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Is Your Floor or Wall Flat Enough for Large Format Tile?

Is Your Floor or Wall Flat Enough for Large Format Tile?

How prepared are you for installing Large Format Tile (LFT) or Gauged Porcelain Tile Panels (GTPT) and ensuring you have a surface that is adequately flat?

It's a big deal and worth considering before you get started

Let’s explore.

Large format tile or gauged porcelain tile panels: Tile sizes are growing significantly!

How to determine the condition of the floor or wall?

What can happen if the surface isn't flat enough?

Questions from Readers:

Why should you never flatten a surface with thin set?

How do wedges and spacers affect surface flatness?

What leveling measurement does a 30"x60" tile require?

Before installing large format tile or gauged porcelain tile panels/slabs, look at the surface! Is it flat enough?

Large format tile or gauged porcelain tile panels: Tile sizes are growing significantly!

Tile sizes are increasing. Large format tile has grown from the old 8” x 8” to 12” x 12”, 12” x 24”, 24" x 48" and beyond. You'll even find Gauged Porcelain Tile and Gauged Porcelain Tile Panels/Slabs (GPT&GPTP/S) ranging from 1 meter x 3 meters (roughly 39” x 10’) up to 1.60 meters x 3.20 meters (roughly 63” x 126”).

Not only are sizes increasing, but larger sized tiles have been fully embraced by residential and commercial building owners and specifiers.

However, there is a difference between Large Format Tile (LFT) and Gauged Porcelain Tile and Gauged Porcelain Tile Panels/Slabs (GPT&GPTP/S).

The newly released ANSI A137.3 provides this definition –

“Tile Panel/Slab: A ceramic tile of size greater than or equal to 1 m² (11 ft²).  A product less than 1 m² (11 ft²) shall be simply a ‘tile’.”

This means that a gauged porcelain tile product that is less than 1 meter x 1 meter (39-3/8” x 39-3/8”) is considered a large format tile.  However, a tile product 1 meter x 1 meter or larger is considered a gauged porcelain tile panel or slab.

As a result, tile installers must know how to accommodate inherent warpage associated with large format tile, as well as how to provide (and get paid for doing so) a surface that will allow these products to be installed without lippage or at least within the allowable tolerances provided in the ANSI documents.

>> See Do You Have Enough Mortar to Accommodate Most Tile Warpage?

The ANSI Standard for Subfloor Surfaces, ANSI A108.02 - section, states in part:

“For tiles with at least one edge 15” (0.38m) or longer, the maximum allowable variation is not no more than 1/8” in 10’ (3 mm in 3 m) and no more than 1/16” in 2’ (1.6 mm in 0.6 m) from the required plane, when measured from the high points in the surface.”

How to determine the flatness of the floor or wall?

Determining the flatness of the floor or wall is relatively easy by using a ten-foot straightedge.

Simply mark the substrate with some method such as circling the low spots and placing an X on the high spots to quickly show where the work is needed to meet the ANSI specification.

Using a combination of cementitious patching compound (either trowel applied or self-leveling) with the recommended primer fill the low spots and grind down the high spots (be aware of and comply with the OSHA guidelines for grinding). This will normally provide a surface that will be suitable for installing tile within the prescribed tolerances.

By the way, you should never use thin-set or large and heavy tile (formerly medium bed) mortars to flatten the substrate.

Use a long straightedge to determine if the trowel applied patch is flat enough for large format tile.

Notice in the image above taken during an Advanced Certifications for Tile Installers (ACT) test, how the installer is using a straightedge to determine if the trowel applied patch is flat enough to receive a 12” x 24” porcelain tile with a 1/8” grout joint and still be within the lippage requirements.

Realize that the allowable lippage for this tile installation (with a grout joint less than ¼”) under ANSI Specification A108.02   section 4.3.7 is just 1/32” (about the thickness of a credit card).

What can happen if the surface isn't flat enough?

If the floor or wall is not flat, the installer has significant difficulty when installing the tile.

The constant lifting and resetting of the tile to ensure that the tile surface is smooth and flat wastes a huge amount of tile. 

When the tile edges are not flat with each other, the resulting lippage is unappealing to view, but it can also be a tripping hazard for those walking across the floor. 

And understand that the installer is responsible for this condition and can be sued for negligence.

Questions from Readers

Why should you never flatten a surface with thin set?

Alex Bergland asks,

Hey there, curious why it is a "never" to level out a wall or floor with thin-set? I'd imagine you can do it up to a certain tolerance but where do I find the guidelines for when thin-set cannot be used in this manner? Thanks

Scott responds,

Alex, thin set and large and heavy tile mortars are designed specifically to install ceramic tile products, but they were never intended to patch or “fix” the wall or floor.  Almost all of today’s mortar manufacturers make cement-based patching materials to fill the low spots and flatten out the high spots on both floors and walls.  These products are formulated to bond to almost any clean and properly prepped surface and dry in about one hour depending on temperature and humidity.  Conversely, if thin set is used to patch a surface, it normally takes twelve to twenty four hours to harden (A lot of wasted time). 

Additionally, if the thin set in applied thicker than the recommended ¼”, it will shrink significantly. 

Unfortunately, some installers set tile over out of flat substrates by filling the low areas with thin set as they go.  This “extra” mortar plus the mortar applied with a notched trowel will be well beyond the manufacturer’s recommendations.  As the mortar begins to set and cure, it shrinks, pulling the tile with it.  This produces post-installation lippage which was not present when the tile was installed.   A simple word of advice, use thin set and large and heavy tile mortars to install tile and patch for making the substrate flat.

