Ask the Experts – December 2016


We have a contractor who used two different tiles. He bought the 4” x 4” tile elsewhere but after installation it appears to have changed color. The tile he bought from another source at the bottom, stayed looking like it did prior to installation. What could have caused this?


glazed-wall-tile-ateThe color of some tiles can change with grouting, for example this can occur with soft, natural stones and unglazed, honed or polished through-body ceramic/porcelain or quarry tiles (and/or if grout release wasn’t used).

It appears the tiles shown in this installation are glazed ceramic. You stated the two sizes of tile were procured from two different distributors. It is possible that even if the tiles are from the same manufacturer they may have been manufactured on different dates or in different lots. That being the case, the glaze may have a slight variation between the two. The glaze also appears to be different on the bullnose tile.

If spare, uninstalled 4” x 4” and 3” x 6” tiles from this job, new from their cartons, are placed side by side, I suspect you may notice the difference in color between them. Have you checked to see if the two tiles are from the same manufacturer and what their date of manufacture and manufacturer lot numbers are? This information should be available on the end flaps of the tile cartons. If the manufacturer, date of manufacture or lot numbers differ, there may well have been a difference in glaze color out of the box that wasn’t noticed until the tiles were installed and grouted

It is important to verify that all of this information aligns between all cartons of tile before beginning the installation.

Mark Heinlein – CTI #1112, NTCA Technical Trainer/Presenter


I need to know what the TCNA standard for slope in a commercial kitchen is.


The minimum slope for floor drains is 1/4” as specified by ANSI A108.01 2.2, which directs compliance with ANSI A112.21. The Universal Plumbing Code also specifies minimum slope of 1/4” per 12”.

The mortar bed is to be of a uniform thickness. ANSI A108.01 and include a chart and discussion of minimum and maximum mortar bed thicknesses based on the service rating of the floor. For a commercial kitchen rated as Extra Heavy/Heavy, the bed thickness is 2-1/2” minimum to 3-1/2” maximum. Appropriate size and gauge galvanized welded wire mesh (i.e. 2” x 2” 16 gauge) must be suspended as reinforcing wire in the mortar bed. If the mortar bed will be in excess of 3-1/2” thick, heavier reinforcing, larger aggregate, richer mix and greater compaction may be required and must be detailed by the specifier.

To keep the mortar bed from becoming too thick or the slopes to a drain too long, multiple drain locations should be planned based on the layout of the kitchen.

Mark Heinlein – CTI #1112, NTCA Technical Trainer/Presenter

Ask the Experts – November 2016

SponsoredbyLaticreteOne of the benefits NTCA offers is its technical staff, and seasoned know-how that can be tapped to address problems and questions. Here’s an example of the profusion of information from the NTCA technical staff for the questions of one Hawaiian homeowner. – Lesley Goddin


I am a homeowner in Hawaii looking at installing a large-format porcelain tile floor (planks are ~9” x ~47”). I have two questions.

My first question is what is the NTCA convention for pricing that tile install? Is the convention for an install price per square foot based on the actual footprint of the rooms (actual tile on floor when job is done) or does it also apply to the tile waste?

As an example:

– Lets say I have a simple 25’ x 40’ room: 1,000 sq. ft., once the tile is installed.

– Lets say I plan on 10% waste. So I purchase 1,100 sq. ft. of tile

Would your convention for the installers bid price be X dollars per sq. ft. based on the 1,000 sq. ft. or based on the 1,100 sq. ft.?

My second question is what sort of waste percent should I expect for that large-format 9” x 47” tile?

Thank you for your time and feedback.


We have three technical trainers on staff, and they are all coming off of extensive travel dates for training programs, so I am taking a shot here on this question, and they will likely provide additional or a slightly varied response.

In my experience on this subject, the labor bid for the job generally covers the 1,000 actual square footage of the room in the bid. However, if you have a 10% waste factor for materials, you would be charged for the 1,100 sq. ft. of material. My customers usually ordered about 3-5% in a straight-cut square pattern and 7-10% with a pattern design, like what you are describing below. This depended on room size and how it is laid out. I will see if they chime in with a different response but this has been my experience in the past.

