If you're asking how to trowel mortar when installing tiling, realize that you are asking a fundamentally critical question. It's so critical that readers like you have made this CTEF Blog article the one with the highest readership since we originally published it on June 7, 2016.
You see, when thin set mortar is incorrectly applied, the installation will many times fail. More likely than not, the tile will break - as you see in the image below.
Not a good situation for the customer and a terrible one for everyone involved in installing tile correctly.
Let's focus then on how to properly apply the mortar.
What function do thin set and large and heavy tile mortars serve?
Thin set and the new large and heavy tile (LHT) mortars serve several functions within a tile assembly:
They bond to the substrate
They support the tile
They bond to the back of the tile.
When these three key elements are in place, the tile installation will provide many trouble-free years.
The way you trowel mortar for setting tile makes a big difference. Porcelain tiles, in particular, can withstand extra heavy service conditions by taking a lot of impact or point load (think of high heel shoes, pianos, refrigerators or pallet jacks) when installed correctly over a sound substrate. When the mortar isn't properly troweled, it creates unsupported space under the tiles which become weak spots that can be easily damaged by the same impact or heavy loads.
Here's how to avoid these issues and create a strong tile assembly.
1. Select the right trowel type and notch size!
Make sure you select the appropriate trowel type and notch size.
When a “v” notched trowel is used, the narrow point of the mortar does not provide the coverage necessary to properly support the tile.
Using a notch which is too small will not allow enough mortar to be applied and likewise will not adequately support the tile.
2. Spread the mortar completely to the layout line!
Make sure to spread the mortar completely up to the layout line.
Installers who stay away from the chalk line to eliminate having mortar squeeze up in the grout joints leave dangerous voids which can result in catastrophic failure, especially in areas subject to high heels, point loads such as pallet jacks, pianos or a refrigerator.
Follow established methods and best practices for installing ceramic tile to avoid long-term issues.
3. Trowel the mortar correctly by playing it straight!
Troweling mortar in one direction in a left to right direction, as seen in the National Tile Contractors Association Trowel & Error video (which you'll find at the end of this article) yields the best mortar coverage on the back of the tile. Place the tile into the mortar moving it in a back and forth motion perpendicular to the trowel ridges. This movement collapses the trowel ridges into the valleys leaving very few if any voids in the mortar which supports the entire back of the tile.
As the NTCA video explains, trowel ridges running in straight lines are much easier to collapse. Furthermore, they assist with air removal to maximize mortar coverage and ensure a strong bond to the tile and substrate.
The first step is to “key in” a coat of mortar into the substrate for a good mechanical bond with the flat side of the trowel.
Then, add more mortar to the substrate combing the mortar in straight lines, all going in one direction. Combing the trowel ridges in straight lines provides better distribution of the mortar.
When installing rectangular (plank) tiles, the trowel ridges should go in the same direction as the short side of the tile. This allows better air release from under the tile since the air travels a much shorter distance when you bed the tile.
Be careful not to leave any voids along chalk lines or between tiles.
Use a trowel that will help you achieve a continuous minimum 3/32” coverage. Larger tiles most often require deeper trowels.
In the photo below, you can see that two adjacent tiles are broken with those pieces being much lower than the unbroken tile around them. The cause of the failure was multi-faceted. The installer did not spread the mortar with a recommended trowel, there was no mortar at the edge of the tile, and the installer was not qualified. The installer in this case had been placing tile for a grand total of three weeks.
Here's the tile installation problem you want to avoid.
Watch this video to learn how to correctly trowel mortar.
The 6:35 minute NTCA video titled Trowel and Error demonstrates how to correctly apply mortar as well as what happens when you don't. Doing it correctly creates a stronger bond between the tile and the substrate; it minimizes lippage, tile breakage and other costly tile installation problems. It also means never using these failure-prone tricks:
Spot bonding with mortar - which is not recommended to install tile. It may be easier to set tiles flat to each other during the installation, but it’s only a matter of time before just the slightest force causes a failure! Not to mention hollow-sounding tile. It means you can't get proper mortar coverage; air gets trapped with nowhere to go leaving the tile unsupported.
Swirling the mortar - this causes voids where the tile is not bonded to the substrate. These voids can result in cracked tile and bond failure under normal use but especially under point load or impact.
Here's the video:
Bonus Tip: Don't forget to back butter the tile!
For large format tile, glass tile, natural stone or any tile set on exterior surfaces, “back buttering” the tile is recommended.
Use the flat side of the trowel to get an even coat of mortar filling all the spaces in the surface of the tile.
After keying in and spreading the mortar, set the tile firmly, moving it back and forth across the trowel ridges at least the same distance as the width of the trowel notch in each direction (e.g., 1/2"x1/2" trowels need the tile to be moved about one inch). Move the tile only in one direction, perpendicular to the ridges, without moving in the opposite direction or twisting the tile.
When you first start setting and then periodically through the installation remove a tile checking for coverage.
The ridges should be collapsed into the valleys. Neither the tile nor the substrate should be missing mortar. Straight trowel ridges collapse with the back and forth motion to eliminate voids, achieving a much stronger bond with complete mortar coverage.
And remember - the larger the trowel, the more back and forth movement is needed.
Questions from Readers about Troweling Mortar Correctly
Since publishing this article in 2016, we have received many questions that we answer below.
Thinset on the tile vs. the wall
In this section, we answer three questions.
Question 1
Can I apply the thinset directly to the tile or should it always be applied to the wall? I'm using a VersaBond polymer modified-LFT mortar. I'm putting 6x6 porcelain tiles on the wall inside the tub/shower area, 12x12 porcelain tiles outside of the tub/shower area and 12x24 porcelain tiles on the floor. Thanks for your advice, Steve.
Steve, Thank you for your question.
Conventional thin set and large and heavy tile (LHT) mortars should always be keyed (mechanically forced) into the substrate, in your case, the tub/shower walls to promote a good bond. Applying the mortar to the back of the tile eliminates this crucial component of first quality tile installations.
Additionally, the mortar should always be spread in one direction, either east to west or south to north. Place the tile into the freshly spread mortar, moving the tile back and forth in a perpendicular direction to the trowel ridges. This back and forth motion will collapse the ridges into the valleys as you can see in the Trowel and Error video above.
A tub/shower area is considered a wet area according to the TCNA Handbook which requires the mortar coverage to meet the required 95%.