TILE BACK BUTTERING EXPLAINED

What Is Back Buttering Tile?

Back buttering is the process of applying a thin, even layer of thinset mortar (or tile adhesive) directly to the back of the tile just before placing it onto the substrate (floor, wall, or backer board).

You typically use the flat side of a trowel to spread the mortar, creating a skim coat that fills the tile's pores, ribs, or any warpage. This is then set into the notched thinset already applied to the surface.

This technique complements the main thinset bed on the substrate – it doesn't replace it.

Why Is Back Buttering Important?

Proper back buttering helps achieve the industry-standard mortar coverage required by ANSI (American National Standards Institute) and TCNA (Tile Council of North America):

• 80% minimum coverage for dry interior areas
• 95% minimum coverage for wet areas (showers, bathrooms), exteriors, or commercial floors

Benefits include:

1. Stronger bond promotes the increased bond strength of the mortar to the back of the tile by keying or burning the mortar into the tile surface.
2. Fills voids created by the embossed pattern on the back of the tile which creates a flat plane.
3. Can fill inherent tile warpage of the tile (as long as the maximum mortar thickness designated by the manufacturer has not been exceeded).
4. Prevents shadowing for glass and translucent tiles.

When Should You Back Butter Tiles?

Note: According to the ANSI Specifications, back buttering is not a requirement.

ANSI A108.5 section 2.5.3 states:

"Average contact area shall be not less than 80% except on exterior or shower installation where contact area shall be 95% when not less than three tiles or tile assemblies are removed for inspection. The 80% or 95% coverage shall be sufficiently distributed to give full support of the tile."

Step-by-Step How to Back Butter Tiles

1. Prepare your thinset – Mix according to manufacturer instructions (no lumps, peanut butter consistency).
2. Trowel the substrate first – Use a properly sized notched trowel in one direction for full ridges.
3. Scoop thinset onto the tile back – Use the flat side of your margin trowel or the flat edge of your notched trowel.
4. Spread a thin, even layer – Aim for a skim coat about 1/16" to 1/8" thick (just enough to cover the back completely and fill ribs/voids). Some pros "burn it in" by pressing firmly.
5. Place the tile – Set it into the substrate thinset, sliding back-and-forth perpendicular to the trowel lines to collapse ridges and remove air.
6. Beat in or twist slightly – Use a rubber mallet or grout float to ensure full contact.
7. Check coverage – After setting a few tiles, lift one to verify 80–95%+ thinset transfer. Make sure, wet thinset is on both surfaces.

Frequently Asked Questions

Is tile back buttering a requirement?

No – it is only recommendation. See above.

Is back buttering the same as medium bed mortar?

No – medium bed is for thicker applications on uneven floors. Back buttering is always a thin skim.

Do I need to back butter small tiles like mosaic?

Rarely – It’s most essential with large, heavy, or porous tiles like natural stone and porcelain.

Can I back butter with ready-mix mastic?

No – only use thinset mortar for back buttering.

Can I do back buttering with a regular trowel?

Yes – a notched trowel works well for spreading mortar on both the substrate and the tile back.