Bart Bettiga, 

NTCA executive director

Bart has provided you correct information. I would add that with a plank tile such as your 9” x 47,” I may consider estimating upwards toward 12% depending on the pattern, obstacles, squareness of the room(s), etc. This is especially critical for special-order material to ensure it is all from the same manufacturing lot. Any remaining tile should be retained as “attic stock.”

There is not a standard pricing convention for labor. A contractor may base the labor rate on square footage, hourly or daily rates. A qualified contractor using Certified Tile Installers will be very familiar with the intricacies of installing large-format tile. Large tile is not necessarily faster or easier or less expensive to install. Additional substrate preparation is almost always required to ensure a quality installation based on tile industry standards.

The NTCA does have member contractors in Hawaii. Please search for them on our web site at:

To locate a Certified Tile Installer please search the Ceramic Tile Education Foundation at:

I hope this helps.

Mark Heinlein – CTI #1112, NTCA trainer/presenter

In my area, most installers are paid for the amount of material they use rather than the square footage that they cover. They do have to handle, cut, and dispose of that tile and usually will include it in their invoice.

As far as percentage waste to figure, two things should be considered: the layout of the area and the pattern to be installed. One single square room usually requires very little waste, while an installation with many corridor and rooms would require more.  Also a straight-lay pattern requires very little waste, while a herringbone pattern or an installation on a 45-degree would require much more. I figure somewhere between 5 and 15% waste considering all the factors just mentioned.

On a commercial project, the spec book often times calls for 5% attic stock to be left on site at completion. If that’s the case you would also want to add that on to your estimated waste amounts.

Robb Roderick, NTCA trainer/presenter

Ask the Experts – October 2016


I have a project in Park Ridge, Ill., which requires replacing the ceramic tile on the floor. Originally the owner decided to install a 12” x 24” tile, but due to sloping issues, such tile cannot be installed. Therefore the owner switched to a smaller tile, a 3” x 3” mosaic. Would the labor to install either size tiles be the same? If not, which one costs more? Please advise. Thank you!


Installing mosaic tile typically requires additional time for the cutting, straightening and handling of many individual tiles and grout joints even if, or sometimes because, they are mesh-mounted.

Installing large format tile such as 12” x 24” on a substrate that does not meet minimal acceptable flatness requirements of 1/8” in 10’ can require additional labor and materials for such activities as grinding and flattening the substrate with appropriate patch or self-leveling underlayment and their primers. Large-format tile (any tile with one side longer than 15”) will also require setting material designed for large-and-heavy tile. Due to any inherent warpage, this may require the use of a larger gauged trowel to assure required minimum bond coat coverage, which may result in the need for additional mortar.

I hope this helps.

Mark Heinlein,
NTCA Technical Presenter
[email protected]


Our rectified porcelain wall tile is three years old. We need to re-grout or caulk these thin grout lines. Which is better: silicone caulking or unsanded grout? For both I would use the caulking gun. Thank you!


Grouting is a complex and integral component of any tile installation. Grouting requires a degree of knowledge and experience with materials, environmental conditions, tile types, tools and techniques. Qualified professional tile contractors possess these traits.

I do not recommend using caulk in the joints between the tiles.

When removing the existing grout, take care not to chip the edges and corners of the tiles. You will want to ensure that you end up with a joint that is at least two-thirds the depth of the tile. Sharp utility knife blades, high quality grout rakes and oscillating tools with carbide grout removal blades are some of the tools you may want to consider using. It often requires a combination of all these tools to do the job.

ate-1Depending on the width of the joints you will want to select a grout that will adequately fill the joints. If using a portland cement-based grout:

  • Use unsanded grout for joints 1/8” wide and less.
  • Use sanded grout for joints 1/8” and wider.

Many manufacturers now make an acrylic, siliconized-acrylic or urethane based “single-component” or “ready-to-use” grout that may be acceptable for all ranges of grout joints. This information will be listed on the grout packaging. Whether the wall is in a dry or wet area, such as a shower, may make a difference for your grout selection.

