Ask the Experts – April 2016


I am installing an indoor/outdoor patio for a continuous look on a cement substrate. The tile is 16” x 48” porcelain. The outdoor portion will be raised approximately 2” with a mud float. Should I use a bonded mud float or install a cleavage membrane? Additionally should I reinforce the float with lathe or not? Finally, I am in Austin, Texas, so I am wondering if I should put a membrane on top of the float to help prevent problems when there is a rare freeze. Thank you for your expertise.


Thank you for contacting the National Tile Contractors Association.

The closest TCNA Handbook for Ceramic, Glass and Stone Tile Installation method to follow for the exterior portion of this project is F103B. This method details a wire-reinforced mortar bed (maximum 2” thick) on top of a drainage mat/system. A drainage mat makes an ideal cleavage membrane since it promotes quicker evacuation of water from the tile/mortar bed system. Refer to TCNA Handbook methods F103B, F111 and F121 for details including specifications for cementitious bond coat mortar.

Sloping away from the structure is required for any exterior area, both for the substrate and the mortar bed. A minimum slope of 1/4” in 12” of horizontal run is required and is the typical specification to meet code. The slope of the substrate and the mortar bed need to match each other to produce a uniform thickness mortar bed. Prevention of water flooding back to the structure is the main reason, but slope and drainage are also a great help in reducing efflorescence issues.

A waterproof membrane is a good idea as long as there is not excessive moisture in or beneath the slab, which can be caused by poor landscape drainage design or clay-type soils for an on-ground installation. Moisture in the slab should be checked before deciding to install a waterproofing or crack-isolation membrane. Most membranes are suitable for moisture presence up 5 lbs/1,000 sq. ft. Some manufacturers state their product can be used up to 12 lbs/1,000 sq. ft. Check with the manufacturer of the membrane to determine what their product can handle. Moisture in excess of 12 lbs/1,000 sq. ft. will require significant mitigation before the project can proceed.

It is critical to pay strict attention to the planning and installing of soft joints and expansion joints. Installation of a membrane does not eliminate the need for proper movement joint placement. Soft joints must be installed in the mud bed and tile in line with the control joints. Expansion joints must be installed in the tile every 8’ – 10’ in the exterior and in the interior section if it is adjacent to a large surface area of windows and glass doors. Expansion joints are also required at the interior/exterior dividing wall and all other interior and exterior walls, cabinetry etc. See TCNA Handbook method EJ-171 for more details.

To ensure acceptable lippage with the installation of the large format tile it is very important to begin with a flat substrate. Your mud bed flatness must ensure a maximum variation in plane of 1/8” in 10’. You will also want to consider a 1/3rd offset layout if the tiles have any warpage. A lippage tuning system may also be beneficial.

I hope this helps!
Mark Heinlein,
NTCA technical trainer

Ask the Experts – March 2016


I have a question regarding polished porcelain mosaic tiles. Polished porcelain is becoming more popular these days with so many “marble” looks being manufactured. I have a client wanting to use a polished porcelain for her shower walls and would like to use the coordinating polished porcelain 1.5” x 2.5” basket weave mosaic for the shower floor (it is not available in matte). Is it ok to use any polished porcelain mosaic on a shower floor? Please advise.


Small mosaics generally do not pose a slip/fall hazard in wet areas, even if they are polished. The grout joints are so closely spaced that they create a type of textured surface that lends good traction, even in wet areas with lots of water on the surface. Mosaics have traditionally been used in shower floors with high success and little risk of liability.

– Michael Whistler
NTCA Trainer


Our company has been asked to look at a marble floor that is staining and “rusting.” This has only started happening within the last year or two of the floor’s 15-20 year life. There seems to be no etching or loss of sheen (which does not rule out chemical absorption, I am aware). We have been assured that the cleaning process/chemicals have not changed. The floor is on concrete, with occupied space below, and no evidence of moisture-related damage in that area’s ceiling. Before tearing out this floor and replacing it, I would like to be able to suggest what may be the cause and possible solution prior to going in and having unforeseen issues. Any assistance would be greatly appreciated.


