Tile & Stone

Trowel Notch and Mortar Coverage for Tile

ANSI mortar coverage rules for tile (80% and 95%), directional troweling, back-buttering, and notch sizes for small and large-format tile.

Updated · 2 min read

For tile, the trowel notch is only a starting point. What the standard actually measures is coverage: how much of the tile's back is in contact with mortar. Pick a notch, set a few tiles, pull them up, and adjust.

Coverage requirements

ANSI A108.5 sets the minimum share of the tile's back that has to be in contact with mortar.[1]

Installation Minimum contact
Interior floors 80%[1]
Exterior and showers 95%[1]
Natural stone 95%, with no voids over 2 sq in and none within 2" of the corners[2]

Pull at least three tiles to inspect coverage.[1]

Choosing a starting notch

Bigger tile needs a bigger notch. Some typical starting points:[1]

  • 6x6" and smaller: 1/4" x 1/4" may be adequate.
  • 18x18": 1/2" x 1/2", or a 3/4" loop notch.
  • Flatter substrates and flatter tile need less mortar to reach coverage.

Directional troweling

How you comb the mortar matters as much as the notch.

  1. Key the mortar into the substrate with the flat side of the trowel.[1]
  2. Comb it with the notched side, all in one direction.[1]
  3. Set the tile and push it across the combed ridges so they collapse and air escapes.[1]
  4. Pull a tile early and check coverage. Adjust notch or technique if needed.

Swirled ridges trap air and leave voids even with a big notch.

Back-buttering

A thin coat on the back of the tile improves the bond and fills low spots. It doesn't count as coverage on its own, though.[2] You still need a full bed on the substrate.

Large-format tile

Large-format tile has tighter rules.[3]

  • Large-format means at least one edge longer than 15".
  • Substrate flatness: no more than 1/8" in 10 ft, and no more than 1/16" in 24".
  • Use a mortar rated for large-format tile (ANSI A118.4 or A118.15).
  • The minimum grout joint is three times the tile's facial dimension variation, and never less than 1/16".

Common mistakes

Most hollow tiles come from technique, not the notch size.

  • Swirled ridges. Comb in one direction.
  • Spreading too far ahead. Skinned mortar won't bond.
  • Never pulling a tile. It's the only way to know coverage.
  • Relying on back-buttering alone.

Hollow tile and lippage troubleshooting is in Lippage, hollow spots and cracked tile.

Trowels we carry

These are square-notch and professional trowels for tile.

References

  1. Laticrete TDS 196: Mortar coverage
  2. Ceramic Tile Foundation: Insufficient mortar
  3. Custom Building Products: Large-format tile FAQ

Linked PDF documents are archived copies saved by PreFloor Tools so they stay available if the publisher moves or removes them. Check the publisher's website for the latest revision.

Tools for this job

Shop by category

More Tile & Stone guides