You will want to grout the joints using an appropriate type of grout float making several passes at 45-degree angles to the joints to ensure you pack the joints full of grout. Then hold the float at a 90-degree angle to scrape the excess grout off the face of the tiles and across the joints, again at a 45-degree angle to the joints to avoid dragging any grout out of the joints. Based on the type of grout you are using, allow it to set up per the manufacturer’s instructions before forming the joints with an appropriate sponge. Then, based on the manufacturer’s instructions, clean the tile with single passes of a lightly damp sponge, once again at 45-degree angles to the tile. Change your grout cleaning water frequently. Again – read and closely follow the grout manufacturer’s instructions for the type of grout you have selected.

I do strongly recommend using a 100% silicone sealant (caulk) in the perimeter joints and any changes in plane (such as where your wall meets the floor and ceiling and where it meets an adjoining wall). Many grout manufacturers make 100% silicone sealant that color matches their grouts. Be sure to use a sealant (caulk) and grout from the same manufacturer.

Depending on the size of the wall, you will want to consider the need for expansion joints (soft joints) within the tile field.

ate-2I suggest you consider hiring a qualified tile contractor to perform this work. In addition to performing the work, a qualified contractor will be able to analyze the existing installation and help you determine the best materials for the job, saving you the hassle of trying to become an expert on your own. You may do a zip code search on either the NTCA or CTEF websites located at these links.

I hope this helps.

Mark Heinlein,
NTCA Technical Trainer/Presenter
[email protected]

Ask the Experts – September 2016

When installing floor tiles, should you tile to the walls or leave a small space for movement and flexibility?

In answer to your question about whether to leave a space where a floor meets a wall – Yes! A gap of approximately 1/4” should be left at all changes in plane (for instance where a floor meets a wall) around the perimeter of the installation. This gap should be present in the underlayment and tile. If no trim will be installed to cover the gap, a “soft joint” can be made with appropriate sealant, or certain trim profiles can be installed to accommodate movement and expansion. This gap should also be left where tile abuts cabinetry, pipes or other permanent fixtures. Any other change in plane such as where a wall meets another wall must also have a soft joint installed to allow for movement and expansion. Also, expansion joints must be properly placed and installed in the tile field depending on the location and size of the installation. Additionally, control joints and saw cut joints in concrete must be honored through the surface of the tile to avoid future cracks in the finished installation. These specifications and the many, many other details related to a successful tile installation can be handled by your qualified contractor and certified labor.

– Mark Heinlein,
NTCA technical trainer/presenter

I have a job we are installing a wet bed on top of concrete and I would like to install an uncoupling membrane on top of the wet bed for crack prevention. My question is, will an uncoupling membrane help to prevent cracks in the concrete from penetrating through to the tile?

There are many products and some installation methods that can help mitigate in-plane cracks from telegraphing to the tile surface.

Manufacturers of uncoupling membranes are best at describing the performance characteristics they warrant their individual products for. If you share with me your geographic location and contact information I can request a technical representative from that company get in direct contact with you, or your preferred uncoupling membrane distributor should be able to put you in touch with a technical representative from the company.

Since it sounds like you are installing a mortar bed, perhaps installing a cleavage membrane to create an unbonded mortar bed system may be an additional solution for you to consider. Details for this type of installation can be found in the Method F111 in the TCNA Handbook for Ceramic, Glass and Stone Tile Installation.

– Mark Heinlein,
NTCA technical trainer/presenter

Ask the Experts – August 2016

SponsoredbyLaticreteThe following conversation took place between a contractor and NTCA technical trainer/presenter Mark Heinlein.


I would like some feedback on a recurring issue with a shower pan that was installed about a year ago (see attached pictures). The curbless shower pan is staying damp and not drying out even after days with electric heat set at 85 degrees.

The installation procedure used was as follows:

  • Prepan liner
  • Membrane over prepan
  • Mud sloped to drain
  • Liquid-applied waterproofing over pan and up inside and outside walls
  • Heated floor (skimcoated)
  • Liquid-applied waterproofing over that

I am looking for a way to rectify this issue and to prevent it from occurring in the future with light colored stone.



I have reviewed the photos you sent and read the description of your installation and have a couple of questions.

Did you install a pre-slope and liner with a clamping ring drain?  Or did you use a bonding flange-type drain, or a divot method?

Are the fixed glass panels mounted with fasteners (i.e. screws) to the floor/pan?

