Using the proper trowel

by Roger

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.

Example of Full 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

types of trowelsTrowels 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

1/4" X 5/16" V-notch trowelTrowel 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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1/4 X 1/2 X 3/8 TrowelWith 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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1/4" X 3/8" square notch trowel

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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1/4" X 1/4" square notch trowel

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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SQUARE NOTCH!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.

1/4" X 1/4" V-notch trowelIf 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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1/4" X 1/4" square notch trowelWith 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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1/4" X 3/8" square notch trowelWith 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’.

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Q

Hey Roger, After getting the floor mat down, leveling everything up and installing the Ditra, my wife would like the tile to continue into the attached laundry room. Because the height is now 1/4″ up because of all of my leveling continuing with Ditra will leave the new area low. 1/4 inch hardi matches the height exactly, but I am assuming it would be a mistake to have any tiles bridge the two different underlayments. What would you recommend?
1.Two layers of Ditra? I can’t find the XL anywhere.
2.One layer of Ditra with thicker thinset underneath
3.Hardibacker with the seam between the two taped with Kerdi?
4.1/8 inch material of some type then thinset and Ditra?
Any suggestions would be great.
Thanks,

Oh and for 12×24 tiles I use 1/2 1/2 trowel right?

Reply

Roger

Hey Q,

You can either use two layers of ditra, ditra with more thinset or use the 1/4″ backer, tape and mud the transition and place a soft joint directly above it.

Yes, 1/2″ trowel.

Reply

Q

As always thank you very much!!!.

if I double the Ditra do I put unmodified thinset between the two layers of ditra as well?
thinset/Ditra/thinset/Ditra/thinset/tile? Whew… that seemed repetitive.

Reply

Tim

I’m using WEDI pre-formed shower base for a curb-free installation. (BTW I wouldn’t do it again, WAY too expensive and it didn’t solve the problems I was trying to avoid. (I wish I had come here before buying all this stuff) I had to build up the surrounding floor to match the edge thickness of the base, and it came up about 3/32 short. My question is, can I rely on thinset to fill in the difference (which will be under a 1/2 inch WEDI board) or do I need to add another layer (of thin plyboard or maybe SLC) to the floor?
Thanks!
Tim

p.s. I checked out your corvette project – man, you have infinite patience and determination!

Reply

Roger

Hey Tim,

Yes, the thinset will work just fine for that. Don’t know about the patience part, but I do have the determination. :D

Reply

Tim

awesome! one more question, how much variation from flat can be accommodated by thinset?

I used SLC to fill in a dip, and the very helpful folks at Lowes told me I didn’t need primer, so the SLC dewatered (I now know) and wouldn’t flow. I had to try and trowel it (actually used a yardstick to try and get a flat surface, since I had no other tools with me) while it was drying at an alarming rate, and it now has many bumps.

Do I need to grind this sucker flat? if so how flat?

thanks!
Tim

hang in there on the ‘vette!

Reply

Roger

Normally about 1/4″ or so, but it’s always better to have it flat first (but you already knew that).

Reply

Terry

Thanks, I’ll get a 3/8″. There is a slight bow. Is back buttering a good idea here?

Reply

Roger

Always.

Reply

Terry

Doing a kitchen floor with 6″x24″ porceline. 1/4 trowel good enough? About how thick should the thinset bed be?

Reply

Roger

Hi Terry,

You really should use a larger trowel. The final thinset height requires a MINIMUM of 3/32″ beneath the tile. I would use a 5/16″ or 3/8″ trowel for tile that size. If they are bowed at all the center of your tile likely won’t even touch thinset with 1/4″.

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Q

Hi Roger,
Thanks for all your help so far. I am on to tiling the bathroom floor now. My floor is osb 23/32 T & g 16″ on center engineered 12 inch joists 14 ft span, I realize the controversy with osb, but also see that some manufacturers allow osb.(sutra, Ardex)
the plan is:
Primer, suntouch heat mat, embed In Ardex k15 slc, unmodifed thinset ditra, unmodifed thinset, tile

Another tile professional suggested cork, heat mat, custom products slc, tile.
Any suggestions?

Reply

Roger

Hi Q,

Nope, your plan is just fine.

Reply

Jim

Hi
I am looking to install 12×24 porcelain tile in a bathroom floor. What size trowel do you recommend?

Reply

Roger

Hi Jim,

Probably a 5/16″ or 3/8″ u-notch.

Reply

Trish

Hi Roger,

My husband has just installed a single row of slate tile in our shower surround. I can visibly see the trowel lines at the top of the slate row and can actually slide a cake tester thru the lines. Lots of air pockets….The tiles were set yesterday. Do the tiles need to be removed or is there a way then can be filled in with thinset? Also how much thinset should be in between each tile? My husband removed all the thinset inbetween each tile.