Victor Taormina states,

I am afraid just don't do something as in, don't use thinset to flatten floors because it will fail does not explain why. As installers we are always looking for the why something has not work. We manage risk of not following standards and practices, customers’ expectations of schedule and budget job site conditions. We do this while attempting to run a business. 75-80% of all wall and floor substrates I work on do not meet tile standards for flatness! You give us one message and product manufactures give us conflicting info, (grout boost or Schluter) and installers have our own experiences which also is often conflicting with manufacture standards and or NTCA standards. I understand the importance educations plays in trying to improve industries operating frame work but there is a long way to go.

Scott responds,

Victor, I believe I have provided the “why” in the above response. 

Not following industry standards and best practices may get the job done in order to meet the customer’s expectations of schedule and budget, but if and when the job fails, the installer is on the hook for the replacement, I will grant you that job site conditions are most times terrible and should never be unacceptable.  But we, as installers, continue to “accept” a filthy work space, bad substrates, and take jobs for less money than the job requires.  We need to stop accepting this poor treatment of tile installers and push for better conditions.  If more installers just said no to the above, these conditions would disappear.  I know that it is not easy, but it needs to start somewhere. 

The only message I (along with many others seeking to improve the tile industry) provide comes straight from the TCNA Handbook and ANSI specifications. 

Realize that these standards are the minimum requirements and that the manufacturers meet or exceed these minimums.  This means that the manufacturer’s guidelines should follow the applicable standard and there should not be any conflict.  As installers, we should follow the manufacturer’s recommendations to the letter so that the warranty which they provide, will enhance our installations.

I further agree with you that we, as the tile industry, have a long way to go.  But, each of us needs to do our part- do it right the first time and help educate the installers “who don’t know what they don’t know.”

How do wedges and spacers affect surface flatness?

Amis Vega comments,

Thank you very much. It is very informative. I have lows and downs on the concrete floor I am intending to tile. It is evident the 600 x 600 tiles are not falling flat, however, I am planning to use the wedges and spacers to level the tiles. Have done a mock and the result is not too bad, tiles are levelled within tolerances, however, I am afraid at some point pressure from the wedges may build and snap the tiles...what is your experiences with these? Any recommendations?

Scott replies, 

Amis, I am not certain that I completely understand your question, but I will give it a shot.

I believe you are describing the lippage control systems or devices, formerly known as edge leveling clips, which are placed between two tiles to bring their top surfaces in line with each other.  These devices work well only if they are used in accordance with the manufacturer’s recommendations along with the ANSI requirements for substrate flatness. 

As noted in the first answer above, the surface to be tiled, in your case the floor, must be flat within 1/8” in 10’ when measured from the high points of the floor.  To achieve this, be sure to use the appropriate patch or self-leveling compound (with the same manufacturer’s primer), but never thin set mortar.

For a large format tile such as yours (approximately 24” x 24”), in a dry area, the required mortar coverage on the back of the tile must be 80%.  The only way you will know if you have met this requirement is to occasionally remove a tile to check the amount of mortar on the back of the tile. 

If the tile installation is completed in this way, any point load such as a woman’s high heeled shoe, a refrigerator, or pallet jack, the tile will not be affected and will stand the test of time. 

What leveling measurement does a 30"x60" tile require?

Hiten Patel writes,

Hi Scott, You discuss and provide very good info for installing large format tile. I am installing extra large tile 30”x60” and have a question.

What leveling measurement should I keep to have good level for this extra large format tile? It is concrete floor on main floor of house in Florida. Should I keep 1/8” for 20 feet or 10 feet? Which will work? Appreciate your help. Thanks Hiten

Here's Scott's answer:

Hiten, 1/8" in 20' is even flatter than the requirement, but the American National Standards Institute (ANSI) requirement for any tile 15" or longer on any one side (Large Format Tile), the substrate must be flat to 1/8" in 10' when using the 10' straightedge measuring from the high points

Flattening the surface to meet this requirement can be accomplished by grinding the high points (always use a HEPA filtered vacuum and dust bonnet on your grinder to meet OSHA guidelines), fill the low areas with the appropriate cementitious floor patch, or a combination of the two.  If the floor requires extensive floor prep, you may want to consider using a self-leveling underlayment (SLU) over the same manufacturer's primer.  

With any large format tile, it is crucial that the floor be within this flatness tolerance. 

Additionally, as the tile size increases, so does the need for a flatter floor.  It is much easier to flatten the floor, then set the tile in your normal way and always meet the ANSI requirement for 80% in dry areas and 95% in wet and exterior areas mortar coverage on the back of the tile.

Also, be certain to keep the lippage between tiles to a maximum of 1/32" (the thickness of a credit card).  With a tile of this size, many consumers expect that there is zero difference from one tile to the next.  As mentioned above, the flatter the substrate, the easier it is to minimize lippage.

I hope this is helpful.

Before installing large format tile or gauged porcelain tile panels/slabs, look at the surface! Is it flat enough?

Before starting your next job, look carefully at the surface that is to receive tile. What will it take to make it flat enough to install large format tile? Identifying substandard jobsite conditions and requesting a change order before the job starts will increase your chances of getting paid for the quality work you have provided.

And, don't even think of trying to “fix” the floor or wall surface as you go with thin-set mortar! It doesn't work and is a bad practice which will almost always result in an unsatisfactory finished product.

Remember, thin set was formulated to set tile and be about ¼” thick (depending on the manufacturer) after the tile is embedded and about ½” thick after the tile is embedded in a large and heavy tile mortar. Using the right product will provide much more satisfactory results than creating a hybrid which does not come with a warranty.  

>> The Tile Installation Experience with Scott Carothers at Coverings

Do your customer and yourself a favor, do it right… the first time.

Thanks for reading,



Download 3 Tip Sheets:  How to Select Your Next Tile Installation Contractor

Note: This article was originally published on January 11, 2017. We updated it on February 26, 2019, and have just added to it.