White marble tiles can contain deposits of iron. As a mineral, iron oxidizes and turns the marble yellow or brownish red when exposed to water or acids, similar to the way metal rusts. It is possible for the oxidation process to occur many years after the tile was installed. The rusting of the tile may have been initiated or accelerated if the tile installation has recently been exposed to a large amount of water or if the chemistry of the wash water has changed.

It is possible to check the water supply for iron. A plumbing supply store should be able to provide a test kit. If iron is detected, the water supply can be chemically treated to remove the iron.

The tile itself can be tested for iron content. A yellowed tile can be removed from the installation and sent to a lab for testing. If there is attic stock tile that has never been installed, it can also be tested. Comparing the test results of the tiles will help determine whether the iron content was native to the tile or introduced through the water supply.

The rusting process is a chemical change internal to the tile and is difficult or impossible to reverse. It may be possible to remove the stain by applying a poultice consisting of a thick plaster of a product called Iron Out, which contains sodium sulfites. Test a small area by applying the paste and keep it damp for several hours. Remove the solution with a wet vac and clean water. Make sure your wash water does not contain iron. If the stain is removed, the entire floor can be treated in this manner. The process will cause some etching and the surface will likely have to be re-polished.

If you determine iron oxidation is not the cause, you may discover the cleaning agents or the cleaning procedure has changed (such as using the same mop and wash water on the marble after being used in a dirty environment). Marble is porous and has naturally occurring pinholes. After years of wear, the surface of the marble may have become less polished and more open to absorption of dirt. Normal traffic or a dirty mop could be introducing dirt into the pinholes.

After you have ruled out iron mineral staining and suspect cleaning is the cause, it is possible to clean the marble with an alkaline solution. Test the alkaline cleaning solution on a small area before attempting the entire floor. The cleaning process will require scrubbing, which will likely dull the surface. The surface can be re-polished.

The size and scope of the project may determine the best course of action. Ultimately, removal and replacement may be the solution.

I will be interested to know what you discover in your investigation and the approach you take to correct the issue.

– Mark Heinlein
NTCA Trainer

Ask the experts – February 2016


Our house is currently under construction. The contractor applied a 1/4” grout for the 7” x 20” wood look-alike ceramic tile. We want to know the standard application of grout for this kind of tile to prevent lippage. Hope to hear from you soon. Thank you.


Grout is an important structural component of the tile installation. Grout joint size is determined by a number of factors such as the grade of the tile, variation in tile thickness, warpage (cupping or bowing) within each tile, wedging (variation in a tile’s facial dimensions), installation pattern, and others. Depending on the quality and grade of the plank type tile you have described, we often see many physical variations that must be accommodated.

While design preferences and final appearance are also important considerations, the installer must factor all the variations to determine the appropriate grout joint width that will produce an installation with allowable lippage. Without personally knowing the factors of your installation, a 1/4” grout joint is not uncommon and may be the most appropriate.

– Mark Heinlein, NTCA presenter


cracked-glassWe have had a lot of difficulty with two tiles that keep breaking with our shower install. We have no choice but to cut the glass tiles because there has to be an opening for the water. We’ve followed all of manufacturer’s suggested techniques: wet drill, drilling from the back etc. Do you have any suggestions?


Glass tile, particularly large format as these seem to be, can be extremely difficult to cut successfully. You stated that you are using wet drilling, so that leads me to believe you are using a diamond core bit (possibly 1-1/4”). You need to ensure that the hole saw is sharp (new diamonds exposed) so undue pressure is not exerted on the tile while being cut. Sharpen a diamond core bit (or diamond blade) by wet cutting several times through a very abrasive material such as an aluminum oxide rub stone or a piece of concrete cinder block. When you wet cut the tile, you need to use LOTS of water, as any heat build-up can result in failure. Also, drill very, very slowly, again to not exert undue pressure on the tile. It seems that cutting from the front AND back of the tile can usually give a successful result. Mark the tile on both sides, wet drill slowly half way through the face side of the tile and stop. Turn the tile onto its face, preferably on a towel to avoid scratching, and slowly wet drill from the back of the tile to meet the cut you previously made. This trick usually works.

