Ask the Experts – April 2014


I am emailing to find out when back buttering the back of larger porcelain tiles started as a standard, and in what edition it was first published.


I would have to physically look in each book but it goes back 15-20 years. ANSI A108 says:

2.3.4 Average contact area shall be not less than 80% except on exterior or shower installations, where contact area shall be 95% when not less than three tiles or tile assemblies are removed for inspection. The 80% or 95% coverage shall be sufficiently distributed to give full support of the tile.

backbuttering2.3.5 If 95% coverage is specified in the project specifications, back butter each tile with bondcoat; or select a notched trowel sized to facilitate the proper coverage. Key the mortar into the substrate with the flat side of the trowel, and comb with the notched side of the trowel in one direction. Embed the tile in the mortar by beating-in, pushing in a direction perpendicular to the combed ridges, or other means to achieve specified coverage. The method used should produce maximum coverage with the corners and edges fully supported. Periodically remove and check a tile to assure that proper coverage is being attained.”

The TCNA Handbook for Ceramic, Glass and Stone Tile Installation suggests directional troweling may help avoid the need to back butter. The requirement is coverage, not how you get it.

David M. Gobis CTC CSI, Ceramic Tile Consultant,
answering on behalf of TCNA


Is it against industry standards to install 16”x24” tile by using thinset on only the tile – without that same application to the concrete floor (post-tension slab)? I just had an installation done that way – all over the house. Now, I have a few hollow areas and grout that “droops.” I assume the “drooping” grout is due to settling into the spaces under the edges of the tile where no thinset was applied to the floor first. I am assuming it is because of the installer trying to take advantage of me via bad shortcut practices. Please advise – I am going to turn him into the Registrar of Contractors for a bad job – and I need to know if this type of tile installation violates industry standards.


Keying the mortar into the substrate is an important part of the bonding process. On post-tension construction, the selection of the appropriate thin-set mortar, movement accommodation joints in the tile work, and/or use of a membrane system are equally important for longevity of the installation.

– David M. Gobis CTC CSI, Ceramic Tile Consultant,
answering on behalf of TCNA

It is amazing to me how many times I have seen this practice. Quite a few “journeyman installers” I hired over the years would install in this fashion, until I caught them at it and instructed them in the correct and only allowable method. I have also seen this in many installers taking the CTI test. Notch troweling the back of the tile and applying to bare concrete (or any substrate) is not approved. Standard practice is to key mortar onto substrate with flat side of trowel, add more mortar, comb notches, then apply tile. With natural stone you are also required to key mortar onto back of tile (back buttering). It would be okay to key in and comb the back of the tile as long as you also keyed into substrate, kind of a reverse process (installers sometimes do this when installing cuts at a perimeter of a project), but applying to a bare substrate is asking for a failure

Michael Whistler,
NTCA presenter and technical consultant

Ask the Experts – March 2014


Can you point me to any publications or standards regarding potential issues related to the installation of ceramic floor tile over post-tensioned slabs on-grade?  This would be for residential tile projects on such slabs. Also, is there a specific mortar (thinset) that should be specified in such a situation?


Even though your slabs are on-grade, because they are post-tensioned slabs they are treated as above-ground slabs for tile-industry purposes. Look to the TCNA Handbook for Ceramic, Glass and Stone Tile Installation for all the various methods for tiling above-ground slabs. Start with method F113A-13 for thin-set method with tile. Most floor methods for concrete substrates show both on-ground and above-ground options. Be particularly careful if using natural stone tile, as a maximum area of 100 sq. ft. is allowed unless the wire-reinforced unbonded mortar bed method is used.

Like all tile systems, expansion joints are required, and are even more important here due to deflection and slab creep that are generally associated with post-tensioned slabs and the site conditions that require them.

Michael Whistler,
NTCA presenter and technical consultant

Try the Ceramic Tile Institute of America. They have produced a paper on that subject. Here is a link to the excellent article:

Bart Bettiga,
NTCA/CTEF executive director


Where do I find the language requiring a post-tensioned slab to be treated as an above-ground slab? Is this a requirement/standard?


