Does Trowel Size Matter?

A common question we receive from Do-It-Yourselfers is, “Does it matter which trowel size I use?

Yes, it absolutely matters, if you want the job done right the first time, so you don’t have to tear something out and do it again!

But, how do you know which trowel size to use for the tile you’re working with?

The short & proper answer is “Use the one that gives you the proper coverage.

That may sound vague and unhelpful to you if you’re less experienced with properly setting tile, so let’s elaborate on that so you can fully understand what that means.

Does Trowel Size Matter? | Tile Shower DIY

Coverage means the area that makes contact between the tile and the substrate. While 100% is ideal, the tile industry standard for ceramic & porcelain tile is 80% mortar coverage in dry areas, and 95% in wet & exterior areas. For natural stone tile, the required coverage is 95%, no matter if it’s installed interior or exterior, or wet or dry areas.

Quite simply, you don’t want voids under the tile. Those voids are air pockets that aren’t bonded to the substrate (the floor or wall material you’re applying the tile to). If it’s a floor tile for example, that unbonded area can lead to cracking when moving something heavy across it like a refrigerator, or can lead to grout cracking and the tile working loose over time with repeated foot traffic.

The coverage you can achieve depends on how flat both the tile and the substrate are. The less flat the tile or substrate, the more mortar you’ll need to not have voids and to fully bond and support the tile…thus needing a larger trowel size to accomplish evenly applying more mortar needed.

Generally speaking, it’s better to have more mortar than needed than not enough.

This point may be a little basic for some, but just so we’re all on the same page, mortar is what bonds (glues) the tile to the floor or wall.

There Are Several Types of Notched Tile Trowels

How Trowels Are Sized

Trowel size is determined by the size and spacing of its teeth. The first number is the width of the notches (the distance between the teeth), and the second number corresponds to the depth/height of the notch. If a trowel only has one measurement listed – i.e. 1/2″ – that means both notch width and height are the same.

When using a square-notched trowel, with equal notch and teeth sizing, the bed of thinset mortar will be half of that size after setting the tile. So, 1/2″ ridges, will become a 1/4″ thick bed of mortar as the tile presses the ridges evenly into the spaces between the ridges.

Sizes of square notched trowels and their uses:

In general, the smaller the tile, the smaller the trowel size; and the larger the tile, the larger the trowel size. But that’s not always the case!

Two tiles of the same dimensions, from two different manufacturers, could require different trowel sizes for proper coverage, depending on tile uniformity, flatness, back side texture, and other factors.

Tile Flatness

Tile manufacturers are allowed by industry standard, a certain amount of tolerance in how flat or “out of flat” the tile is. Most large format tiles (tiles with any side greater than 15″) especially will be slightly concave, warped, or cupped.

If you have a tile that is cupped, this means that if you were to apply mortar to the substrate evenly and equally, and set your tile, then pry it up to check for coverage, you would have coverage all around the edges but none toward the middle of the tile.

You can quickly get an eye for how flat or cupped your tile is by holding two pieces face to face and pressing together one corner. If the tile is cupped, the opposite corner will spread apart. If so, measure that gap and divide by half to determine the amount your tile is cupped. If for instance, you have a 1/4″ space between the tiles, that would equal a 1/8″ amount of cupping, and you’ll need to add that much mortar to the bed in order to get proper coverage.

That’s where back buttering also helps…

Back Buttering

Back butter or flat trowel the back side of the tile – tiles are all made differently depending on the manufacturer, with varying textures and surfaces on the back sides of the tile (such as honeycomb, diagonal grid, smooth, etc.), so applying a uniform thin coat of mortar fills the low spots and gives an even surface of mortar to bond to the mortar on the floor.

While back buttering is not an industry requirement, it is highly recommended. It can ensure you get rid of a film of dust on the back side of the tile and can increase bonding by 50%.

Use The Right Mortar Too

Using the appropriate mortar is crucial as well. Thinset is a specific kind of mortar which by it name implies it can set and bond in a thin layer. For the more commonly used Large Format Tiles today, use a mortar specifically designed for them, such as Mapei Ultraflex LFT – where the LFT refers to Large Format Tile.

Proper Troweling & Tile Setting Techniques

Direction of Notches

How You Hold The Trowel

Setting The Tile

Recapping The Proper Steps Of Installing Tile

  1. Flat trowel mortar to the substrate first: grab some lumps of mortar from your bucket, apply to the substrate, and then spread evenly with the flat side of the trowel, which ensures all low areas are filled evenly. This process is referred to as a Key-In or Burn-In of the coat of mortar into the substrate.
  2. Comb mortar with the notched side of the trowel on the substrate:
  3. Back butter the under side of the tile:
  4. Place tile on substrate and move slightly side to side to press into place:
  5. Lift the first tile laid to check for coverage: use a putty knife or trowel to pry up the tile from one side, properly adhered tile has a lot of suction from the mortar.
  6. If coverage is acceptable, continue. If not, move up to the next size trowel and repeat the process.

Videos You May Find Helpful:

This is a training video put out by Schluter, a leading company in the tile supplies industry. They discuss industry standards I’ve covered in this article, along with other helpful bits of information, and demonstrate quite well how to properly trowel mortar and set tiles.

Another helpful video, not the highest quality or most refined presentation, but you could find some of what he shares to be helpful in understanding why to use certain trowels, mortars, and such.

Here’s a video from a professional tile setter, as he shares from his experience when he wants to use certain trowel sizes, and demonstrates how to properly trowel and set tile.


References & Resources:

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