One further point: many diamond core bits on the market come with a masonry type pilot bit. This bit needs to be removed for glass tile (and really any tile harder than a 4” x 4” or 6” x 6” talc body tile). Begin cutting with the core bit by holding the drill at a slight angle to the face of the tile, and with a steady hand, begin the cut. After cutting in about 1/16”, start slowly rotating the drill more and more upright until you have created a completely circular kerf. You may then begin drilling perpendicular to the tile (exerting very little pressure).

You probably do not have an abundance of extra tiles, so hopefully this will work for you on the first try. If you still experience cracking of the tile, you may need to enlist the help of an abrasive waterjet cutting company. Most cities and even towns have one these days. Just mark the tile, take it to the waterjet guys, and for a nominal fee they will make your cut. This method of cutting puts almost zero pressure on the tile.

I also noticed from your photos that the tile had cracked under an installed plumbing fixture. Tub spouts usually screw into place, and it is very easy for an installer to slightly over-tighten the fixture to get a nice tight fit, resulting in cracking. It is much better with glass tile to leave the fixture one revolution loose and to use a 100% silicone sealant (caulking) between the fixture and the face of the tile.

Hope this helps, and feel free to call me with additional questions.

– Michael K. Whistler, NTCA presenter

Ask the Experts – January 2016



I have a small tile job at the TIA (Tampa International Airport). The general contractor (GC) is directing us to start the work with temporary lighting that is not very good. We have asked that they place either the permanent lighting or temporary light “representative” of the permanent lighting so that we can see exactly the conditions for the installation.

I could have sworn that I read something at some point with regard to tile being installed in the lighting in which it is to be used.Can you help me with some type of literature on this item?



Thank you for contacting the National Tile Contractors Association. I am happy to provide you with some information to help you respond to your GC regarding the installation of tile under poor temporary lighting conditions that are likely not representative of the permanent lighting.

We are not aware of a requirement for the GC to install the permanent lighting before tile work commences. However, you are correct to be concerned about the location of permanent lighting and its potential effect on viewing the finished surface of the tile installation. The angle of natural or manufactured light on finished tile work may cause an acceptable installation to have an unacceptable appearance.

While there is no specific method that addresses installation of lighting, the 2015 edition of the TCNA Handbook includes a statement about various types of lighting and their effect on wall and floor tile installations. When proper backing surfaces, installation materials, installation methods, location of light sources and certain lighting techniques are not carefully coordinated, shadows and undesirable effects may be apparent on finished ceramic tile installations.

The 2015/2016 edition of the NTCA Reference Manual includes an extensive discussion of critical lighting effects on tile installations. This discussion is found in Chapter 5, “Special Installation Procedures,” and addresses many problems along with tips and preventive measures tile contractors can take or recommend before the installation begins. Also included is a series of photographs that boldly illustrate the effect lighting can sometimes have on a tile installation.

The success of your particular installation will depend on a variety of factors such as: flatness of the substrate (which will have a profound effect on the flatness of the finished tile surface); the type of tile being installed; the pattern the tile will be installed in; the inherent warpage of the tile; the grout joint width; any out-of-tolerance lippage of the finished tile installation, etc.

As required by ANSI, we encourage you to construct a mockup of the area to be tiled and illuminate it with an accurate representation of the permanent lighting installation. Have the GC or architect review the mockup and accept it in writing before beginning the tile installation.

If you are a member of the NTCA, please review pages 139 – 144 of the NTCA Reference Manual. There you should find all the information you need, including a sample letter you will be able to reformat as your own company’s formal communication to the GC and/or the owner.

If you are not a member of the NTCA, please visit our website at or contact me and I will be happy to assist with your application.