I believe the language is in the method listed in the TCNA Handbook. If you look at Recommended Uses in F113A-13, for example, it states “For above-ground structural slabs and other floors subject to movement and/or deflection where thin-bed installation of tile is desired.” I understand your slab is on-grade, but there is generally a reason to use post-tensioned slabs on-grade, like expansive clay soil, or other sub-slab site conditions. Unfortunately, using a post-tensioned system rarely cures all of the site condition problems. Plus, it adds a few new problems into the equation from the post-tensioning and its effect on a tile installation; therefore the need for separate and stricter installation methods.

Michael Whistler,
NTCA presenter and technical consultant

Ask the Experts – February 2014

SponsoredbyLaticreteIf you are seeking technical help on a project, expert members and staff at NTCA are at the ready to provide you with the information you need to create a successful, enduring and beautiful install for your clients. Residential or commercial, interior or exterior, feel free to email
[email protected] to get your questions answered and gain valuable professional support for your project.

Here’s an example of a NTCA member reaching out to the network at NTCA for clarification on installing tile over stucco.


0214_AtE_stuccohouseI would like some feedback on a project for one of my designers.

We have an exterior stucco home on which the designers want to install a chiseled-edge, travertine stone accent in three front areas where the windows are. We plan on doing a modular pattern with 8”x8,” 8”x16,” 16”x16” and 16”x24” pieces, 3/8” thick.

The stucco is in great shape – it’s just been pressure washed. My only reservation is that I am not sure if the stucco is painted or if the color is in the finish. There’s no way to find out from builder, as they are no longer in business!

Any ideas would be welcome.


Many manufacturers allow installation over sound stucco, and yes, the caveat is that it must NOT be painted. Since you will be tiling over areas, choose one (or all) that will receive tile and do a small demo of the existing stucco. You don’t need a very large piece (maybe 1”x1”x1/4” thick) that will allow you to determine if the stucco is solid color throughout. Pulverize a portion of your piece to see. Generally, a solid color throughout indicates unpainted stucco, but check for a skin of paint on your sample piece.

Michael WhistlerNTCA technical consultant and presenter

My advice is to reach out to your local installation material manufacturer rep and have him specify the means of setting the stone. I often use my manufacturer reps in the same capacity and have them write the specification. This should protect you in the event of a failure. The tricky part with this is determining whether the color is mixed into the stucco or is it painted. I’m sure there are ways to determine such. Good luck and hope all is well!

Buck Collins, Collins Tile & Stone, Aldie, Va., NTCA Five Star Contractor

Can you use a grinder with a diamond blade and scarify the face? If you can, this is a good bet no matter what the coating is. There are also wire wheels that can be attached to a grinder that are not as invasive as a diamond blade. Buck is right: get a system from a manufacturer that will stand behind their product. Hope this helps.

John Cox, Cox Tile, San Antonio, Texas, NTCA Five Star Contractor

Ask the Experts – January 2014


We were wondering if you could advise us as to whether we should install support brackets for our granite countertop and breakfast bar. The overhang is 4’ W x 1’ D x 1” thick.


It is always a good idea to have more than “sufficient” support for overhangs. Corbels are the easiest solution, and can be made in almost infinite styles and out of almost any solid material, including the stone you are using for your countertops. Plate steel, bolted to the top of the bar framing, is also a good solution. Some installers say additional support is not needed, but I have seen many, many failures in unsupported countertops, usually because a person (like an electrician or painter, or the homeowner changing a light bulb) has climbed up there.

Michael K. Whistler, presenter/technical consultant NTCA


I am installing .197” thick ceramic penny-round tile over cement backer board on a bathroom floor. I am having trouble with too much thinset oozing up through the grout joints. I am using a 1/4” x 1/4” square notch trowel. Can you recommend the proper trowel size and technique for this application? Once the thinset has been applied and scraped, should I flatten the ridges to minimize the thinset oozing up?

