Ask the Experts – October 2015


I’m writing you from the Tamp aarea. I’m a current NTCA member and have followed your writings for quite sometime. I’m a 30-plus year tiler and remodeler. I’ve run into a bit of a problem and I’m stumped, and wondering if you can help me out.

Recently I tiled a shower with 6” x 12” glass over waterproofing membrane. I’m getting some cracked tile on the field walls. First it was one but now I’m up to three. I used a 1/32” grout line and grouted with unsanded. Of course my setting material was unmodified. So please chime in as to why I might be having trouble. I would like to find a resolution before I make my repairs.

This glass product is some of the hardest glass I’ve ever worked with. Difficult to snap cut and wet saw cut with glass blade – it kept cracking in various places. I’m not sure if the quality of the glass poses a problem. Thanks for your help, thoughts and opinion.


Based on your description, it is likely surface tension or lack of flexural value in the glass and shrinkage of the thinset. Installing impervious glass over an impervious membrane causes a very long cure cycle. I have seen glass crack months after it was installed. Cracks when you cut say surface tension and shrinkage to me. It would be pretty expensive to prove that out but pretty sure it could be proven.

David M. Gobis CTC, LLC, Ceramic Tile Consultant


A condo complex called me to look at their elevators. Currently, they have five elevators with 1” x 1” mosaic that is crumbling after 10 years of use. They want to know what my suggestion is to replace it.

What have you seen normally go into an elevator? These are on the Gulf Coast and see lots of exposure to water. I see problems with the type of underlayment I would choose as well as the ability to adhere it to a metal floor. There is 3/4” between the elevator floor and the bottom of the door. I need material and installation recommendations.


I get this question fairly often or slightly different versions of it. Most often the cause of cracking tiles and grout joints can be attributed to excessive substrate deflection.

The elevator cabs chosen for these construction projects are not designed for tile or stone floor finishes. The manufacturer of these elevator cabs will list acceptable floor finishes that usually only include soft goods such as vinyl, carpet, and wood products. In order to be considered for tile or stone the substructure should be constructed in such a way as to not to deflect or “bend” more than a small amount under a concentrated heavy load. There are elevator cabs that are designed to meet these minimum requirements but they are usually much more expensive so they are not chosen in most construction

As for installing tile in these most common elevator cabs that are not designed for such, it is risky and not recommended.

There are products available that may reduce the risk of cracking tile and grout joints such as epoxies, but no warranties from these manufacturers are available. I hope this information helps.

Gerald Sloan, NTCA Trainer


I’m a San Diego handyman. I hired a licensed tile contractor to tile some shower enclosures standard 32” x 60.” I have 15 years experience and installed floors and showers myself.

ATE-1015The installer basically put large +/- 2”x 2” by 2-1/2” high blobs of thinset on the back corners and center of the tile (5) and pushed it onto the wall, leveling the tile with the adjacent pieces as he went. It does not stick out proud from the finished wall because he used 1/4” backer board on the studs. I have only seen 1/2” used but the manufacturer’s website does “approve” 1/4” for walls. Nowhere in the code could I find mention of substrate thickness.

I am concerned about the structure and more importantly the grout. It’s expected to just rely on the thin edges of the tile to hold it. I won’t be using him again but the union guy backing this installation method has me confused. Is this an approved type of installation?


You are correct to be concerned about this issue.

Industry and manufacturer standards recommend 80% coverage at dry areas and 95% coverage in wet areas and exteriors (both with all edges and corners supported). It is almost impossible to achieve these high coverage percentages using the “five spot”

Unfortunately “five spot” usage has become more prevalent in the tile industry in recent years due in part to the choice of large-format tile for the project, as well as the installer trying to install these large-format tiles over substrates that are not within proper flatness tolerances.

These choices by many installers are causing an inordinately high percentage of job failures. As far as using 1/4” backer directly over studs 16” o.c., I am uncertain whether that is an allowable practice according to the manufacturer.

Michael Whistler, NTCA Trainer