One of the most frequent questions I get asked is ‘What size and type of trowel should I use for…?’  The proper answer to that is ‘whichever trowel gives you the proper coverage for your particular installation’.

So there really isn’t one perfect answer to that question, a lot of factors are involved. But I’ll try to help you out.

Proper thinset coverage

The first thing you need to know is what constitutes proper coverage.  As stated in ANSI A108.5 3.3.2 for installation of tile on floors; “Average uniform contact area shall not be less than 80% except on exterior or shower installations where contact area shall be 95% when no less than three tiles or tile assemblies are removed for inspection. The 80% or 95% coverage shall be sufficiently distributed to give full support to the tile with particular attention to this support under all corners of the tile.”

Let me translate that for you:

Proper coverage of any tile in a dry area (bathroom floors, backsplashes, fireplaces, etc.) is 80%. Proper coverage of any tile in a wet area (showers) or outdoors is 95%. This includes complete coverage beneath all corners of the tile. You check this by installing a tile, then removing it to check the CONTACT of the thinset on the back of the tile. If, after checking three different tiles in three different areas of the installation, you have that percentage of coverage then you have proper coverage.

Correct complete thinset coverage

Always aim for 100% coverage. More is always better. The photo to the right is an example of complete coverage (thanks Rob).

If you have less than that percentage of coverage you have a couple of options. You can either back-trowel the tile as well as the substrate (back-troweling means combing lines of thinset on the back of the tile as well, not just skimming it with the flat side of the trowel which is called backbuttering), or you can switch to a larger trowel. Both methods will give you more coverage. In the case of back-troweling it will double the amount of thinset beneath your tile.

So those are your coverage requirements and how to check it. Now onto different trowel types…

Types of trowels

Trowels come in a lot of shapes and sizes, even goofy lookin’ ones that look like something out of a Saw movie. The three basic types you should know are the square notch, U-notch and V-notch.

If you look at the horrible graphic I made (my photoshop skills are like a monkey with ten thumbs opening a banana with the keyboard…) you’ll notice that a V-notch will leave the least amount of thinset on your substrate, the U-notch more than that and the square notch will leave the most.

You would be able to visualize this if those horrible graphics were to scale and all the same size. Which they aren’t. Whole monkey-thumbs keyboard-banana thing…

With any mosaic tile you normally want a v-notch, with any large format you normally want a square notch. With average sized tile (12 inches square up to 17” square) you’ll want a u-notch or square notch. With most of my installations I use a square-notch trowel.

Sizing trowels

Trowel measurements can be confusing – some have two numbers, some three. So you need to know how to read them.

V-notch trowels normally only have two numbers. The first number is the width of space between the teeth, the second is the depth of the notch.

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With square notch and U-notch trowels  the first number is the width of the teeth, the second is the width of space between those teeth, and the third number is the depth of the space between the teeth. Like this:

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If it only has two numbers it means the width of the teeth and the width between the teeth are the same and the second number is the depth of the notch, like the one on the left.

This is the most common sizing you’ll find on a trowel.

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If it only has one number, or a trowel is only referred to with one number (as I often do when recommending a trowel) it means that all the measurements are the same. This is what I commonly refer to as a ¼” square notch trowel, even though the graphic says U-notch.

Monkey, thumbs, banana. Yeah.

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Here:

Better?

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Which trowel to use

As I stated at the beginning there is no one answer to that. All I can do is tell you what I normally use with what size or type of tile.

If I am installing smaller mosaics (smaller than 2” square) I normally use a ¼” V-notch trowel. This is a ¼” X ¼” trowel.

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With mosaics or regular tile larger than 2” square I will use a ¼” square notch trowel ( ¼” X ¼” X ¼”).

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With tile 12” x 12” up to 18” x 18” I’ll normally use a ¼” x 3/8” trowel ( ¼” X ¼” X 3/8”)

With tile larger than 18” I’ll either use the ¼” x 3/8” and back-trowel the tile as well, or use a 3/8” X 3/8” or ½” X ½” square or U-notch.

Those are general guidelines and will work with most installations. You NEED to check your coverage with your installation to ensure that you have proper coverage. If you do not have proper coverage you need to back-trowel the tile or use a larger trowel.

The trowel I normally have in my hand is a ¼” X ¼” X 3/8” square notch, I use it for most of my installations (and to irritate random pets…).

This is another one of those questions that, if you ask 50 different contractors, you’re likely to get 50 different answers. For instance the photo of the tile with full coverage at the top is from Rob. It is a 13″ X 13″ tile and he used a 1/2″ x 1/2″ trowel. For most guys it is a personal choice. As long as you get proper coverage there is no wrong answer. The above are simply guidelines and my personal choices. It is up to you to determine whether your trowel is the proper choice for your installation or not.

I can’t see your installation from here. And you know what happens to your dog if you get improper coverage, right?

If you don’t you need to read my blog more.

Just sayin’.