I saw a reference to your standards about flattening the ridges when using small mosaic tiles less than 3”, but I want to make sure I am applying the right amount of material. I have not been able to get any recommendation from the manufacturer.


You are absolutely correct that flattening the notch trowel ridges is the way to stop squeeze-up in the joints while still getting full coverage on the tile. Make sure you comb the notches all in one direction before flattening. You will also find that beating in the sheets with a beating block, wood float or hard rubber epoxy float will help in this process. Not only will this help eliminate squeeze-up, you will have a flatter installation.

Be careful when using a v-notch trowel, as most do not provide sufficient thinset between the tile and substrate. Maybe try a “worn” 1/4” x 1/4” and flatten the ridges, or a 1/4” x 3/16” trowel.

– Michael K. Whistler, presenter/technical consultant, NTCA


I live in Orange County, Calif.,  and want to start a tile business. I’m not sure what licenses are needed. I have not gone to school for tiling and have no experience. I want to know how to go about this please.


A tile contractor in California needs to have a C54 State Contractors License. You must demonstrate that you have previous experience to be eligible to apply to take the test.

The best and most common route to becoming a tile setter is to find a job working for a tile company as a helper where you will learn all the basics of the tile trade. If, after a time, you prove yourself as a dependable helper, most companies will allow you to advance to apprentice status, whether formal or informal, and will begin letting you install tile and teaching you proper methods. After several years, you can usually advance to journeyman tile setter where you will be responsible for installing projects from start to finish.

If at this point you decide to take on the business part of the job – in addition to the installation side – then it’s time to study for and take the C54 test. Once acquired, you need only get a business license from the city where you work and find some clients.

After you are properly trained, it would also be a good idea to take the Ceramic Tile Education Foundation test to become a Certified Tile Installer. This positions you as a purveyor of qualified labor and builds your customers’ trust in your skills and what you can offer them. And of course, you may want to consider joining and becoming active in the NTCA to keep your knowledge of products and methods sharp, as they inevitably change and evolve over time.

Michael K. Whistler, presenter/technical consultant, NTCA

Ask the Experts – December


Here is the age-old question: Should I use caulk or grout to seal a toilet to a tile floor? Whatever is used will go halfway around the toilet. The goal is to protect the front of toilet from “fluids” and to prevent toilet from rocking. What do you recommend and why?


toilet_ATEToilets are designed to be removed and replaced. The tile floor is meant to be permanent. Toilets should not rock on a flat tile floor, but in the real world they often do rock. Most plumbers use wood or plastic shims to steady the installed toilet, and cut the shims with a utility knife so they do not protrude beyond the toilet base. Using grout or sealant to fill the space between tile floor and toilet base is your choice, though in my opinion, sealant is the much better choice. Grout, when cured, will not withstand the movement that will be present with toilet use, so cracks will develop. Also, grout has no water-resistant properties whatsoever. Sealants remain flexible, have sufficient bond strength and do give some water resistance. Be sure to use a sealant that meets ASTM-C920 performance stan- dards (like a 100% silicone) to get the longest-lasting sealant joint. — Michael Whistler, NTCA presenter/technical consultant


I have some tile and it has this statement on the packaging: “Please verify shade, calibre and grading. Claims regarding these items cannot be accepted after tile setting.” Can you tell me what shade, calibre and grading refer to?


boxlabel_ATEShading and calibre (also spelled caliber) refer to the color and size of the tiles, respectively. Each box of tile should be marked with its shade and caliber. Your job is to ensure that all boxes have the SAME shade and caliber before they are installed. If you use dif- ferent shades or calibers, you will be using tiles of different color and size – generally not pleasing to the eye. Every tile run has multiple cal- ibers and shade lots that are sorted and boxed at the factory, but it is possible to receive more than one caliber or shade within an order. You must check before installing so you can either mix the different lots together to avoid blocking, or reject the different lots and try to get all matching lots (which may or may not be possible). Grading is the designation on the label on each box that shows whether the tiles meet ANSI-A137.1 specifi cations (shown as “STD” or “standard”), or if the box contains lesser-quality seconds. – Michael Whistler, NTCA presenter/technical consultant

Ask the Experts – November

Our soap dishes fell off the tile tub surround, and were repaired by two different installers (using the term loosely). One was reset by using only grout to place it. The other was reset by punching a 2 1/2” to 3” hole in the green board and filling it with grout (not thinset) only. Are my concerns founded that now that the moisture barrier has been breached, the grout can wick moisture into the wall?