Please let me know.

I suspect that water may have pooled in low spots beneath the tiles that has caused the minerals in the soft marble to stain.

– Mark Heinlein,
NTCA technical trainer/presenter


Yes, there was a pre-slope drain with a clamping ring drain.  Workers installing the shower door said they drilled only through the tile for the shower door. There is really no area for water to pool due to the mosaic tile being installed with a small notched trowel.

If they penetrated through our waterproofing and mud bed, do you think that could have created the issue by the shower door?


Yes. Very likely the screws were longer than the thickness of the tile. If the liquid-applied waterproofing applied to the surface of mud bed / heat mat installation was penetrated, water can seep into the screw holes and wick into the bond coat and up into the stones. By what I can see in the photos, it appears this is the area where the staining has occurred.

Required mortar-bond coat coverage in a wet area, especially for natural stone, is 95% minimum. It is even more critical to have full-coverage bonding of soft stone on a shower floor. Were you able to get full coverage with a small notched trowel? There may be voids in the bond coat that are filing with water and staining the tile from beneath.

I also have some concerns about encapsulating the mud bed and heat system with a liner at the bottom and one or two layers of liquid applied membrane on top. How is the topical liquid membrane making a waterproof connection to the clamping ring drain?

Please take a look at TCNA Handbook Methods B421-15 and 422-15 for details regarding bonded waterproofing membranes on a sloped mortar bed shower receptor.

If you find there is no water entering through the screw holes, and if you have a good connection of the liquid waterproofing to the clamping ring drain, and if you have full-mortar bond coat coverage under the tile, this may be an example of when water stains soft stone tiles in shower pans in many types of installations. This phenomenon is currently being researched by an NTCA Technical Sub-Committee.

– Mark Heinlein,
NTCA technical trainer/presenter

Ask the Experts – July 2016


I am trying to find out if tile can be installed over tile. I have a ranch house in Florida built in the late ’70s with an original slab floor and terrazzo. Ten or so years ago the then-owner installed 12” x 12” tiles over the terrazzo. We want a new tile floor but do not want to remove the current tile. I know it’s “all about that base,” and our current tile floors are solid as a ROCK, level, not a single hollow or loose spot anywhere. As a matter of fact, it is almost impossible to get this tile off! No contractor will lay tile over tile, but I have read many, many articles online – from contractors – that say it can be done. What do you say? Thank you!!!


Thank you for contacting the National Tile Contractors Association.

Cementitious terrazzo is really just a type of mortar bed that has been ground very smooth. The only problem with going over it is the terrazzo is usually highly finished or waxed, with multiple layers. This finish must be completely removed and the terrazzo re-ground to open up its pores before tile installation. When properly prepared with the right materials, it becomes an excellent substrate for a new tile surface.

A qualified tile contractor is the best person to determine whether your existing tile installation is well bonded to the terrazzo.


Twin City Tile Co. Ltd., of Kitchener, Ontario, was responsible for restoring the Registry Office of the Waterloo Region, which was constructed in 1938 and designated as a Heritage Landmark by the Historical Society. This included the original terrazzo floors, many of which consisted of nine cement colors and eight different colors of terrazzo chips of various sizes, with intricate geometric patterns and three different thicknesses of zinc and solid brass strips. Although you wouldn’t want to tile over a terrazzo floor of this quality and beauty, terrazzo CAN be an excellent substrate for tile, given the proper preparation by a qualified tile contractor.

Tile-over-tile is a method in the Tile Council of North America’s TCNA Handbook for Ceramic, Glass and Stone Tile Installation, providing the bottom layer of tile is well bonded and properly prepared to accept the bonding materials for the new layer of tile. This can be done by scarifying or grinding the surface of the existing tile and/or by applying an appropriate primer that will allow the new setting material to achieve a proper bond. Grinding the surface of existing tile can create a lot of dust and may release potentially hazardous particles into the air from the materials used in the glaze. It is best to have this work performed by a certified, experienced professional installer. Such an installer will also be familiar with the primers and setting materials that will work best for this type of installation.

