While of course we can't say definitively what the issue is without "seeing" what's underneath, the tile should not move, period. I'm going to share a document I received this morning from Sal DiBlasi, a seasoned tile professional who we work with on an industry committee. Our committee met yesterday and discussed the problems associated with "spot bonding". I think Sal's explanation is clearly stated. Sorry for the long read......."A tile is applied to a surface, be it a floor wall ceiling, whatever, has one critical element that must be done properly if that tiled surface is to endure over time. That critical element is the bond of the tile to the surface to which it is applied. This might seem like a simple thing, and for the most part it is, but the execution is not as simple as it might seem. We can talk about surface preparation and the requirements that must be met, but it all comes down to how mortar is applied to the surface, and to the tile and how well the two bond together. If a tile is not well bonded, then it does not take long before telltale signs of installation failure start to appear. Some of these signs are loose cracking grout, hollow sounding tile, tile that seems to move or rock when it is walked on, cracked tile, broken tiles, tiles that have splotchy stains or uneven shading that seems to come from behind the tile. These problems are not limited to floors but can also manifest on walls, especially wet areas like a shower stall. In many cases when a tiled surface exhibits these issues the culprit is very likely “Spot Bonding” To understand what spot bonding is, and why it is a very bad idea, it might be helpful to look at what a is required to properly bond a tile to a suitable substrate. First the proper type of mortar must be selected, then the mortar must be applied to the surface by first “keying” the mortar to the surface using the flat side of the trowel to force the mortar into the pores of that surface, then the mortar is spread and gauged with the notched side of trowel to create an even bed of mortar to which the tile is laid and set. In a dry area the required coverage of the set tile is 80% in a wet area or for natural stone it is 95%. Dry set Mortar, often referred to as “Thinset” has properties that allow it to be used up to a certain thickness, in most cases no more than 3/8” if the mortar is applied at greater thicknesses it will start to shrink and affect the bond strength of the tile to the substrate. Spot bonding is the practice of applying thick globs of mortar to the back of the tile, one in each corner, and one in the center. There are variations in the number of “Globs” applied to the tile, sometimes 9 or even more if the tile is very large. This method is used because it allows the installer to create a flat even tile surface over a substrate that is not flat and unsuitable for tile. A flat tile wall, or floor can easily be achieved with zero prep of the substrate and in a short amount of time. It is efficient, easy and get the installation finished. As we talked about. Mortar has a maximum bond coat thickness and this “5 Spot” method will always require that the mortar be applied at a thickness far beyond that it was designed for. The results will usually be that the mortar will shrink, pull away from the tile and leave it with very little bond to the substrate, also the wall or floor will have very little support, have very large voids under the tile, will have no impact resistance and be prone to having cracked loose grout, tiles will easily debond crack and break and leave the tile surface with a hollow sound. End result a failed tile installation in short order."
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tile/wallpaper
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