I dislike being the bearer of bad news, but there is probably no good way to re-attach those soap shelves. First, your tile was installed over a paper-covered gypsum product. Although common years ago, this method has not been allowed in wet areas for quite some time.

Second, tile and grout systems are not waterproof, or even water resistant. In fact, the tile system generally pulls water into the substrate (even if it is sealed). Over time, the paper on the gypsum board begins to degrade, and delaminate from the soft gypsum core below. Almost nothing will stick to this raw gypsum for long.

Not seeing the shower in person, I cannot unequivocally say that you are due for a new shower, but soap shelves falling off is usually the first sign of the end for this type of system. What usually follows is grout cracking and tiles falling off.  When these symptoms occur, you will generally find that in removing the old tile and green board there will be much degradation of the board, and likely mold, since the paper and gypsum in a wet, warm environment are perfect food for mold.

Michael K. Whistler, NTCA presenter/technical consultant

(from an architect)
Should I expect wall and floor grout lines to meet as best practice from a tile installer?

It is not a written industry standard that when using the same tile on walls and floors, or a modular tile (where more than one smaller tile plus grout joints equal the size of one large tile), all grout joints should align at vertical to horizontal tiled surfaces. Unless specified in contractual language, it is more a bonus of using a highly-skilled and quality-conscious contractor. This is the art of “layout” and can sometimes take nearly as long as the actual laying of the tile. There are times when it is physically impossible, as in the case where angled walls meet floors, but generally a quality craftsman will have nearly all grout joints aligned.

Michael K. Whistler, NTCA presenter/technical consultant

I was wondering what the best floor backer board is for porcelain and ceramic tile.

All the backer boards are good.  Each has different properties that may be needed for a specific project, such as thickness, dimensions of sheets available, ability to use on exteriors, weight, etc. Also keep in mind that if a cementitious backer board or unit (CBU) is specified for a project, substitutions are usually allowed with other brands of CBUs. But other TYPES of backer board, like foam, water-resistant faced gypsum, fiber cement or others will be difficult to substitute.  Not that any are bad, you just need to follow specifications.

So find a type you like, familiarize yourself with the manufacturer’s instructions and go to town!!

Michael K. Whistler, NTCA presenter/technical consultant

Ask the Experts – October 2013


I have a question about a new home constructed in 2012. We have porcelain tile over cement backer board over LP 3/4 floor decking. The backer board was installed with thinset to the OSB and screwed down. Tile was then thin set to the cement backer.

AtE-OctI have issues with loose tiles in two areas: kitchen and master bath. An engineer has assessed the I-joists and beams and found no movement or deflection. The contractor wants to blame the radiant floor heating, but I have hundreds of square feet of tile unaffected by the radiant heating. Any thoughts? Attached are a couple of interesting pictures.


Thank you for including the pictures. They make this an easy diagnosis. Your tile installer did not include movement accommodation joints (or insufficiently-sized joints) in your tile job.

Tile expands and contracts, and at a different rate from the substrate below. Every method shown in the TCNA Handbook for Ceramic, Glass and Stone Tile Installation requires movement accommodation joints. Interior dry areas need joints no more than 20’-25’ in each direction; exteriors, wet areas and interiors exposed to sunlight (i.e. where south-facing windows occur) 8’-12’ maximum. Glass tile and any radiant-heated tile need reduced distances, and in all areas, perimeter joints (where tile meets walls, cabinets or any dissimilar plane or surface) must be left open or filled with a flexible sealant. Even if the field distances are not exceeded, not including perimeter joints can cause this failure.

