More mortar isn't more coverage
What a day of peeling up tile at the Ceramic Tile Education Foundation taught me about trowels, back-buttering and the half-inch notch.
About 10 to 15 years ago, tile got big. Fast. And almost overnight, the advice on every box and every counter turned into the same three words: half-inch trowel.
That advice came from a real problem. Back then, the worry was that installers weren't putting enough mortar under the tile. A 1/2" x 1/2" square-notch trowel fixed that. It puts down a lot of mortar.
But putting down a lot of mortar and getting good coverage are two different things.
How we tested it
At the Ceramic Tile Education Foundation, we ran coverage tests side by side. The setup was simple, and I'd recommend it to anyone who wants to see this for themselves:
- Back-butter the tile with white thinset.
- Spread gray thinset on the floor with the trowel you're testing: 1/4", 1/2", V-notch and others.
- Set the tile, then peel it back up.
With two colors, you can see exactly where the mortar from the floor met the mortar on the tile, and where it didn't. There's no arguing with it.
What we found
The half-inch trowel made sure there was plenty of mortar under the tile. What it didn't do on its own was get great coverage. Those tall ridges need a lot of work to collapse: moving the tile back and forth, a vibrator, a sander or a rubber mallet. Skip that work and the air stays trapped, which leaves hollow spots and, sometimes, failed installs.
Back-buttering helped the mortar transfer to the back of the tile. But it didn't guarantee coverage by itself either.
The surprise was the trowels with a 1/4" x 3/8" equivalent: Euro-style trowels and slant-notch (lay-flat) trowels. They got better coverage than the half-inch square-notch. The slant-notch trowel did so well that it's the one I'll be recommending for the rest of my career.
80% minimum mortar coverage for dry interior floors. 95% minimum for wet areas and exteriors. Large format tile should aim for 95% everywhere.
What to do on your next job
- Choose the trowel for coverage, not volume. Bigger isn't automatically better. A slant-notch or Euro-style trowel often gets you there with less fight.
- Back-butter. A flat skim coat on the back of the tile helps transfer and fills the tile's back pattern.
- Work the tile in. Whatever trowel you use, move the tile back and forth across the ridges so they collapse and the air gets out.
- Pull a tile and look. It takes ten seconds and it's the only way to really know.
You can still use a half-inch trowel and get a great install. You just have to put in the work to set the tile. The tool doesn't do it for you.
The Krummy Floors tile calculator recommends a trowel from your tile size, warns you when a trowel is too small for large format tile, and tells you how many bags of thinset to buy, back-buttering included. Find it on Etsy.