Ask the Architect

A vibrating floor is a concern


Q. We are redoing our largest bathroom, and our interior designer and contractor are planning to put 12-inch-square tiles on the walls and the ceiling across the room. It should be nice, but we’re wondering if there’s something that needs to be done that we should know about, since we wonder if the tile can’t come down. The floor above is bedrooms and bathrooms, and we notice a little shaking in the floor when people are walking around. Should we be concerned?

A. Yes. As with any construction, you should always be concerned, or at least educated enough to be confident, about what others are doing to your home. Never take for granted that the people you hire just know it all. Nobody knows it all, and with the internet, it’s possible to see examples to answer your question. The funny thing is that I went to the internet to see if anything has changed with tile adhesives and best materials before answering your question. What I found were a number of different how-to videos, and nearly every one of them left out important information. People only tell you what they know.
There are multiple steps to installing wall and floor tile, and ceiling tile installation is similar, with the exception that a few more steps are required. The backing material attached to your ceiling joist structure, referred to as the substrate, must be non-porous and not typical sheetrock (gypsum board), because sheetrock is laminated with a facing of paper that will saturate and peel off. Most internet videos didn’t mention that. I’ve seen entire walls, covered with tiles, suddenly drop to the floor, sometimes years after installation, ending up in a messy heap.
The structure must be rigid, and not moving as you described. Not one of the videos questioned whether the structure or backing material was correct for the job. In the building codes, the amount of floor sag, called deflection, can’t be more than a half-inch over the entire length of the structure. If you have floor movement, the floor must first be calculated and stiffened before the substrate boards are added. The “experts” seemed to take that for granted. Maybe because it is perceived that it would cost too much to know by hiring someone qualified to figure that out.
If the structure isn’t stiffened correctly, movement will potentially loosen tiles and, aided by gravity, those ceiling tiles can fall. The substrate I recommend is a cement board, fiberglass reinforced (to lighten the load). There are several available. The most common are Wonderboard or Durrock. These boards have slightly rough surfaces to receive mastic which will be applied, and adhere well by increased surface “teeth” texture.
The mastic, as described by manufacturers, should be a lightweight, “no sag” adhesive. Not one mastic manufacturer I investigated even mentioned ceilings in their directions or warranty, for obvious reasons. So have the structure evaluated and continue to ask questions before you start. Good luck!

© 2022 Monte Leeper. Readers are encouraged to send questions to, with “Herald question” in the subject line, or to Herald Homes, 2 Endo Blvd., Garden City, NY 11530, Attn: Monte Leeper, architect.