability" and creating a potential for moisture to be trapped under the stone. "Resin backing" must be either completely removed or marble has to be installed with Method #2 (bonded waterproof membrane shower pan with epoxy adhesive and grout).

>> See INSTALLATION OF MARBLE IN WET AND DRY AREAS

12) What about impregnating sealers?

To achieve predictable results with no moisture discoloration NO sealer/impregnator can be applied to marble installed with a water-in water-out method otherwise it can trap moisture that will try to evaporate from inside and under stone.

>>See STONE IMPREGNATING SEALERS IN WET ENVIRONMENTS? BAD IDEA by Dr. Frederick Hueston of Stone Forensics.

Only marble installed with Method #2 has to be sealed with a very permeable ("breathable") impregnator like StoneTech Bulletproof to increase the water-repellency of the method and to prevent epoxy resins from blocking the pores of stone.

Only marble installed with Method #2 has to be sealed with a very permeable ("breathable") impregnator like StoneTech Bulletproof to increase the water-repellency of the method and to prevent epoxy resins from blocking the pores of stone.

>> See ZBIZ 2020 STONE PANEL - HOW TO INSTALL MARBLE ON SHOWER FLOORS?

13) What grout to use?

To provide proper water evacuation and moisture evaporation within the traditional dry-packed mortar bed shower pan DO NOT USE epoxy/urethane/quartz grout or "highly stain-resistant" sanded grout because such products, having very low porosity, can increase the chances for moisture entrapment within the dry-packed mortar bed under the stone. Such products are a great choice for ceramic, porcelain, and glass tile but not for marble on shower floors. In my opinion, best grout choice would be a simple grout (use caution when applying sanded grout to avoid scratching of stone).

If you doubt the performance of stone tiles installed with simple setting materials, then look at the following picture of a 20-year old marble shower that was installed with such materials and looks almost perfect after so many years because of basic maintenance & care procedures the homeowner provides.

{{attachment:4645043}}

14) Maintenance & care

Educate your customers about basic maintenance & care of their newly installed natural stone shower pan and provide them with a quality PH-neutral cleaner.

For more information see A Guide to the Care & Cleaning of Natural Stone by Natural Stone Institute.

{{attachment:4645044}}

Guest bath

COMPANY

EXPLORE HOUZZ

CONNECT WITH US