A well-experienced, qualified, certified tile installer/contractor will know and understand the methods detailed in the TCNA Handbook and will be able to examine your existing installation and determine the best approach for the new tile. Look for a contractor who is a CTEF Certified Tile Installer and a member of the National Tile Contractors Association. I have included links below to help you find one near you. A contractor who is a member of the NTCA has a direct connection to us for any technical advice and support if needed.

If this is an above-ground construction or on a wood frame subfloor, consideration must be made to support the weight of the new tile installation. A qualified tile contractor can assist with this, but an additional contractor or engineering assessment may be required. Floor-height transitions to other areas must also be considered. The contractor you eventually hire should discuss this with you.

To locate an NTCA member contractor:

To locate a Certified Tile Installer:

I hope this helps!

Mark Heinlein
NTCA Technical Trainer/Presenter

Ask The Experts – June 2016

I have a bathroom that was just retiled: floor, tub and shower walls, using ceramic tile, and a polymer-modified, cement-based, sanded grout with a stain-resistant additive. There are a few areas where the grout has set up like powder and can be rubbed out. I’m looking for some reasons that could cause the grout to set up like powder and fall out.

Selection, mixing, installation, curing, cleaning and environmental conditions at the time of grouting all play a critical role in the success of the installation. There are many potential reasons for this to have occurred.

Most are related to improper preparation, mixing, proportioning of powder and liquid, slaking, re-tempering or curing of the grout. Environmental conditions at the time of grouting and curing such as exposure to hot air flow, direct sunlight, a very dry atmosphere or freezing could also cause this condition to occur.

Over-washing of the grout immediately after packing the joints or cleaning cured grout with acidic cleaners may contribute to this problem.

In some instances, a very highly-absorptive tile or foreign material in the joints may cause rapid dehydration of the grout that may lead to this happening.

Without knowing the particulars at the time of your grout installation it is not possible to narrow it down further to a specific cause or group of causes.

– Mark Heinlein,
NTCA Technical Trainer

That is very helpful. What would the needed fix be regarding the powdery grout? Does it have to be removed and re-grouted?

It may be possible to slowly rehydrate the grout by misting with water and covering with kraft paper for several days and re-misting as needed. However, since an additive other than water was used to mix the grout, I would contact the grout manufacturer and ask their opinion regarding rehydration. Otherwise, it will likely be necessary to remove the grout. When removing the grout, take care to not chip the edges of the tiles or damage the waterproofing system under the tiles. – M.H.

This is actually a tool question. I am looking to purchase a snap cutter and wet saw capable of handling larger format tile (up to 48”). Any suggestions?

It certainly is important to have the right tools for the task at hand and to invest in quality tools that will perform well for a long time.

There are many manufacturers that produce the type of equipment you are looking for. While I can’t provide a recommendation for a particular make or model, I can give you a listing of some of the tool and equipment manufacturers that currently sponsor Partnering for Success and are Workshop/Educational Program Trailer sponsors. Some of them make the type of equipment you are looking for.

I recommend you take a look at what these manufacturers have to offer and talk to other craft persons that may have experience with similar tools.

Here is a list, but it may not be all inclusive:

  • Alpha Professional Tools
  • Corona Bellota
  • Dewalt
  • European Tile Masters
  • Husqvarna
  • Mark E Industries
  • Marshalltown
  • Miracle
  • MLT
  • Progress Profiles
  • QEP
  • Rubi Tools
  • Russo Trading Company
  • SGM
  • Tuscan Leveling

It doesn’t appear that you are a member of the NTCA. As a professional tile contractor you will find membership to be extremely rewarding and can get in on the effort to achieve Qualified Labor status and grow your professional potential. There is more information at or you can contact me for information how to join this amazing group.

Thanks for the contact and good luck with your research!

– Mark Heinlein,
NTCA Technical Trainer

Ask the Experts – May 2016


Is there any TCNA or industry information that indicates that rounded-top porcelain cove base is not meant for situations where tile is installed on the walls above the base (since it has a rounded top)? I’d also like to know how the cove base is to be installed in conjunction with floor tile. I stated in my RFP to “install metal trim strips when coordinating porcelain tile pieces are not available.” Given that the cove base option for a selected tile series has a rounded top and foot, this causes a potential for an unclean install. The options that the contractor has given us are:

  • Using the wall base and filling in the rounded top with an enlarged grout joint,
  • Cutting the wall tile and butting it to the floor tile with either a grout or caulk joint at the connection, or
  • Field-cutting the rounded tip of the wall base off

These are not preferred options for the government, as these lead to maintenance issues down the road for the facility.