Mark Heinlein, NTCA Technical Trainer

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

Ask the Experts – September 2015


I’m contacting you regarding talc on the back of tiles. Since there are no standards or guidelines regarding talc on the back of tiles from the manufacturer, what does your organization suggest for the tile installer? Are they required to clean this off, or can it be installed as is?


The talc is actually called “kiln release” and is used to stop tiles from fusing to the conveyor belts in tile firing kilns. Usually there is not enough to create a problem, but when there is too much, which will create bonding failures, you have two choices: wash the backs of all the tiles and allow to dry, or back butter all the tiles with the flat side of the trowel.

– Michael Whistler, NTCA presenter


I have a question regarding the TCNA Handbook. Is it required to have a soft joint between the floor tile and the vertical tile bullnose base?


According to Method EJ- 171 in the TCNA Handbook, you must provide movement accommodation at all perimeters as well as all changes in plane. So if you install your floor tile first, you must provide an open space (or sealant-filled joint) between the floor tile and the wall (or cabinet, toe kick, tub, etc.). Then set the tile base on top of the floor tile, thereby covering the visible gap.

If you are installing the base first, you still must have perimeter movement accommodation, accomplished by leaving the joint empty of mortar and grout and filling with compressible backer-rod and sealant, or use a pre-manufactured perimeter movement joint (available from a number of manufacturers). These details are shown in Method EJ-171 in the TCNA Handbook.

– Michael Whistler, NTCA presenter


We are dealing with a 70-plus hollow-tile issue. Our villa was built and the tile installed in 2011. The hollow tile started in January 2015 with two hollow tiles. As of May 2015, there are 70-plus hollow tiles. We would like to know the cause of the problem and the appropriate resolution.


Hollow tile is usually caused by bond loss. The timeline you describe is relatively common when tile is not provided with appropriate movement accommodation, an important part of any tile installation.

– Dave Gobis, CTC, CSI, Ceramic Tile Consultant

INQUIRY, continued

The original installer concluded the installation was good but the concrete settled, therefore the tile detached and became hollow. With this being the situation, he said this is what is supposed to happen (meaning the tile should become hollow rather than cracking) and that we are lucky we don’t have more cracked tile (about 10 now). He wants to inject the 70-plus hollow tiles with epoxy and replace the cracked grout and drill holes from the injection. This view is supported by the builder with whom the floor is under warranty until December 2015.

The two independent tile companies both concluded that the tile was improperly installed and needs to be replaced to address the underlying problem and that when tile is installed properly this should not happen (meaning 70 hollow tiles).

Our villa is in Venice, Fla. There is about 900 sq. ft. of ceramic tile. There are hollow tiles in all six tiled rooms with one room not being connected to the others. The hallway is where two hollow tiles started and now nearly all the tile in the hallway are hollow with cracked grout.

If you could advise as to the proper procedure to correct this tile issue, we would greatly appreciate it. Please contact us if you have any questions or need any clarifications. Thank you for your help.

ANSWER, continued

Your original installer is uneducated, ill informed, and incorrect.

I can’t speak for the other two tile contractors but they know more than the first one.

Injecting is something done to mask installation shortcomings. Based on your pictures, there is minimal bond to the surface, plus I still believe, a lack of movement accommodation. This is a very common problem and one that occurs frequently in Florida. I have been to Florida numerous times for this issue for several years now. Based on what I see replacement is probably appropriate.

– Dave Gobis, CTC, CSI, Ceramic Tile Consultant


Ask the Experts – August 2015 “Green Issue”


I was referred to you through my tile supplier. I am a general contractor and I would like to ask you a question regarding a former project of mine.

I took on a tile job for a person in an electric wheelchair. The chair weighs several hundred pounds (plus the amount of the person, probably 170 lbs-180 lbs). Before I started, the old tile in her hall, kitchen and entryway had broken, resulting in loose tiles. She had told me that her wheelchair was the cause of the damage. Those tiles were 12” x 12”, installed long ago. She wanted new tile installed throughout the house. I installed 18” x 18” ceramic tiles. I used flexible thinset, and 1/4” cement board. I staggered the board and used screws (in the correct sequence) as recommended by the manufacturer’s instructions. The joists are 24” center. Expansion joints were used throughout.