Many tile installers do not know the industry standards or have never experienced this failure, because it does not ALWAYS occur, but unfortunately for you, this is part of the learning curve for your installer. It generally does not take but one or two of these failures for an installer to learn the importance of including movement accommodation and spending the time up front to educate his clients (because there is generally an additional fee to perform this step within a tile project).

And please note that the thinset type is not a factor as long as it is suitable to tile and substrate. Even if there was a thinset that was 50 times stronger, the forces exerted by expansion cycling would still overcome it. Typically the thinset itself shears, but if it were stronger thinset, it would likely change the shear point to the substrate surface or the thinset/substrate interface.

The requirement to include movement accommodation joints is included in each installation method within the TCNA Handbook, and the specifics on placement and construction of joints is in section EJ-171. The TCNA Handbook is available on our website at for sale. It is really a standard that every tile installer should own.

Michael K. Whistler, NTCA presenter/
technical consultant

Ask the Experts – September 2013




I have been in a newly-built home for the last year. Upon possession of the home, air bubbles were noted in the grout. The tiler mixed new grout and applied it over the top of existing grout. After almost a year, cracks were discovered in the grout. Water presumably penetrated the grout cracks and allowed moisture behind the tiles and onto the mortar.

AtE_septThere was a secondary issue where only plywood was used and tiled over without a waterproof membrane. When attempting to rectify the cracks, the builder indicated that the cracks were due to the movement in the exterior wall from the settling of the home. Most of the grout was removed with a utility knife (therefore not completely removed). A couple of days later, a white crystal substance could be seen growing out of the grout lines. Thinking back, this was observed prior to the grout removal and was seen growing on the surface of the grout.

I wonder if you can provide any insight as to the cause of this growth, which I can only assume is efflorescence. As the manufacturer of similar products, I would be very interested in your opinion as to how to remedy the situation, and advice if you have ever heard of or seen such a reaction. It has been seven weeks since the grout was removed and it continued to produce this growth.


After looking at the pictures you sent, it appears that the tile was bonded directly to plywood walls with an organic adhesive (mastic). This is not an approved method according to The American National Standards Institute (ANSI) or The Tile Council of North America (TCNA)  for installing tile in a wet area such as a shower stall.

No tile or grout should be considered waterproof – even if a tile and grout sealer has been used. While some tiles such as porcelain are impervious and only absorb a small amount of water other ceramic tiles may absorb much more water – up to 30% of their weight. This absorption allows the water to transfer through to the back side of the tile, thereby requiring  a water management issue to be addressed. 100% waterproofing behind the tile is not required in all cases, but for a successful tile installation, a water-management system is required along with appropriate use of products.

For instance, a house with a shingled roof doesn’t have a waterproof roof. But if the appropriate shingled products are used, and a skilled professional properly installs these products according to ANSI standards or the product manufacturer’s written directions, a successful long-lasting installation will be achieved.

I’m sorry, but it appears necessary to remove all tile work in the wet areas and replace tiles using any of the several methods found in the TCNA Handbook for Ceramic, Glass and Stone Tile Installation for tile installation in shower receptors. Plywood and wet tile don’t mix well and will most likely continue to be problematic. This combination will most likely produce cracking grout joints and cracked or loose tile and also offers a perfect opportunity for mold and other bio-organic growth.

The National Tile Contractors Association (NTCA) web site offers a list of NTCA tile contractor members that are located all across the nation ( – click on Find a Tile Contractor or Consultant). I highly recommend a tile contractor that is a member of the NTCA, because members are well-informed on standards and updates in our industry.

– Gerald Sloan, NTCA presenter/consultant


Ask the Experts – August 2013


Oh my gosh! I inadvertently sprayed Lime-A-Way® on travertine! How do I correct the problem? Thank you for your professional advice.


You are definitely not the first person to say “oops” after spraying an unsuitable cleaner onto stone tile. It is actually a pretty easy fix, though probably not inexpensive.

