The method of applying thin-bed and medium-bed mortars has changed over the years. Today many installers use (or should be using) the straight line troweling method as describe in the National Tile Contractors Association (NTCA) Trowel and Error video.
This method requires that the mortar be keyed (or forced) into the substrate with the flat side of the trowel and then notched in one direction, either left to right or up and down. The tile is then moved in a perpendicular motion to the mortar ridges. This back-and-forth motion lays the ridges into the valleys, normally yielding full mortar coverage on the back of the tile. This process has been in use for well over 20 years and has solved many of the mortar coverage issues previously found in the field.
With the ever-growing size of tiles found in the marketplace, the need to achieve full mortar coverage has become even more important. In order to achieve full mortar coverage, the air found in the valleys of applied mortar mustfind its way to the nearest grout joint. Common sense would tell us that finding the route to the closest joint would speed this process. This is especially necessary with rectangular-shaped tile.
While still using the straight line troweling method, we need to modify our technique just a bit. The change is that the mortar is now spread the same direction as the short side of the tile. By doing this, the distance the air must travel is reduced. Moving the tile in the back-and-forth motion will bring the ridge into the valley, thereby forcing or “burping” the air out from under the tile. If you listen carefully during this process, you can actually hear the air being removed. As you can see in the attached photo, the air is gone, the ridges are collapsed into the valleys and full coverage is achieved.
While this method normally works really well, there is one more critical step in the process. Occasionally lift an installed tile to verify that you’ve got the coverage you need. This simple test will ensure you are well-covered and provide the quality your customers deserve.