I have installed many jobs with these specs in the past without any issue whatsoever. The customer has said some of the tiles have loosened and the grout is cracking. I suspect this is due to the wheelchair (which is, of course, out of my control). I am looking for your opinion if you can share it with me based on this information.


There is no industry method for 1/4” backer over 24” centers. I understand it is done often and if all the stars are aligned and everything done correctly it might work. However, with a rolling wheel load extra precautions would be required. The PSI of a wheel is much greater than normal foot traffic. The deflection between 24” centers is too great. Flexible thinset doesn’t compensate for lack of a supporting structure. You need another layer of plywood at a minimum and I would consider some bridging as well to stabilize the truss/joist.

David M. Gobis CTC CSI, Ceramic Tile Consultant


I’ve been setting tile on and off for the past 11 years, and it wasn’t several years after getting started that I learned about the importance of using expansion joints in tile. That’s thanks to working under Mike Hearn, one of the few Certified Tile Installers in Atlanta, Ga.

I’d like to get your opinion on one project. I just moved to Guayaquil, Ecuador with my wife to be closer to her family. My father-in-law is in the final stages of building a house here where all the flooring and bathrooms are tile. They are nearly finished installing 24” x 24” rectified porcelain upstairs and 24” x 24” non-rectified porcelain downstairs. Each level is approximately 1000 s.f. and is tiled continuously throughout the entire floor.

On the rectified tile, the joint looks like maybe 1/32” and on the non-rectified it looks like about 1/16”. I emphasized to my father-in-law on several occasions the importance of expansion joints.  I’ve even pointed out how you can see them used here in malls and many commercial applications.  Well, they did not use any – and they grouted in hard around the perimeter.

thermal-heatThe subfloor is concrete and the walls are concrete as well. The first thing he said when I walked in was that he checked on it, and because here the temperature variations are minimal, there was no need for expansion joints. This is frustrating because they could have at least left a gap around the perimeter without any aesthetic change since it will be covered by base.

So I did some research on temperature variations here in Guayaquil. The average monthly temperature variation is only 5 degrees Fahrenheit, that’s the difference between the lowest average monthly temperature and the highest average monthly temperature. The average daily variation, and also the max, from day to night is 16 degrees Fahrenheit.

The only thing I could think to do about it now would be to cut out expansion joints around the perimeter of the rooms, but I’d like to get your opinion before I confront my father-in-law.

I know in the end this is his problem, but having worked in the industry I hate to see so much money invested into projects that will likely fail prematurely.


I had an argument like this on a project in Hawaii. They said the other projects had been in more than 20 years with no problems and chose to not use soft joints. A few years later one of those projects lost bond.

Temperate climates are helpful, but:

• Soils do move

• Tile gains size long term with moisture absorption from the slab, and cleaning.

• Sun exposure creates unequal thermal mass.

• Large tile has a fraction of the grout joints smaller tile does. Compression strength of grout is, maybe 1,500 lbs to 3,000 lbs. Compression strength of tile, laterally, is 25,000 lbs – 30,000 lbs. Big tile, fewer sacrificial grout joints. My experience is if you have a 12” x 12” and a 24” x 24” in the same installation, like a border, the 24” will go first. I see this a lot on malls where they have a fair amount of footage to observe.

• Small joints or butt joints have nearly no buffer of sacrificial grout.

Based on temperature range alone, this is likely not a problem. But, everything in a building moves and it all moves at different rates. My opinion would be this is low risk – not no risk – unless it is wet soil and sunny, then raise it a few notches. The picture attached shows the different temperature early in the morning on a floor that received sun exposure (yellow) and the shaded area (purple).

I have been down to Mexico a half dozen times in recent years in the Southern part of the country for bond loss due to expansion issues.

