How thick can you apply thin set mortar?

The type of mortar used in a thin set application depends on the tile type as well as the substrate to receive the tile. I.E. ceramic tile installed on cement board would only require a mortar with the designation of ANSI 118.1 on the bag, but a porcelain tile to be installed over cement board would require a designation on the bag of 118.4. from the American National Standards Institute.

ANSI 118.1 is an unmodified mortar and ANSI 118.4 is modified. Porcelain tile requires a mortar that slows down the curing process. ANSI 118.4 mortars do not slow down drying time, only the cure time. Typical tile installation and grout routines remain the same but the total cure takes longer overall. This process can interfere with submerged applications, high humidity or cold environments and some membranes of a tile assembly.

Some tile types may require ANSI 118.15 or improved modified thin-set mortar. This type of mortar is used on glass tile in addition to porcelain, installations requiring greater shear strength, and improved adhesion or tensile strength.

There is also ANSI 118.11 which is used to adhere tile to Exterior Grade Plywood. There are instances where it is necesarry to bond to plywood. For the most part though an installer should look for a way to avoid directly bonding to EGP. Wood is less stable than cement board. It may move at a different rate than the tile covering during thermal changes which may lead to de-bonding or cracking of the tile.

When installing over different types of membranes you may need a specific mortar type over another, despite the tile type. Some waterproofing membranes require a modified mortar meeting or exceeding ANSI 118.4, whereas other manufactures may designate only unmodified mortars when used over their impervious membrane, even if the tile is porcelain. Always check with the membrane manufacturer when in doubt.

A thin-set mortar installation is a method of tile installation, not a product.  A bag of mortar that reads" Thin-Set Mortar" is indicating that this particular mortar should be used in conjunction with the thin-set/thin bed method. A Thin-set mortar installation is the most common form of adhering a ceramic tile to a given substrate in North America. The important distinction between a thin-set application and any other mortar application is thickness, it's in the name, thin-set: to be installed in a thin layer.

There are many things to consider and identify prior to the application or even the purchase of thin-set mortar. Ask yourself these questions just as a specifier might:

  • What is the substrate type; Is it concrete, wood, a membrane or other?
  • Will the installation be submerged? Will it be wet intermittently?
  • What is the tile composition? Is the tile vitreous, semi-vitreous,non-vitreous, or impervious?
  • Will the mortar color matter? Is the tile translucent, opaque, etc.
  • Will the mortar need to be tinted with a colorant to compliment the finished grout work.
  • What type of traffic will the finished floor receive such as foot, carts, automobile, or otherwise, be used in the given area? While the wear resistance matters less in regards to mortar selection it does matter for the sake of environmental exposure. Impact and shear strength are an important consideration in commercial, and some residential applications.
  • Is the weight of the completed assembly an important factor in relation to the area receiving the installation?
  • Does the project time frame allow for a proper cure time for the selected mortar before the floor, walls, shower, etc is put into service?
  • Would a light weight mortar reduce installer fatigue over the course of an installation and/or decrease the expected mortar slump?
  • Is rapid setting mortar required/desired?
  • Does the stone or tile have a mesh back?
  • How much square footage will the mortar cover depending on the trowel size?
  • Is the mortar compatible with a given membrane, as the membrane manufacturer sees it?
  • What warranty might the mortar have? Does the warranty only apply as part of a total manufacturers system?

The Tile Council of North America Handbook specifies that an LHT mortar may be 3/4 inch thick (nominal) after the tile is embedded utilizing the thin set method. Providing the manufacturer indicates 3/4 on the bag. Thin set mortar has limitations on how thick it can be applied. This mortar type is also used when the bond coat will be over 3/16ths of an inch thick. Thin set mortar should not be used to true, plumb or level a substrate, in almost all circumstances. Manufacturers do not disclose when it may be suitable to true small imperfections because purchasers of the product may abuse, or overdue product limitations. And as such manufacturers are careful to have ridgid boundries when designating limits. LHT stands for Large Heavy Tile. This type of mortar was formerly referred to as medium body mortar or Large Format Tile mortar. These mortars are specifically designed to have an increase thickness when the tile is fully embedded onto the substrate and are used not only to support a heavy tile by reducing sag, slump, or drop during the initial cure, but to also achieve greater mortar coverage requirements if there is warpage in the tile. With most tile that exceeds 15 inches in length there is often warpage to one degree or another.

LHT mortars may have a "T" beside their ANSI designation i.e. ANSI 118.15T, this indicates that the mortar is thixotropic.

Mortars with this property have a resistance to flow unless acted upon, which increases mortar stability during the initial drying process. They are non-Newtonian, similar to ketchup.

On almost any tile project there is always the need for mortar. Fat mud, deck mud, concrete, and adhesive to name a few. The most common type is adhesive meant to be installed in a thin bond coat often referred to as thin set or dry set mortar. An installer will tell a foreman "I'm gonna need ten more bags of thin set" and what he is actually conveying is "I need ten more bags of mortar adhesive to install this tile using the thin set method."

