Lack of weep holes, uncollapsed mortar ridges may cause stone darkening in showers

An ongoing issue with stone shower installations is that sometimes stone showers mysteriously discolor or appear to not drain. There are various conditions and situations when this appears, and a range of explanations why this happens.

Recently a NTCA Five-Star Contractor had a problem with a Calacatta marble tile shower, installed with 4” x 4” tileable drains, utilizing a topical waterproof membrane system. The contractor reported that the waterproofing used is not leaking, the slope is done according to code and the thin-set mortar was chosen appropriately for the installation system and white marble. These topical membrane systems and drain assemblies are sometimes used in lieu of a traditional mortar bed shower pan application with weep holes.

The contractor contacted the manufacturer rep as well as the tile supplier, asking, “We are wondering if the water infiltrates the white marble from the exposed edges around the drain enclosures, and because of its porous nature, and because of the waterproofing underneath, it is taking much longer to dry.” 

Mark Heinlein, NTCA Training Director, enlisted the help of other industry experts to provide an answer, as well as giving his expert opinion.

“There is indeed concern in the tile industry regarding the situation you described,” Heinlein said. “The NTCA’s Technical Committee has been giving this issue serious consideration and has appointed a subcommittee to gather data in an attempt to quantify and hopefully identify the nature of the problem. To help me respond to your question, I have addressed Mr. Martin Brookes and Mr. Steve Young, who have been the persons actively pursuing the causes of this issue.”

Martin Brookes, of NTCA Five-Star Contractor Heritage Tile and Marble in Mill Valley, Calif., offered these thoughts: “The problem with this type of shower pan assembly is that there is no accommodation for a weep system and all water is evacuated topically. This in turn could create a problem around the drain with light colored stones or transparent tile,” he said. “The moisture can’t evacuate like a traditional water-in/water-out shower pan liner with a weep system. In addition, since the stone material is directly bonded to the membrane, allowing no room for error in the installation process, water can pool under the tile especially if the ridges are not fully collapsed. It is my opinion that if this type of system is used with the light colored stones or transparent tile, proper slope, mortar coverage and troweling techniques must be used to prevent any water accumulating underneath the tile surface.” Steve Young of Steve Young & Associates, White Plain, Ga., concurred.

Heinlein provided the contractor with documentation they could present to the client to address a similar problem on other showers on a current project.

“Page 252 of the 2018 NTCA Reference Manual is a sample letter (with photos) to prospective clients regarding this matter,” Heinlein said. “I have also spoken with James Woelfel, Chairman of the NTCA’s Technical Committee. James concurs with Martin’s assessment in that it is likely the troweled mortar ridges under the tile on the pan are not fully collapsed. This allows water to run under the tile through the bond coat and on top of the waterproofing membrane until it hits the drain assembly, where it then begins to pool in the uncollapsed ridges and is further absorbed by the tile. Additionally, there are some concerns that the stone could be sourced from quarries around the world and may be a similar species as Calacatta Marble, but may not have the same properties as Calacatta Marble.”

The contractor was elated at the information and solution to the mystery. “Thanks so much all!” she said. “We are so grateful to have such accessible expert resources!”

Editor for TileLetter, TileLetter Coverings, TREND and TECH publications.

Lesley Goddin has been writing and journaling since her first diary at age 11, and drawing and sketching since she could hold a pencil. Her penchant for observation led to her becoming a paid professional as a trade journalist, publicist and is editor for TileLetter. She has also written for Guideposts, Walls, Windows and Floors, Floor Covering Weekly, and Low Carb Energy.