– David M. Gobis CTC CSI, Ceramic Tile Consultant

Ask the Experts – July 2015


I have a new construction (2008) post-tensioned slab where very small cracks have developed in my travertine tile flooring (ground floor). I had a contractor come out and he said that the standards recommend installing a slip sheet before applying the tile. Is there a professional article that I can use to back up my assertions to the original tile contractor that there should have been a slip sheet?


Since your installation was done in 2008, that year’s industry standards must be consulted. The 2008 TCNA Handbook for Ceramic, Glass and Stone Tile Installation states in method F113-07 in the Limitations section that “Method F111 is the preferred method over precast concrete floor systems, post-tensioned concrete floor systems and other floors subject to movement or deflection.”

Method F111 is an unbonded (includes cleavage membrane) wire-reinforced mortar bed, minimum 1-1/4” and maximum 2” thick. Hope this helps.

Michael Whistler, NTCA presenter


Could you direct me to a simple method to calculate the regulated flexural strength of cementitious floor tiles (i.e. equation or formula) for various areas of application (i.e. residential use vs. commercial use vs. industrial use)? There must be an ASTM standard for these floor coverings and a simple way to determine these flexural strengths depending on the length/width/thickness of the tiles. European standards use EN 14411 with the following formula: Breaking force F(N) = 2 x ß x h² x b /3 x L in which:

1. ß is flexural tensile strength of the tile (N/mm2)

2. h is tile thickness in mm

3. b is tile width in mm

4. L is tile length in mm

Thank you for the time that you will allocate to this information request.


There is no standard for cement floor tile, though there have been discussions over the years.

Relative to your question about flexural value, the U.S. does not address that issue in ceramic tile well. In addition to EN 14411, there is EN 14617 for agglomerated stone. I have found the 14617 standards of greater value when considering cementitious floor tile, primarily in the area of dimensional stability. On flexural value, architects typically put the cementitious tile in section 09300 and treat it as floor tile. When I get a claim on cement tile, one of the tests I often use is ISO 10545-4 because it has a standard for flexural value for ceramic tile. While ceramic is >30 N/mm2 and porcelain is 35 N/mm2, I often find cement tile anywhere from 10 to 17 N/mm2. This requires a surface with less curvature than a conventional tile to be successful. I have had well over a dozen large claims on cementitious floor tile. In the majority of them this was an issue and the dimensional stability was a problem as well.

David M. Gobis CTC CSI, Ceramic Tile Consultant

Ask the Experts – June 2015


I am a contractor from Northern California and have been setting tile for 23 years. I did a job where two shower walls cracked. The ceramic tile with a high gloss finish, bought from a local distributor, cracked from the inside. No one is sure why these tiles cracked in many directions. The grout lines did not crack, just the tiles, looking like a crackle or crazing under the surface.

The contractor board and its expert came in. He said I used the wrong materials and did not let the mortar bed cure long enough. He did not say how long a mortar bed should cure and he did not ask me how long before I started to set the tile.

crazingThe materials I used for my float bed include a moisture vapor retarder and 20-gauge stucco netting, stapled into the wall. All these products were bought from my local distributor, and are the same materials sold at all the tile stores in my area.

The inspector said I used the wrong materials. I have used these same materials for 28 years. I was in the tile union when I started and papered and wired apartments in the Bay Area. These are the only products I know, so I was very surprised when I was told I am using the wrong materials. If so, why are the tile stores all selling the wrong materials?

I have spoken with the Northern California Tile Institute and was told that in northern California 20-gauge, one-inch galvanized stucco netting stapled to substrate is acceptable in residential applications not to exceed nine feet in height. This is the application I use as well as most of the guys around here.

I am confused. I do not believe that is the reason the tiles cracked. If you could help me in any way, I would greatly appreciate it. I could give more information if you need it.


You are correct that the method you describe using stucco netting (chicken wire) is considered an “Allowed Local Practice” in your area and is allowed when a local building inspector comes to check your rough-in. A few Northern California counties and cities are pretty much the only places in the USA that do allow this method.