Do you have any idea how I can respond to this or help with any industry or TCNA info? Thanks so much!


There is no such language in the TCNA Handbook or in any of the ANSI manuals.

I believe that the #3 solution is the proper method, and how my contracting company usually accomplished this detail if round top cove was the only cove available (if cove was required). Be sure to stone the cut, and although it will have a slightly different appearance than a factory edge, it will not be a maintenance issue. Since the tile cove is round footed, it is designed to be top-set.

There is another option that is very effective, if somewhat more expensive. Profile and edge manufacturers make stainless steel coves (with corner trims available) with different sizes available to match the thickness of your tile. Very beautiful, easy to install, and cool, too!

If cove is not required by code, then just use the field tile at all inside corners with a joint filled with foam backer rod and ASTM C920 sealant (100% silicone or single part urethane). Be sure that the tile is not set tight and that the joint is completely free of mortar and grout.

– Michael Whistler,
NTCA technical trainer


We are currently working on a project that includes 30,000 sq. ft. of penny round tiles that were manufactured in Japan. It’s our understanding that penny rounds are classified as a specialty tile and therefore, very little criteria has been set regarding the mounting of them. We are having some issues with the specified product and need a third party to evaluate the mounting of the tile. Can you offer some assistance or point us in a direction regarding mounting issues with mosaics, in particular penny rounds?

experts-01 Attached are pictures of some of the mounting problems, including inconsistent spacing, sheets not being square and excessive mesh where the sheets meet up with one another. Another problem is that the individual tiles release from the mesh backing as soon as they get wet (from thinset).

may-experts-02Two manufacturers’ reps were here last week to review the problems. Their solution is to either send us a video or one of their Japanese installers to show us how to install the tile. As you may imagine we took exception to their suggestion. We have plenty of experience with the installation of penny round tile and have processed through many issues regarding sheeting and wall-washing concerns. What we are in search of, is some guideline or criteria that we can show ownership so they can assess their expectations more in line with industry standards. Any assistance you can share with us would be greatly appreciated.


You certainly seem to understand all of the issues and have the experience to install penny round mosaics, which are difficult at best. The tiles you have been provided appear to have an especially flimsy mesh backing, and the inconsistent spacing and water-soluble adhesive is not going to help matters.

Obviously, a proper substrate that meets minimum deflection and flatness requirements and using the correct mortar and troweling method to achieve a minimum of 80% coverage on each tile with no squeeze-through will be critical.

I’m almost thinking that to have the manufacturer send their installer to show you how it’s done (for the duration of the project) might be a way to bring the manufacturer on board with some liability for the installation.

Has all of the tile been manufactured and delivered? Have you discussed the matter with the owner and architect?

Other than what Katelyn has provided below and the general workmanship requirements found in the TCNA Handbook and ANSI standards, I am not immediately coming up with anything that I can send you. I will check with my colleagues and get back to you.

Perhaps the best approach will be to hire a recognized consultant to come in and serve as a third party to view the tiles and installation area and scope of work and provide you with a written opinion.

– Mark Heinlein,
NTCA technical trainer


From a third party testing lab’s perspective, there is no way to measure mounting variation of penny round, mounted ceramic mosaic tiles. The mounting variation test methods for mounted tile are only performed on square mosaic tiles. I’ve included an excerpt from ANSI A137.1 Specification for Ceramic Tile below:
9.5 Test Method for Mounting Variations
9.5.1 This method is only valid for mosaic tiles with the following characteristics:

  • Nominally square
  • Nominal sizes of 1 inch x 1 inch (25.4 mm x 25.4 mm) to 3 inches x 3 inches (76.2 mm x 76.2 mm)
  • With straight edges

Our lab does not provide reports based on expert opinion so you may need to hire an independent consultant if that is something you are looking for. Please let me know if you are interested in an independent consultant and I’d be happy to send you the contact information for one.

– Katelyn Simpson,
TCNA laboratory manager

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

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