Reply

Roger

Hi Trish,

The tiles are fine. Ideally there should be a solid, thin layer of thinset behind the tiles. But the tile on the wall will not be walked on, it’s only holding it on the wall. There doesn’t need to be any thinset between the tiles.

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John

Doing a bathroom floor with 2″x2″ mosaic. 1/4 x1/4 sqr notch or V notch trowel? Backbutter or no? What is the best way of leveling/seating the tile? Plywood with a rubber mallet?

Regards

John

Reply

Roger

Hi John,

v-notch. Embed the mosaics with your grout float. Plywood and a mallet work well too, but the grout float is easier and just as effective. No backbutter on mosaics.

Reply

Sean

Hi Roger,

The water test on my Kerdi installation went 100% awesomely – practically no drop in the water level, thanks to your guide.

Quick question about shower wall tile. I wasn’t able to get my substrate (Hardibacker) totally flat, and there is a seam between Hardie boards on the side wall where one board sticks out maybe 1/8″ more than another. Can I remedy this by using a bigger-notched trowel? What size would you recommend (13″x13″ square porcelain tiles)? And what size trowel should I use for the spots that are totally flat?

Thanks a bunch, Roger. You gave me the confidence to totally tear apart my ugly bathroom and make it look great with tile.

Cheers,
Sean

Reply

Roger

Hi Sean,

Yes, you can make up for that by using a larger (3/8″ x 3/8″) trowel. I normally use a 3/16″ for most of my tile.

Reply

Patrick

Roger,
I am planning on placing 1/8 (grout line) of silicone caulking where the floor tile butts against the baseboard molding and tub. Is this correct?
Should the floor tile be below, level or above (thickness of tile) the baseboard molding?
I will be installing my floor tile diagonally. You mentioned that you should start with a full half tile. It doesn’t have to be exactly in the middle of the door way. Correct? You just want a full half tile and then position it as best as possible so it leaves the largest cuts on the perimeter.
I will be cutting the door casing and jam by under cutting the wood taking into consideration the thickness of the tile and thinset. Is this the correct method?
Regards,
Patrick

Reply

Roger

Hi Patrick,

1. Yes.
2. As close to level as you can get it.
3. Correct, it does not need to be exactly in the center of the doorway. Yes, largest perimeter cuts.
4. Yes, absolutely correct.

Reply

Rick

Roger,

When installing 1″ x 2″ Mosaic tile on a shower floor, what size/type trowel should I use? Your earlier post states that you use a 1/4″ x 1/4″ V-notch when installing this type of tile. But does it matter if it is installed on the shower floor compared to a dry location? Second question (or is the third)? If the tile is 1/4″ thick and I use a 1/4″ x 1/4″ V-notch trowel, for measurement purposes can I assume that the finished height of the tile will be approximately 3/8″ from floor substrate (top of final Deck Mud) to top of tile surface?

Reply

Roger

Hi Rick,

Shower floor doesn’t matter, same trowel. It will likely be about 5/16″ high, a v-notch compresses a bit more than 1/2.

Reply

Q

Roger,
When I tape and mud the seams of my cement board and Niches does it matter if I use the same thin set as my final thin set brand and type etc?

I plan on using ultra flex 2 or Ultraflex LFT, but I have a half bag of versa bond that I am considering using for the tape, unless it should be the same make and type.

Reply

Roger

Hey Q,

Doesn’t matter what type of thinset you use at all, you can use the versabond.

Reply

Q

Hi Roger,
Still stuck in my Niche.
The tile is 11 7/8. I am laying the wall tile over the front edge of the niche lining tiles. If my opening is 13.25 from cement board to cement board and then I subtract 3/8+3/8+1/8+1/8 I would be 1/4 inch too large. If I added an additional piece of 1/4 hardibacker to make up the difference would I be getting too close.
The biggest unknown is my thinset thickness. I assume I will likely use a 1/4 inch trowel for the tiles that line the niche. I am also assuming that it will compress down to 1/8″ But I could be wrong. Should I make the niche size 1/4 to 3/8 inch too large as I have it and build the excess with thinset or should I try to reduce my overall size with hardibacker by a 1/4 inch and hope my thinset compresses enough.
I wanted to include the extra material under my liquid membrane if I was going to add it.

THANKS!!!!!!!!

Reply

Roger

Always make is larger. You can always put more thinset behind your tile, you can not compress tile to make it smaller.

Reply

Brian

Hi Roger.

Stumbled onto your sight and poked around a bit. Wanted to commend you on your knowledge and skill set. I am a tile contractor for almost 30 years and as you know it is not an easy task to find another installer who takes the time to ” do it right ” Your advice is spot on, you are a blessing to your readers.

Brian

Reply

Roger

Thanks Brian. :D

Reply

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