What is buttering (back-buttering) and when is it used?

• A. It is applying grout to the back of the tile.
• B. It is tapping tiles to set them.
• C. It is applying a sealant to the tile back.
• D. It is applying a thin coat of mortar to the tile back (or both tile and substrate) to improve bond for large-format or challenging surfaces.

Answer: (D)

Buttering (back-buttering) means applying a thin, even coat of mortar to the back of the tile, and sometimes also to the substrate, to improve adhesion for large-format or otherwise challenging tiles. The idea is to create more contact with the tile and fill any ridges or gaps on the tile back (and on an uneven substrate), so when you press the tile in, there are fewer voids and a more uniform bed. This technique is especially helpful with large-format tiles, very smooth backs (like glazed porcelain or glass), or substrates that aren’t perfectly flat. It’s not about grout or sealant. You’ll use back-buttering when the tile size, back texture, or substrate condition could otherwise lead to bond problems or noticeable lippage.