You need to contact a stone restoration or stone re-polishing company. Try the Yellow Pages first, and if you don’t have a listing, call local tile distributors, local tile contractors, or local granite countertop fabricators, since this is a skill that some installation companies have. In some cases just the affected area can be ground and re-polished to blend indistinguishably with the surrounding original stone, but in some cases all the stone needs to be hit with the final grits to give an even appearance.

If this is a shower, all the better. Re-polishing stone done wet precludes the major dust that comes with dry grinding and dry polishing within your home. My company would have the polisher strip down to a pair of swim-trunks to do the wet polishing inside a shower, and once finished with the polishing process, wash down the shower and himself, and squeegee off the walls and shower glass.

Hope this helps.

Michael K. Whistler, presenter/technical consultant, NTCA



In the July 2013 Ask the Experts, the answer to this question was incorrectly stated, due to an assumption that the inquiry included concrete structures. This is the correct question and answer.


Can ceramic tile floor and wall finishes be installed in commercial modular buildings (which are constructed offsite, then hauled to and set up onsite) using the standard TCNA tiling methods, or are there special requirements for this type of building? These are wood-framed modular buildings (similar to those used for school classrooms), which would have restrooms with ceramic tile floor and walls. Are there any special tiling requirements to accommodate the movement of the manufactured building while being transported and set onsite? Maybe ceramic tile would not be a good choice here?


Any tile installation requires a certain stiffness, or lack of deflection. If in moving the structures it can be pre-determined or engineered that the completed tile installation will not be subject to any bending or twisting, it would probably work. I see this as highly unlikely, though.  Perhaps the structures could be moved into place and tile installed after the structures are permanently installed on their foundations.  Tile is a very good heavy-service finish, but it does require a very stable substrate.

– Michael K. Whistler, presenter/technical consultant

Ask the Experts – September 2012


We just installed a stone tile shower floor. It was sealed twice and shortly after white stuff kept showing on the floor even though the walls of the shower and the bathroom are of the same tile. We then removed the sealant and re-sealed it. Again shortly after this white stuff comes out. I have used an efflorescence cleaner which removed it, but the white stuff keeps coming back. Can you tell me what is happening and how we can fix it?

Thank you in advance for all your help.


Without being there on site and doing several destructive tests to determine the actual cause of the problem, I can give you several of the most common causes for the condition that you have described. First of all, given the cleaning product you have described, it sounds like soluble-salt efflorescence. This soluble salt is a natural product that is present in the Portland cement-based products used most often to produce the setting bed that is under the tile installation and the grout material that is used to fill the grout joints.

Natural stone tile may contain these mineral salts as well, especially if the natural stone is a softer material such as marble or limestone. These mineral salts may, and most often do, migrate when exposed to water or water vapor. As a way to control or minimize this efflorescence, water management is of the utmost importance. Careful consideration of the materials being used should be taken into account. Efflorescence that occurs after the tile or stone installation is in service is very difficult to stop or even control. Time and use will eventually cause all the “free mineral salts” to be exhausted and the problem will go away, but to fix this problem from the beginning will most likely require a removal of the entire shower floor, and may even require removal of portions of the walls or all the walls as well.

Starting fresh with a new shower framed and ready for tile or natural stone, the methods for proper shower assemblies can be found in the TCNA Handbook for Ceramic, Glass and Stone Tile Installation. These methods show pre-sloped shower pan assemblies, weep hole protectors, references to American National Standards Institute (ANSI)  for proper mixes and measures for setting beds such as 1-to-4-ratio of Portland cement to washed masonry sand used in many shower pan assemblies. The TCNA list cautions when using soft natural stone products in wet areas – if efflorescence may be of concern – a porcelain or ceramic tile that is made to resemble natural stone may be a safer choice.

A few things the NTCA Reference Manual has to say about the causes efflorescence are as follows:

  • Soluble salts from the Portland cement-based products brought to the surface by capillary action where there is water or moisture present.
  • Contaminated water or sand containing soluble salts.
  •  Excessive mineral content in the water used for maintenance.

I hope this information helps.

Gerald Sloan, NTCA trainer

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