This practice was developed in Northern California by large union shops for tiling tub splashes at a time when tile above a tub extended no higher than 5’0” above the floor height. Also, the tile used with this method was almost exclusively 4-1/4” x 4-1/4” or 6” x 6” white body tile, a very lightweight material. At this point in time, almost all of the companies that developed this method have abandoned its use due to the high failure rate.

There is a faction of Northern California Tile Contractors and Inspectors that are pushing to exclude the use of this method altogether, due to the high risk of failure that affects everyone involved in the project: specifier, general contractor, tile contractor, tile distributor and property owner.

Unfortunately for you, when you use the “Allowed as Local Practice” defense, if you ever do experience a failure, all the risk is yours.  The tile industry standards (TCNA Handbook for Ceramic, Glass and Stone Tile Installation, and ANSI A108/118) make no mention of stucco netting as an approved material. In fact, minimum 2.5 lb. per square yard galvanized wire lathe is the specified reinforcement material. Also, as a California C-54 Licensed Contractor, you are obligated to follow the TCNA and ANSI A108/118 standards, since they are the basis of all regulations regarding tile installation in California.

The cracking of your tile could have been caused by several problems, or a combination of problems such as mortar mixed too rich in portland cement, with an excess of lime, too fine sand, too wet- all could be contributing factors. Also cure time could be a culprit, as you mentioned. ANSI A108 specifies that a minimum of 20 hours at 70 degrees F should be sufficient, but cure times of up to 10 days are desirable. This pre-curing before installation allows the mortar bed to shrink and move prior to covering with tile. Many times stress cracking in tile does appear if the mortar bed is not allowed to pre-cure properly.

I am sorry you are having a failure, but this should sway you that following the TCNA and ANSI standards can truly be in your interest.

Michael Whistler, NTCA presenter and trainer

Ask the Experts – May 2015


My husband and I are at a loss. I was hoping you might be able to recommend a local recognized tile consultant (in Houston, Texas). We moved into a brand new home approximately 15 months ago. We have had a number of stone/tile issues that we have been trying to get resolved.

The one at hand at the moment is that we have an exterior archway that leads to our front door. It stands approximately 15’-18’ tall. It is covered with stacked stone. Individual pieces have been falling off for at least the past 10 months.

We have discussed this issue with the builder and he is insistent that this is normal. We have a 5- and 8-year old at home who are constantly running through these archways to get to the yard/basketball hoop on the driveway. I am fearful that they will get hurt one of these days. “It’s completely normal” is not an excuse I am willing to accept and anyone that offers that as an excuse makes me uncomfortable with being able to remediate such an issue. I would appreciate any professional advice you may be able to offer.


Of course this is not “completely normal.” If even one piece falls off it is considered a failure.

Unfortunately, much manufactured veneer is incorrectly installed. Exterior installations generally require the most care and planning, as well as following the industry standards, since they require the highest performance due to extreme conditions.

Prompt action is advised, as you are correct about danger of possible injury. Since you are already aware of this problem, your liability is increased. Perhaps retaining an attorney to send a letter to your general contractor could help as a first step, since he is being unreasonable. It is entirely possible that in your GC’s experience this IS a normal occurrence, but this would be due to using the same mason or tile setter that repeatedly uses a faulty installation method. This project needs to be repaired, which should probably include tapping each piece firmly with a rubber mallet to ensure it is bonded.

If you receive no satisfaction through a lawyer, it would be time to contact a forensic consultant, but be warned, their fees are usually quite high. Then you go to court. It would be much preferable to get the GC to pony up and fix an obvious failure in your home.

Michael K. Whistler,
NTCA presenter/technical consultant

As a follow-up to this inquiry, the NTCA webmaster responded to the homeowner, connecting her to a local NTCA Recognized Industry Consultant who is working with her on a resolution with her builder. In addition, advice was offered to look into the possibility of a warranty on her home or a “waiting period” where grievances can be filed after the purchase of a home.

For information and technical advice, email [email protected].

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