A description on a bag indicating the product is dry set mortar is conveying the product is to be installed on a dry or fully cured substrate. As opposed to tile being installed on wet, or green concrete which would be a wet set method and shares no part of a thin-set method. Always read the product description as well as the mixing directions. You only have to do it once, it will make the products workability, and curing predictable, and that can save you a headache.

Did you know that organic adhesive or ready-to-use dispersion adhesives "mastics", are installed in the thin set method? That means a bucket of mastic/glue may indicate that it is a thin set. Well, we know what they mean, It's not a thin set mortar, but it can be used in a thin set method. Dispersion adhesives, mastics, designated D1 are only for dry, interior locations. They are not to be used in a shower or tub receptor. D2 are for interior only in intermittent wet areas. I have been installing for almost 25 years and have not found a good instance to use mastic when a mortar will do the job.

Yes it can. When thin set is applied too thin for a particular tile or substrate, a failure can occur. Most bags of thin-set mortar have a guide that recommends the proper trowel size for their mortar. In general you want the thin-set mortar to be a thick as possible when the tile is embedded without exceeding the manufacturers guidelines or creating a mess with bleed through coming up through the grout joints.

An example of appropriate trowel size would be;

A 1/2 inch X 1/2 inch notch trowel for a 3/8ths thick tile on a concrete substrate. When the ridges on a 1/2 X 1/2 notch trowel are collapsed, the thin-set mortar bed is usually about 3/16ths thick.

A thinner mosaic tile could use a trowel as small 1/4 inch X 1/4 inch notch on top of a concrete substrate that is expected to have foot traffic. An even smaller notch could be used for a mosaic if the mosaic was to be installed in a dry area that does not get walked upon. In general, keep the thin-set bed on the thicker side as long as it does not create a bleed through problem for the mosaic.

Knocking down the ridges prior to embedding the mosaic tile is always a good idea. It makes a bigger notch possible without all the bleed through. This method is used primarily on clear glass, but this technique has a benefit for all mosaic tile. See the video below.

What other performance and workability characteristics might a mortar have?

When thin-set is installed too thick the chance of failure is increased. Thin-set mortar will shrink and even curl during the curing process. Excessive curling will weaken the mortars ability to bond to the tile and the substrate. Too much thin set can also be heavy which may cause further drop during the initial set following placement of the tile on horizontal applications. Curing times are extended with excessive amounts of mortar sometimes greatly extended. Especially when a modified mortar is used on porcelain tile or other impervious materials.

Spot bonding tile with dollops, dabs, dots, etc, is one of the single worse things an installer can do in regards to the application of thin-set mortar. A host of problems are created and the fact that the spot of mortar is too thick is just one of them.

What happens if thin-set is too thick?

Can thinset be too thin?

What is a thin-set mortar?

method distinction and product limitations.

 Written by Michael Weaver

The photo above is a good example of what proper mortar coverage looks like on the back of the tile and on the top of the substrate when checking during the installation. Lifting tiles periodically to check coverage is important.

In conclusion.

There are so many different mortars, substrates, tile types, as well as system configurations to consider when selecting a mortar. Whenever you are unsure, you should always contact the manufacturers technical line. As a pro I have done it many times over the years to confirm what I believed was true. This needs to be done in the planning phase and not five minutes before the job is about to begin. They want you to succeed, and so do I!

How do I know what type of thinset mortar to use?

Be aware of improper mortar application

The proper application of the thin-set mortar can not be overstated. Please review our page on spot bonding to avoid catastrophe, and watch the Trowel & Error video for a demonstration on proper mortar application. Directional troweling is extremely important as a tile becomes larger than 8x8 inches square.

My favorite mortars are light weight mortars. Oh man, moving ten 30lbs. bags is so much better than even five 50lbs. bags. Light weight mortars also reduce the drag that is added to a tile hanging on a vertical surface. Light weight mortars are often ANSI 118.15T so they have improved strength and are very helpful on glass mosaic installations.

Sometimes a mortar is chosen for how bright white the color of the mortar is when it's cured. A truly bright white is desirable when installing clear or translucent glass tile and mosaics. Glass tile mortar is often brighter and contains much finer aggregate. Some mortar manufacturers have adhesives that can be mixed with color packets to create a custom color that will match or compliment the grout. Always choose a mortar by the adhesive manufacturer that is approved for use in glass tile installations. It will look better, bond correctly, and be easier to work with.

Mortars with a reduction in dust during the mixing process are gaining traction. When mixing several bags of mortar an apprentice may be exposed to silica dust. There are dust removal devices on the market that work very well, but depending on how much room the mixing area has, the vac can be impractical or crowd an already tight workspace. Dust extraction is great, but a reduction in dust created is even better.

There are mortars specifically for dealing with flex. Flexible mortars are usually incorporated into one or another type of modified mortar. For the most part any perceived flex prior to tile installation in the substrate should be resolved by correcting the substrate. Flex or multi flex materials are for areas expected to go through thermal stress and intermittent load fluctuations. Not to correct an inappropriate substrate.


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