What is Thinset and What Trowel Do I Use?

Thinset mortar is a blend of very finely graded sand, cement, and a water retention compound that allows the cement to properly hydrate. Using the thinset method tile is adhered to the substrate with a thin layer of this mortar.

The terms thinset, thinset mortar, dryset mortar, and drybond mortar are synonymous. This type of cement is designed to adhere well in a thin layer - typically not greater than 3/16th thick. For example, a 3/8" notch trowel will produce a 3/16th inch thick coating after the tiles are pressed in to the cement. While very minor adjustments in height can be made, this method is not appropriate for adjusting the level or flatness of a surface - rather the tile will follow the plane of the substrate.

The American National Standards Institute (ANSI) defines the properties of thinset mortar in the A118.1 specification

Size matters but it is not your only determining factor. One of the most common mistakes is using one trowel on all tile! There are no standard trowel size for tile installation, since it depends on the size and type of tile you install. Each type of tile may require a differently sized trowel. 

Tile A is fairly flat with minimum warping (bowing), you will meet the proper coverage requirement using a 3/8” square-notched trowel. 

NOTE: A minimum of 85% total coverage beneath a tile for a dry area installation (most floors, fireplace, etc.) and 95% minimum total coverage in a wet area (showers, tub surrounds, etc.). There is more to that requirement such as complete coverage beneath all four corners of the tile, but those are the basics.

What trowel size is proper for the installation depends on both the tile itself and the substrate. How flat is the tile and substrate? The less flat the tile and or substrate are, the more thinset mortar you need beneath it, which means a larger notch size.

How much thinset mortar is required in the finished installation. A finished installation requires a minimum of 3/32” beneath the finished installation, aiming for a 1/8” minimum makes it easier to calculate the proper tile trowel size.

Choosing between the Square and U Notch is a matter of personal preference, you can achieve similar results.  Selecting the correct size notch is paramount as you will require a larger U notch to meet minimum standards.

A square notch will create square ridges, these ridges tend to collapse over on each side when you embed the tile into the bed. Pushing the tile down into the bed of square ridges causes the ridges to fold over from the top of the ridge, then spread out.

A U-shaped ridge, on the other hand, spreads out when embedding a tile into the bed. Pushing a tile into the bed of rounded ridges causes the ridge of thinset to spread out, from the bottom, with less possibility of trapping air.

The difference in trowel notch shapes determines how easily the tile becomes fully embedded in the bed of thinset. Some people prefer the U notch because it maybe easier to wiggle a tile into a bed of thinset however although it maybe easier you must also use a larger notch to achieve the same thickness beneath the tile.

V Notch Trowels have a saw tooth pattern with points. They are typically used for small tiles, such as  small mosaics, glazed wall tile up to 4-1/2". V-notch trowels dispense less mortar than square or U-notch, they are also primarily used for many wall applications. 

The Euro Notch and Slant Notch Trowels were designed to form notches in the mortar that would collapse on themselves and leave less pockets. These trowels are new for the tile industry and have grown in popularity due to the increase demand for large format tile.